Changes in Production Practices Used for Disease Management in Blueberry Nurseries in Georgia, USA, Over a 15-year Period

Authors:
Jeremy C. Haralson Department of Plant Pathology, University of Georgia, Athens, GA 30602, USA

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Phillip M. Brannen Department of Plant Pathology, University of Georgia, Athens, GA 30602, USA

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Walt Sanders Department of Plant Pathology, University of Georgia, Athens, GA 30602, USA

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Harald Scherm Department of Plant Pathology, University of Georgia, Athens, GA 30602, USA

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Abstract

Surveys of blueberry [rabbiteye blueberry (Vaccinium virgatum) and southern highbush blueberry (Vaccinium corymbosum interspecific hybrids)] nurseries in the State of Georgia, USA, were conducted in 2007 and 2022 to determine the prevalence of and associations among propagation practices, especially related to disease management. As indicated by the reduction in surveyed nurseries in 2022 (7) compared with 2007 (18), the Georgia blueberry nursery industry has consolidated. However, cultural disease management practices have generally improved in these remaining nurseries. In 2007, in nurseries where cuttings were grown in containers, 77.8% reused containers and 66.7% did not sterilize them before use. The growing medium [pine (Pinus sp.) bark] was reused for subsequent production cycles in 29.4% of nurseries, although such reuse of media tended to be associated with production in beds as opposed to containers (P = 0.08). Nurseries used well water in 88.2% and pond water in 11.8% of cases. Cuttings were grown on benches (vs. the ground) in slightly fewer than half of the nurseries. In contrast, all nurseries surveyed in 2022 grew their cuttings in containers, used well water, and had increased bench use, albeit only slightly. Although all nurseries reused containers, only 28.5% did not sterilize containers before use, and only 14.3% of nurseries reported reusing media. Most nurseries surveyed in 2007 (83.3%) were on a routine, calendar-based fungicide program using a.i. targeted primarily against aboveground diseases (blights and leaf spots) and secondarily against soil-borne water molds (Phytophthora and Pythium species of the Oomycetes class). In contrast, 42.8% of those surveyed in 2022 were on a 2- to 3-week spray schedule, 42.8% used fungicides on an as-needed basis, and 14.4% were on a monthly schedule, indicating that fungicide scheduling varied dramatically among the remaining nurseries.

With the rapid expansion of Georgia’s (USA) blueberry (Vaccinium sp.) industry in the past 2 decades (Fig. 1), the demand for rooted cuttings increased exponentially. This prompted many growers to begin propagating cuttings both for their own use and for sale to other growers. The first published accounts of blueberry propagation were made in a bulletin from the US Department of Agriculture Bureau of Plant Industry entitled “Experiments in Blueberry Culture” by F.V. Coville (1910). Coville (1921) later published “Directions for Blueberry Culture, 1921,” laying out the basic procedures for the propagation and production of blueberries as a horticultural crop. In the intervening years, little changed in blueberry propagation methods (Cline and Mainland 2008; Haralson et al. 2021). Many recommendations have been made based on Coville’s work, but no preferred method has been established for either propagation per se or disease management during propagation. Thus, it is not surprising that growers and nursery operators in Georgia have applied a wide range of methods that combine elements from many different sources, resulting in wide variation in propagation methods (Haralson 2009).

Fig. 1.
Fig. 1.

Blueberry area in Georgia, USA, from 2001 to 2020 (University of Georgia Center for Agribusiness and Economic Development 2001–20); 1 acre = 0.4047 ha.

Citation: HortTechnology 33, 3; 10.21273/HORTTECH05154-22

Blueberry propagation is achieved primarily by rooting either softwood or hardwood cuttings, although softwood cuttings are used almost exclusively in Georgia. For hardwood propagation, cuttings measuring 4.5 to 5.5 inches long with a diameter of 1/4 to 1/2 inch are taken from the previous season’s growth in late January to early February (Haralson et al. 2021). Softwood cuttings can be taken in May to early June and late July to early August in Georgia. Softwood cuttings are cut from apical shoots from the current season’s growth and should measure between 4.5 and 6 inches in length (Cline and Mainland 2008; Haralson et al. 2021). An advantage of softwood is the speed at which rooting occurs; softwood cuttings can be rooted in 6 to 8 weeks (Haralson et al. 2021), compared with hardwood stock that can take as long as 6 months to produce a rooted cutting. In addition, hardwood cuttings also generally have lower rooting percentages and are at higher risk for stem blight (Botryosphaeria dothidea), as compared with softwood cuttings. However, softwood cuttings are subject to numerous plant diseases (Haralson et al. 2021, 2022). Registered chemical control options are limited (Haralson et al. 2013); therefore, using best-management cultural practices is important to disease suppression during propagation.

Although softwood cuttings are the primary choice for propagation material in Georgia, many different propagation systems and methods are used by growers (Haralson et al. 2022). Propagation systems can be separated into two main types: open and closed. A closed system is defined by an enclosed growing environment such as a greenhouse or shade-house, whereas an open system is exposed to the natural outdoor environment. Preliminary farm visits (2000–05) indicated that a range of growing media, containers, and pest management practices are used in blueberry propagation.

Diseases have been a particularly difficult challenge for blueberry propagation (Haralson et al. 2013, 2021, 2022). In other regions, the four main diseases affecting blueberry cuttings are Cylindrocladium root rot [Calonectria ilicicola (Cylindrocladium parasiticum)], Phytophthora root rot (Phytophthora cinnamomi), Pythium root rot (Pythium sp.), and Rhizoctonia root rot (Rhizoctonia sp.) (Cline 2004; Haralson et al. 2013; Schilder and Cline 2017). In Georgia blueberry nurseries, the predominant pathogens are C. ilicicola and Rhizoctonia sp., with water molds (Phytophthora and Pythium species of the Oomycete class) so far contributing little to plant losses from disease (Haralson et al. 2021). C. ilicicola is spread primarily through contaminated plant material or propagation substrate, and reuse of propagation media has been strongly linked to Cylindrocladium root rot in Georgia and North Carolina, USA, rooting beds (Cline 2004; Haralson et al. 2022). Likewise, a similar trend was observed in Georgia for Rhizoctonia root rot (Haralson et al. 2022).

With regard to fungicide use, efficacious fungicides for blueberry propagation are limited due to lack of US Environmental Protection Agency registrations. Fludioxonil (Cannonball WG; Syngenta Crop Protection, Greensboro, NC, USA) is registered for use against Rhizoctonia and Calonectria, but this may be the only currently registered fungicide with a blueberry propagation use pattern on the label. Azoxystrobin (Abound FL, Syngenta Crop Protection), which is known to have activity against Rhizoctonia, is registered for use on blueberry, but it is not registered specifically for a propagation use pattern. The use pattern language is often debated when selecting fungicides, but a conservative approach to fungicide use would dictate that the use pattern must be on the label to allow for a legal application, especially for enclosed structures.

To better understand the diverse methods of propagation found in Georgia and as a baseline for developing best-management practices, a survey was conducted in 2007 to document production and disease management methods in use. To further document changes in the industry and production practices following the first survey and subsequent efforts to educate producers regarding disease management practices, a second survey was conducted in 2022.

Materials and methods

Survey collection

Survey data were collected from 18 blueberry propagation nurseries located in Appling (3), Bacon (5), Brantley (1), Clinch (5), Pierce (2), and Ware (2) counties (key areas of Georgia, USA, blueberry production) in mid-Jun 2007. Subsequently, a second round of survey data were collected from seven blueberry propagation nurseries in Bacon (5) and Pierce (2) counties from Dec to mid-Feb 2021 and 2022. Both surveys were collected by means of farm visits or by phone through use of a questionnaire—the same questions being used in both surveys. Producers were interviewed about their current propagation methods to document the diversity in production methods, review current industry trends, and assess disease risk based on propagation practices.

Survey instrument

The survey consisted of the following questions: 1) How many cuttings do you take every year? 2) How many are rabbiteye blueberry (Vaccinium virgatum) and how many are southern highbush blueberry (Vaccinium corymbosum interspecific hybrids)? 3) Are they hardwood or softwood cuttings? 4) What time of year do you take cuttings? 5) What percentage of your cuttings generally root? 6) Do you use rooting hormone? 7) Do you employ an open or a closed propagation system? 8) Do you use benches or grow at ground level? 9) Do you use containers, and if so, what type? 10) Do you reuse containers? 11) If so, do you sterilize the containers? 12) What kind of medium is being used for propagation? 13) Do you reuse media? 14) What is your source of irrigation water for your propagation system? 15) What type of irrigation system do you use (mist heads, impact sprinklers, etc.)? 16) What fungicides do you use and at what rates are they applied? 17) What is your spray schedule? 18) What disease(s) do you generally see in your propagation system?

Statistical analysis

The collected survey data were summarized (means for continuous variables, frequencies for categorical variables), and associations between selected practices were analyzed using chi-square contingency tables using statistical software (JMP Pro version 16.0; SAS Institute Inc., Cary, NC, USA).

Results and discussion

As expected based on preliminary observations, the two surveys revealed a maturation of the blueberry industry over the 15-year interval between them. Indicators of this include the recent leveling-off and slight decline in blueberry acreage (Fig. 1), the decrease in producers of propagated plants, and the more uniform propagation methods as compared with 2007. The 2007 survey gave insights on how growers adapted their propagation systems based on what they observed in neighboring nurseries, but also shed light on the lack of uniformity between them. In contrast, nurseries surveyed in 2022 used similar practices in all but a few aspects. To illustrate the diversity present in the propagation systems surveyed, the production methods of six nurseries are outlined for comparison.

Propagation system examples

A typical example of an open propagation system was located in Bacon County, GA, USA, in 2007 (Fig. 2). This nursery eliminated construction costs by growing propagated plants under a pine (Pinus sp.) stand, which acted as a wind break and provided shade for the cuttings. Softwood cuttings were produced in “trade gallon” (3-qt) plastic containers on ground covered with a ground cloth barrier to suppress weed growth and help prevent direct ingress of pathogens from the soil into the pine bark rooting medium. Softwood cuttings were taken in May and June. Impact sprinklers were used for irrigation, and the water source was a deep well. In 2006, this nursery produced 200,000 softwood cuttings, two-thirds of which were rabbiteye blueberry and one-third southern highbush blueberry. The disease management program consisted of fungicide sprays every 7 to 10 d with a rotation of captan, pyraclostrobin, and thiophanate-methyl. The grower reported 90% rooting using this system, which concurred with observations at the site. However, poor site drainage in low areas tended to result in pooling water from the irrigation system, which led to lower rooting success in certain portions of the nursery. In years with heavy rain, this drainage issue could pose serious problems.

Fig. 2.
Fig. 2.

Open, containerized blueberry propagation system using impact sprinklers in Bacon County, GA, USA. Note the standing water and potential for pathogen introduction from soil and water. The inset shows the number of cuttings set per pot; individual plants will be repotted for additional growth and sales.

Citation: HortTechnology 33, 3; 10.21273/HORTTECH05154-22

Another open growing system, located in Clinch County, GA, USA, in 2007, used very different propagation methods. As before, softwood cuttings were propagated in trade gallon containers with milled pine bark as the medium, but the containers were grown in full sun on ground cloth under an overhead mist system connected to a deep-well water source. Softwood cuttings were taken in September during the second flush of vegetative growth, and these were treated with the fungicide captan only when disease was observed. Calonectria was recovered from this nursery (Haralson et al. 2022).

The grower reported producing 200,000 softwood cuttings in 2006 with 80% rooting success. At this nursery, overcrowding was a major concern, as more than 20 cuttings were grown in each container, and of these, approximately one-third had died. Overcrowding results in excessive competition for nutrients and water, and any substantive increase in temperature can result in stress and plant death. Overcrowding, along with sun exposure (overheating) on the sides of the containers, was likely responsible for the failure of these softwood cuttings. The producer’s estimated 80% rooting success seems overly optimistic considering issues observed.

A similar nursery was surveyed in 2022, which produced 300,000 softwood cuttings, 20% of which were rabbiteye blueberry and the remaining 80% southern highbush blueberry. This nursery used many of the same tactics: an open propagation system with trade gallon pots and a pine bark rooting medium. Trays were also used to start softwood cuttings, which were then moved to pots, preventing the overcrowding problems of the second 2007 nursery. The system also used a mist head irrigation system with well water. Softwood cuttings were taken in both spring and fall from May to June and August to September, and 80% to 95% rooting success was reported. A key difference with this operation was the 2- to 3-week spray schedule employed. This producer declined to share details of chemicals sprayed.

A good example of a closed propagation system was found in Pierce County, GA, USA, in 2007. Softwood cuttings were grown in a shade-house on benches under overhead mist (Fig. 3). The grower used open propagation flats filled with finely milled pine bark as a propagation medium, and he reported production of 80,000 softwood cuttings with equal proportions of highbush blueberry and rabbiteye blueberry and 90% rooting. Softwood cuttings were taken during the first flush in late May to mid-June, and these were subjected to a 10-d spray schedule with rotations of captan, mefenoxam, benzimidazole, and phosphorous acid fungicides. On-site observations confirmed high rooting percentages, and most softwood cuttings observed were asymptomatic of any diseases. Benches appeared to promote good air flow, and containers were well-drained while retaining sufficient moisture to prevent softwood cuttings from drying out. Benches also made working with cuttings much easier, as no bending was required.

Fig. 3.
Fig. 3.

Closed, containerized blueberry propagation system (shade-house) in Pierce County, GA, USA, with cuttings produced in flats located on benches. Bench systems allow for better sanitation between propagation runs, and groundwater/soil is less likely to be a source of pathogen inoculum.

Citation: HortTechnology 33, 3; 10.21273/HORTTECH05154-22

Another closed system, visited in Bacon County, GA, USA, in 2007, used entirely different methods. The propagation house was constructed of fiberglass with canvas sides to allow cross-ventilation (Fig. 4). Softwood cuttings were propagated in raised bark bed troughs ∼2.5 ft high and 3 ft wide, which ran the length of the structure. The bottom of the bed was lined with drainage tile covered by a 1-ft sand layer. This was covered by a thick layer of finely milled pine bark, which acted as the propagation medium. Softwood cuttings were irrigated by a mist system controlled by a microleaf mist controller. Softwood cuttings were taken in June toward the end of the first vegetative growth flush, but sometimes a second crop of rooted softwood cuttings was produced after the second flush in August to September. The fungicide program consisted of weekly rotations of captan and thiophanate-methyl. Both Calonectria and Rhizoctonia species were subsequently recovered from this nursery. The grower reported producing 350,000 softwood cuttings with an 85% rooting percentage, which appeared overly optimistic in light of several issues observed. Among these, the propagation beds were severely overcrowded with little room for airflow among softwood cuttings. The close proximity of cuttings also appeared to facilitate disease spread through plant-to-plant contact and through the medium. The second factor that increased disease risk was the reuse of medium for a second crop of softwood cuttings later in the year. Losses in these beds were severe, and diseased cuttings were prevalent, with ∼30% of the propagation bed area showing diseased plants.

Fig. 4.
Fig. 4.

Closed, bark bed-based blueberry propagation system in Bacon County, GA, USA. Bark bed systems can be constructed outside or inside of greenhouses. They are difficult to sanitize if bark is removed and replaced, and because of the cost of bark, growers are often tempted to simply reuse the bark for the next round of propagation.

Citation: HortTechnology 33, 3; 10.21273/HORTTECH05154-22

Closed propagation systems in the 2022 survey were very similar to those described previously, but with notable improvements in sanitation. Good sanitation is accomplished by removing and discarding old media from flats, scrubbing and washing off remaining debris, soaking the flats in 10% bleach solution for 1 h or more, and discarding the bleach solution after a single use. Although there may be means of sanitizing spent media, no producers in either survey attempted to sanitize media; instead, the media was either discarded or simply reused.

A nursery in Pierce County, GA, USA, surveyed in 2022 reported taking 40,000 softwood cuttings per year from May to July. Flats were used with a pine bark medium at ground level. The irrigation system was by mist heads connected to well water. A 90% rooting success was reported. Flats were reused after being sanitized in a bleach solution and the media was discarded after a single use. These precautions provided a much more sanitary space for softwood cuttings to grow, which could account for the nursery’s reduced spray program. The fungicide program used captan and phosphorous acid fungicides on a monthly or as-needed basis. They reported observing Phytophthora and Rhizoctonia root rots as the main disease problems encountered.

A second closed propagation system surveyed in 2022 from Bacon County, GA, USA, was very similar. This nursery reported taking 30,000 softwood cuttings per year, all southern highbush blueberry with 75% to 80% rooting success. These were taken in early spring and treated with rooting hormone. Every practice previously discussed for the Pierce County nursery was used. They used flats with pine bark medium at ground level with a mist irrigation system on well water. This nursery also discarded media after a single use. Also in line with the Pierce County nursery was the reduced spray schedule. This producer reported using captan and fludioxonil on a 2- to 3-week schedule. Root rot, although not specified, was again reported to be the main cause of disease in this operation.

Prevalence of specific propagation methods

One factor complicating data analysis was the relatively small database of nurseries available for survey, especially in 2022 (n = 7). Several factors may explain the decline in nursery numbers during this 15-year period. Although Georgia’s blueberry acreage was considerably larger in 2022 than in 2007, acreage has declined slightly in recent years; in contrast, 2007 was at the beginning of a rapid acreage increase (Fig. 1), which required large numbers of nursery stock for expansion. Furthermore, other US states (e.g., Florida) are now providing a substantial number of blueberry plants for the Georgia industry. Still, the observations and data collected during the course of the survey shed light on the situation in the remaining southern Georgia blueberry nurseries (Table 1).

Table 1.

Prevalence of propagation practices in blueberry nurseries in southern Georgia, USA, based on surveys conducted in 2007 and 2022.

Table 1.

Types of softwood cuttings produced

There are two major types of blueberries produced in southern Georgia: rabbiteye blueberry and southern highbush blueberry (Williamson and Lyrene 2004). Based on the results of this survey, nurseries have made a shift to producing more southern highbush blueberry softwood cuttings than rabbiteye blueberry, which was predicted at the time of the 2007 survey. In 2007, 14 of the 18 nurseries surveyed responded with the approximate number and type of blueberry grown. These 14 growers produced ∼1.6 million softwood cuttings in 2007, of which 75% were rabbiteye blueberry. Of the seven nurseries surveyed in 2022, six responded with the number and type of blueberry grown. These six growers produced ∼424,000 softwood cuttings, of which 14% were rabbiteye blueberry. The remaining softwood cuttings were 81% southern highbush blueberry and 5% Legacy, a northern highbush blueberry cultivar (V. corymbosum). Historically, ∼90% of blueberry plantings in Georgia have been rabbiteye blueberry (Scherm and Krewer 2003), but more recently the popularity of southern highbush blueberry cultivars has drastically increased because of their earlier harvest date and coincident market advantage. This, in addition to the fact that southern highbush blueberries are sometimes grown in high-density beds (thereby requiring a larger number of nursery plants for initial establishment), may account for the lower proportion of rabbiteye blueberry cutting production.

Regarding the timing of cutting collection, 12 growers in 2007 took softwood cuttings during the first flush in May and June, whereas four used softwood cuttings from the second flush in August and September, and only two engaged in hardwood propagation (taking hardwood cuttings in February and March). The time the cuttings were taken was dependent on the schedule of the blueberry farm. Softwood cuttings were usually taken immediately after harvest, without regard to the type of blueberry (i.e., rabbiteye blueberry or southern highbush blueberry). Most growers chose not to use rooting hormone, as previous experience had shown little benefit on their rooting percentages. This is consistent with observations from North Carolina, USA (Cline and Mainland 2008). When reviewing the data from 2022, softwood cuttings are still the most popular form, as all seven nurseries surveyed took most of their softwood cuttings in the spring, and three reported that they also take softwood cuttings in the fall. No hardwood cuttings were reported. Four of seven reported use of rooting hormone, but their reports for rooting success were similar to those who chose not to use it.

Nursery size

For the purposes of this paper, a small nursery was defined as one that propagated 70,000 or fewer softwood cuttings per year, whereas a nursery propagating more than 70,000 softwood cuttings was defined as a large operation. A slight majority of nurseries (60.0%) surveyed in 2007 were large operations (Table 1). In contrast, a majority of nurseries (71.5%) that provided survey responses in 2022 were classified as small. This proportion will fluctuate slightly from year to year, as some of the growers produce cuttings based on their own needs, hence demand could drop considerably after their new plantings have been established.

Open versus closed systems

Open growing systems are exposed to the elements, although some use man-made or natural wind breaks to prevent moisture loss. In closed structures, softwood cuttings are grown in a greenhouse or shade-house where moisture can be controlled more easily. Based on the 2007 survey (Table 1), the two general production types were represented roughly equally, with closed systems being slightly more prevalent (52.9%). The 2022 survey yielded similar results, although the open propagation systems were slightly more prevalent (57.1%). Summer droughts may give growers almost total control of moisture levels in open propagation systems. However, an open system may prove problematic in wet years, as was the case in Georgia when reports of reduced rooting percentages followed heavy rains in 2001 (Krewer and Cline 2003). Closed propagation systems are the best option for growers who plant rooted cuttings annually. Whereas the initial expense might be high, the reductions in risk due to better environmental control will likely pay off over time.

For the 2007 data, a chi-square contingency table analysis revealed a significant association (P = 0.003) between nursery size and the use of open vs. closed propagation systems. Specifically, large-scale production (>70,000 softwood cuttings per year) was associated with the use of closed systems for propagation (Table 2). Indeed, among large-scale nurseries, 77.8% used closed systems, whereas none of the small-scale nurseries used closed systems. This relationship was not surprising, as most large-scale growers have more resources to invest in permanent or semipermanent structures. Conversely, small-scale growers primarily grow cuttings on a temporary basis and have less capital to invest in infrastructure. However, by 2022, some small-scale nurseries used closed systems, whereas the two remaining large-scale producers did not (Table 3).

Table 2.

Frequency of southern Georgia, USA, blueberry nurseries (n = 18) with select propagation practices in 2007.

Table 2.
Table 3.

Frequency of southern Georgia, USA, blueberry nurseries (n = 7) with select propagation practices in 2022.

Table 3.

Use of benches

Slightly fewer than half (47.1%) of the surveyed nurseries in 2007 produced softwood cuttings on benches rather than at ground level (Table 1). Chi-square analysis showed a trend (P = 0.09) for an association between propagation systems (open vs. closed) and the use of benches, whereby propagators who used closed systems also generally used benches (Table 2). In the 2022 survey, there was a near equal representation of benches and ground systems; four nurseries used benches and three did not (Table 1). Surprisingly, open systems favored growing the softwood cuttings on benches (75.0%), the exact opposite of the first survey. This trend is difficult to explain, although benches alone (open system) are cheaper than benches in closed systems. As overall sales have been reduced, economic considerations may be driving these decisions.

Container versus bed production

The decision whether to grow cuttings in continuous propagation beds or in containers is a very important consideration. The 2007 survey results showed that the two types were represented roughly equally, with containerized production being slightly more prevalent (Table 1). Five of the eight nursery operators who used bed systems chose to build their beds on benches or raised off the ground on cinderblocks. Containers also were either placed on benches or left on the ground, but there was no statistical association between container use and bench use (Table 2). For ground-based containerized systems, some type of barrier (usually ground cloth) was used in most cases to prevent direct contact of the containers with the soil. Of the containers chosen for propagation, the standard trade gallon was by far the most popular option, being used in six of the nine container-using nurseries. These containers are inexpensive, reusable, and easily available; provide sufficient room for multiple softwood cuttings per container; and are deep enough to prevent the formation of zones of saturation around the roots. The other containers in use were deep cell packs and large propagation flats, and these met requirements needed for good rooting as well. Of those nurseries using containers rather than beds in 2007, 77.8% reused their containers. Of concern was the fact that only 33.3% of these nurseries sterilized the containers before reuse, providing potential ingress points for pathogens to enter propagation systems. The 2022 survey revealed that the industry has shifted toward plastic container propagation, as 100% of the seven nurseries surveyed reported doing so. Of these, 71% reported using trade gallon pots, and the remaining nurseries reported using large flats. Every nursery reported reusing their containers, but 71% of those nurseries sterilized their containers to control disease—a major improvement from 2007.

Propagation media

Historically the medium of choice for propagating blueberries was aged sawdust from sawmills (Cline and Mainland 2008). Over the years, most sources of sawdust have disappeared, forcing propagators to choose substitutes. Raw pine bark is now the primary propagation medium used in blueberry nurseries. Indeed, the 2007 survey indicated that raw pine bark was used in 89% of Georgia blueberry nurseries; only two nurseries used other types of propagation media, with one having access to aged sawdust whereas the other used a peat-bark-perlite mixture. The grower using the mixed medium used deep cell packs, which allowed him to use much less medium than those who used either trade gallon containers or bedding systems, while also making the best use of the more expensive medium.

The almost exclusive use of raw pine bark as a propagation material is one of the few consistent factors illuminated in the 2007 survey. This observation continued into the 2022 survey, with 86.0% of surveyed nurseries using either raw or aged (composted) pine bark. The only nursery not using pine bark was using a standard store-bought growing medium with flats to reduce media usage. In both surveys, the quality of the bark varied greatly among operations. Composted pine bark is highly recommended as a propagation material, and it has been shown to suppress many pathogens, including species of Phytophthora, Pythium, and Rhizoctonia (Hoitink and Boehm 1999). Notwithstanding these benefits, composted bark is rarely used (Brannen PM, unpublished data). Composted bark takes more time to produce, and it would be required in greater volume as compared with raw pine bark, rendering it more expensive.

Research has shown that reuse of media and propagation in beds results in a significant increase in diseases caused by Calonectria and Rhizoctonia species during blueberry propagation (Haralson et al. 2022). With regard to disease management practices, research and education efforts can at least in part be linked to a positive change in grower behavior over the past 15 years. Reuse of propagation media has been reduced drastically since 2007, when 29.4% of nurseries reported reusing media (Table 1). In the 2022 survey, all but one of the surveyed nurseries reported discarding media after a single use. The single nursery not doing this was the smallest of the nurseries surveyed. This positive change could also be attributed to the large number of nurseries now using containers instead of bed systems. Data from the 2007 survey reveal that growers who used containers were the least likely to reuse media, with only 11.1% (1 of 9) of this subgroup doing so. Based on a chi-square analysis of the data, a trend (P = 0.08) toward the reuse of media by growers using bedding systems (as opposed to containerized production) was noted (Table 2). Bark decomposes and settles by the end of the growing season, and if the grower uses a ground-based, continuous propagating system, there is a great temptation to simply add fresh bark to the top of the bed without removing the remaining bark from the previous year. Pathogen inoculum would increase with each reuse cycle. In addition, as bark ages and breaks down, drainage is compromised, and the disease-suppressive qualities of the bark are much reduced (Hoitink et al. 1999). The 2022 survey results indicate that bed use has been reduced, and as a result, growers are less likely to be tempted to reuse potentially pathogen-contaminated media from the previous year.

Growing in containers has additional benefits, besides simply allowing easy disposal of used media. Containers provide barriers against the spread of soil-borne pathogens; in a continuous rooting bed, pathogens can spread rapidly throughout the bedding system via plant-to-plant contact (Cline 2004; Haralson et al. 2021). When containers are used, the diseased plants can simply be removed with the container, rather than going through the laborious process of removing the infected softwood cuttings from the bed and then digging out the infected bark in an attempt to prevent further spread.

Water sources and irrigation systems

Water sources are another area of improvement over the 15-year survey period. In 2007, most nurseries (88.2%) used wells as a source of irrigation water, with the remainder using pond water (Table 1). In 2022, 100% of nurseries reported using well water. As water mold diseases can be spread through contaminated pond water (Thomas et al. 2005), the widespread use of wells is encouraging. One fact that nursery operators must keep in mind is that most well water in the survey area has a high pH. As blueberry plants are acid-loving members of the heath or heather family (Ericaceae), the alkalinity of the water must be taken into consideration when irrigating this crop.

Nursery irrigation occurred primarily by mist systems, with 12 of the 18 propagators in 2007 and all propagators in 2022 using this system. The remaining propagators in 2007 used large open systems using less expensive impact sprinklers, which appeared to work well, especially where adequate shade and protection from the wind were provided to help prevent moisture loss. As long as softwood cuttings are provided with a constant film of water on their leaves for the first 3 weeks after collection and sticking, they should root. However, nurseries using impact sprinklers are more vulnerable to desiccation because this system relies on fewer spray heads to cover a relatively large area when compared with an overhead mist system, and a single clogged sprinkler head can result in significant losses. Softwood cuttings can suffer irreversible damage if left dry for as little as 30 min (Cline and Mainland 2008).

Cutting failure and fungicide use

Most nursery operators provided only a vague estimate of rooting success, often in the range of 80% to 90%. However, observations made during the course of the survey stood in contrast to the numbers estimated by the growers, with both under- and overestimation taking place. These data are not tracked closely by many growers, as would generally be the case in most ornamental nurseries. Growers do not typically address rooting success in terms of disease vs. other causes, but where failures occur, the vast majority are related to disease issues, as opposed to failed irrigation or other physiological issues, although these do occur as well (Brannen PM, unpublished data).

A major reason for cutting failure was the tendency toward overcrowding cuttings in both container and bed propagation nurseries. The standard recommended spacing is between 1.5 and 2 inches to allow for adequate airflow and root growth (Cline and Mainland 2008). Many producers grow softwood cuttings with a spacing of 0.5 inch. In some extreme cases, growers placed as many as 25 cuttings in a single trade gallon container, which at most should hold 14. Overcrowding increases competition among cuttings for moisture and space for developing root systems. Indeed, many of the cuttings in overcrowded beds died, having never formed sufficient roots due to lack of water and space to grow. In several cases in which samples were taken for pathogen isolations (Haralson et al. 2022), softwood cuttings were difficult to remove from beds or containers because roots had intergrown with neighboring plants. To separate the cuttings, the young plants must be separated by either cutting or tearing the root system. This leaves numerous potential entry points for disease on replanting. In situations in which Cylindrocladium root rot caused severe losses, cuttings were often planted too close.

In Georgia and North Carolina, USA, Calonectria is the primary pathogen of blueberry propagation (Cline 2004). In azalea (Rhododendron sp.), Calonectria enters nurseries primarily as leaf spots on cuttings from the field (Linderman 1973), and a similar pathway of introduction into the nursery environment is assumed on blueberry (Schilder and Cline 2017). Once introduced, the pathogen can spread rapidly due to its multiple modes of reproduction via conidia, ascospores, and microsclerotia. A significant association between the reuse of propagation media and the presence of Calonectria has been established for Georgia blueberry nurseries (Haralson et al. 2013). Between bedding periods, Calonectria can survive in the propagation medium as either mycelia or microsclerotia. Also, given the limited number of fungicides registered for use on blueberry cuttings, this pathogen poses an important threat to the blueberry propagation industry.

Likewise, Rhizoctonia species are common pathogens in ornamental and forest nurseries (Harris et al. 1994), but Georgia blueberry nurseries also have significant issues with this pathogen (Haralson et al. 2022). Rhizoctonia is ubiquitous in soil and plant debris, and it is most likely introduced by direct soil contact with media.

Water molds, although found in lower frequency, should not be dismissed as a threat. As mentioned previously, these pathogens can be introduced through irrigation water, but because they also survive in debris in or near the ground, they can be introduced directly or through splash to propagation materials (Haralson et al. 2021). Several chemicals are registered for use against water molds: fosetyl-Al (Aliette WDG; Bayer CropScience, Research Triangle Park, NC, USA), potassium phosphite (ProPhyt; Helena Chemical Co., Collierville, TN, USA), mono- and di-potassium salts of phosphorous acid (Agri-Fos; Agrichem, Oak Brook, IL, USA), and mefenoxam (Ridomil Gold EC, Syngenta Crop Protection) are all registered and have proven activity on water molds (Brannen et al. 2009). However, many if not all of these fungicides may not meet the strict label interpretation for a propagation use pattern, although they are registered for use in blueberry plantings.

Efforts over the years to better educate blueberry propagators about potential disease threats seems to have had some effect. Disease management in the surveyed propagation systems appeared to consist primarily of fungicide applications to control diseases that affect the upper portion of the cuttings, such as gray mold (Botrytis cinerea), Phomopsis twig blight (Phomopsis sp.), Septoria leaf spot (Septoria albopunctata), and anthracnose leaf spots (Colletotrichum sp. and Gloeosporium myrtilli). In 2007, the most commonly used fungicides were captan products. Other a.i. used by at least three of the 18 nurseries included mefenoxam, phosphorous acid, pyraclostrobin, and thiophanate-methyl. Several additional compounds were used less commonly (Fig. 5).

Fig. 5.
Fig. 5.

Fungicide usage in surveyed blueberry propagation nurseries in southern Georgia, USA, in 2007 and 2022.

Citation: HortTechnology 33, 3; 10.21273/HORTTECH05154-22

Whereas many growers are using broad-spectrum fungicides that can control many fungal groups, including water molds (phosphorous acid and mefenoxam) and Rhizoctonia species (azoxystrobin), it must again be mentioned that none of these chemicals are registered for this usage pattern (i.e., for use in blueberry propagation). This is a major concern revealed by this survey. As very few chemicals are registered for use in blueberry propagation, much work needs to be done via education of the growers concerning potential threats to their crop and to get chemicals registered for legal use in this industry. The 2022 survey revealed that the pool of popular fungicides has been reduced, but captan products are still, by far, the most popular. Other a.i. include mefenoxam, phosphorous acid, pyraclostrobin, and fenbuconazole (Fig. 5). Based on research conducted by Haralson et al. (2013), registrations of two fungicides, triflumizole and fludioxonil, were expanded to include blueberry propagation use patterns. Although both gained some popularity after 2007, fludioxonil alone remains labeled.

Conclusions

The foundation of any horticultural industry is healthy plants. A streamlined and standardized method of propagation is needed to maintain the viability of the Georgia blueberry industry. Although other factors may be involved as well, propagation research and outreach efforts have been implemented in Georgia over the past ∼15 years, and the industry has generally moved closer to standards that would result in fewer disease problems during propagation. Propagation in containers and the discarding of used media have become much more prevalent, which helps decrease the spread of disease. Mist head irrigation systems, along with the use of well water, have also seen an increase. Overall, the propagation industry has reached a mature state and is greatly consolidated in the time between the two surveys. The blueberry industry as a whole in Georgia has also matured in the time between the two surveys, with less new plantings. This sentiment is reflected in the change in blueberry acreage (Fig. 1). Data from Georgia Farm Gate Value Reports (University of Georgia Center for Agribusiness and Economic Development 2001–20) show that blueberry acreage peaked in 2017 and has been in slow decline since. Although this is characteristic of a maturing agricultural industry, both domestic and international competition have also placed downward pressure on growth. As better-yielding and disease-resistant cultivars are released and old plantings are replaced, the demand for high-quality cuttings will continue, although in the near term, this demand is not likely to require such large quantities of nursery stock as previously observed. With the widespread use of fungicides that are not registered for use in blueberry propagation, continued research is needed to determine the key pathogens associated with cutting failure as well as optimal chemical control programs. Strong recommendations should still be made against the reuse of propagation media, as it has been associated with Cylindrocladium and Rhizoctonia root rots. Recommendation of containers over traditional bedding systems should be emphasized, as traditional propagation beds encourage media reuse. Based on this survey, the groundwork for future standardized recommendations for the production of disease-free blueberry cuttings can be laid.

Units

TU1

References cited

  • Brannen PM, Harmon P & NeSmith DS. 2009 Utility of phosphonate fungicides for management of Phytophthora root rot of blueberry Acta Hortic.810 331 340 https://doi.org/10.17660/ActaHortic.2009.810.43

    • Search Google Scholar
    • Export Citation
  • Cline B. 2004 Fungal pathogens associated with blueberry propagation beds in North Carolina Small Fruits Rev.3 1-2 213 221 https://doi.org/10.1300/j301v03n01_21

    • Search Google Scholar
    • Export Citation
  • Cline B & Mainland M. 2008 Blueberry propagation 56 62 Proceedings of the 42nd Annual Open House and Trade Show, North Carolina Blueberry Council Clinton, NC, USA

    • Search Google Scholar
    • Export Citation
  • Coville FV. 1910 Experiments in blueberry culture US Dept Agric, Bur Plant Ind Bull 193

  • Coville FV. 1921 Directions for blueberry culture 1921 US Dept Agric Bull.974 https://doi.org/10.5962/bhl.title.108248

  • Haralson JC. 2009 Pathogens associated with blueberry cutting failure in south Georgia nurseries and their control (MS Thesis) Department of Plant Pathology, University of Georgia Athens, GA, USA

    • Search Google Scholar
    • Export Citation
  • Haralson JC, Brannen PM, NeSmith DS & Scherm H. 2013 Chemical control of Cylindrocladium and Rhizoctonia root rots in blueberry propagation Crop Prot.44 1 5 https://doi.org/10.1016/j.cropro.2012.09.017

    • Search Google Scholar
    • Export Citation
  • Haralson J, Brannen PM & Oliver J. 2021 Propagating disease-free blueberry plants from softwood cuttings Univ Georgia Bull 1540. https://extension.uga.edu/publications/detail.html?number=B1540 [accessed 22 Jan 2023]

    • Search Google Scholar
    • Export Citation
  • Haralson JC, Brannen PM & Scherm H. 2022 Survey of potential root pathogens in softwood cuttings collected from Georgia blueberry nurseries Plant Health Prog.23 87 92 https://doi.org/10.1094/PHP06-21-0094-S

    • Search Google Scholar
    • Export Citation
  • Harris AR, Schisler DA, Neate SM & Ryder MH. 1994 Suppression of damping-off caused by Rhizoctonia solani, and growth promotion, in bedding plants by binucleate Rhizoctonia spp Soil Biol Biochem.26 263 268 https://doi.org/10.1016/0038-0717(94)90166-X

    • Search Google Scholar
    • Export Citation
  • Hoitink HAJ & Boehm MJ. 1999 Biocontrol within the context of soil microbial communities: A substrate-dependent phenomenon Annu Rev Phytopathol.37 427 446

    • Search Google Scholar
    • Export Citation
  • Hoitink HAJ, Nameth ST & Krause MS. 1999 Control of Phytophthora and other major diseases of ericaceous plants Ohio State Univ Ext Factsheet HYG-3073-99.

    • Search Google Scholar
    • Export Citation
  • Krewer G & Cline B. 2003 Blueberry propagation suggestions https://smallfruits.org/files/2019/06/03BlueberryPropagationSuggestions.pdf [accessed 22 Jan 2023]

    • Search Google Scholar
    • Export Citation
  • Linderman R. 1973 The role of abscised Cylindrocladium-infected azalea leaves in the epidemiology of Cylindrocladium wilt of azalea Phytopathology.64 481 485

    • Search Google Scholar
    • Export Citation
  • Schilder AMC & Cline WO. 2017 Cylindrocladium rot 19 21 Polashock J, Caruso FL, Averill AL & Schilder AC. Compendium of blueberry, cranberry, and lingonberry diseases and pests APS Press Saint Paul, MN, USA

    • Search Google Scholar
    • Export Citation
  • Scherm H & Krewer G. 2003 Blueberry production in Georgia: Historical overview and recent trends Small Fruits Rev.2 4 83 91 https://doi.org/10.1300/j301v02n04_09

    • Search Google Scholar
    • Export Citation
  • Thomas PA, Seymour RM, Pennisi BV & Stegelin FE. 2005 Greenhouse*A*Syst: Water recycling and water reuse assessment Univ Georgia Ext Athens, GA, USA

    • Search Google Scholar
    • Export Citation
  • University of Georgia Center for Agribusiness and Economic Development 2001-2020 Georgia farm gate value report Blueberries https://caed.uga.edu/publications/farm-gate-value.html. [accessed 22 Jan 2023]

    • Search Google Scholar
    • Export Citation
  • Williamson JG & Lyrene PM. 2004 The Florida blueberry industry: A decade of growth Proc Fla State Hortic Soc.117 234 235

  • Fig. 1.

    Blueberry area in Georgia, USA, from 2001 to 2020 (University of Georgia Center for Agribusiness and Economic Development 2001–20); 1 acre = 0.4047 ha.

  • Fig. 2.

    Open, containerized blueberry propagation system using impact sprinklers in Bacon County, GA, USA. Note the standing water and potential for pathogen introduction from soil and water. The inset shows the number of cuttings set per pot; individual plants will be repotted for additional growth and sales.

  • Fig. 3.

    Closed, containerized blueberry propagation system (shade-house) in Pierce County, GA, USA, with cuttings produced in flats located on benches. Bench systems allow for better sanitation between propagation runs, and groundwater/soil is less likely to be a source of pathogen inoculum.

  • Fig. 4.

    Closed, bark bed-based blueberry propagation system in Bacon County, GA, USA. Bark bed systems can be constructed outside or inside of greenhouses. They are difficult to sanitize if bark is removed and replaced, and because of the cost of bark, growers are often tempted to simply reuse the bark for the next round of propagation.

  • Fig. 5.

    Fungicide usage in surveyed blueberry propagation nurseries in southern Georgia, USA, in 2007 and 2022.

  • Brannen PM, Harmon P & NeSmith DS. 2009 Utility of phosphonate fungicides for management of Phytophthora root rot of blueberry Acta Hortic.810 331 340 https://doi.org/10.17660/ActaHortic.2009.810.43

    • Search Google Scholar
    • Export Citation
  • Cline B. 2004 Fungal pathogens associated with blueberry propagation beds in North Carolina Small Fruits Rev.3 1-2 213 221 https://doi.org/10.1300/j301v03n01_21

    • Search Google Scholar
    • Export Citation
  • Cline B & Mainland M. 2008 Blueberry propagation 56 62 Proceedings of the 42nd Annual Open House and Trade Show, North Carolina Blueberry Council Clinton, NC, USA

    • Search Google Scholar
    • Export Citation
  • Coville FV. 1910 Experiments in blueberry culture US Dept Agric, Bur Plant Ind Bull 193

  • Coville FV. 1921 Directions for blueberry culture 1921 US Dept Agric Bull.974 https://doi.org/10.5962/bhl.title.108248

  • Haralson JC. 2009 Pathogens associated with blueberry cutting failure in south Georgia nurseries and their control (MS Thesis) Department of Plant Pathology, University of Georgia Athens, GA, USA

    • Search Google Scholar
    • Export Citation
  • Haralson JC, Brannen PM, NeSmith DS & Scherm H. 2013 Chemical control of Cylindrocladium and Rhizoctonia root rots in blueberry propagation Crop Prot.44 1 5 https://doi.org/10.1016/j.cropro.2012.09.017

    • Search Google Scholar
    • Export Citation
  • Haralson J, Brannen PM & Oliver J. 2021 Propagating disease-free blueberry plants from softwood cuttings Univ Georgia Bull 1540. https://extension.uga.edu/publications/detail.html?number=B1540 [accessed 22 Jan 2023]

    • Search Google Scholar
    • Export Citation
  • Haralson JC, Brannen PM & Scherm H. 2022 Survey of potential root pathogens in softwood cuttings collected from Georgia blueberry nurseries Plant Health Prog.23 87 92 https://doi.org/10.1094/PHP06-21-0094-S

    • Search Google Scholar
    • Export Citation
  • Harris AR, Schisler DA, Neate SM & Ryder MH. 1994 Suppression of damping-off caused by Rhizoctonia solani, and growth promotion, in bedding plants by binucleate Rhizoctonia spp Soil Biol Biochem.26 263 268 https://doi.org/10.1016/0038-0717(94)90166-X

    • Search Google Scholar
    • Export Citation
  • Hoitink HAJ & Boehm MJ. 1999 Biocontrol within the context of soil microbial communities: A substrate-dependent phenomenon Annu Rev Phytopathol.37 427 446

    • Search Google Scholar
    • Export Citation
  • Hoitink HAJ, Nameth ST & Krause MS. 1999 Control of Phytophthora and other major diseases of ericaceous plants Ohio State Univ Ext Factsheet HYG-3073-99.

    • Search Google Scholar
    • Export Citation
  • Krewer G & Cline B. 2003 Blueberry propagation suggestions https://smallfruits.org/files/2019/06/03BlueberryPropagationSuggestions.pdf [accessed 22 Jan 2023]

    • Search Google Scholar
    • Export Citation
  • Linderman R. 1973 The role of abscised Cylindrocladium-infected azalea leaves in the epidemiology of Cylindrocladium wilt of azalea Phytopathology.64 481 485

    • Search Google Scholar
    • Export Citation
  • Schilder AMC & Cline WO. 2017 Cylindrocladium rot 19 21 Polashock J, Caruso FL, Averill AL & Schilder AC. Compendium of blueberry, cranberry, and lingonberry diseases and pests APS Press Saint Paul, MN, USA

    • Search Google Scholar
    • Export Citation
  • Scherm H & Krewer G. 2003 Blueberry production in Georgia: Historical overview and recent trends Small Fruits Rev.2 4 83 91 https://doi.org/10.1300/j301v02n04_09

    • Search Google Scholar
    • Export Citation
  • Thomas PA, Seymour RM, Pennisi BV & Stegelin FE. 2005 Greenhouse*A*Syst: Water recycling and water reuse assessment Univ Georgia Ext Athens, GA, USA

    • Search Google Scholar
    • Export Citation
  • University of Georgia Center for Agribusiness and Economic Development 2001-2020 Georgia farm gate value report Blueberries https://caed.uga.edu/publications/farm-gate-value.html. [accessed 22 Jan 2023]

    • Search Google Scholar
    • Export Citation
  • Williamson JG & Lyrene PM. 2004 The Florida blueberry industry: A decade of growth Proc Fla State Hortic Soc.117 234 235

Jeremy C. Haralson Department of Plant Pathology, University of Georgia, Athens, GA 30602, USA

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Phillip M. Brannen Department of Plant Pathology, University of Georgia, Athens, GA 30602, USA

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Walt Sanders Department of Plant Pathology, University of Georgia, Athens, GA 30602, USA

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Harald Scherm Department of Plant Pathology, University of Georgia, Athens, GA 30602, USA

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Contributor Notes

P.M.B. is the corresponding author. E-mail: pbrannen@uga.edu.

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  • Fig. 1.

    Blueberry area in Georgia, USA, from 2001 to 2020 (University of Georgia Center for Agribusiness and Economic Development 2001–20); 1 acre = 0.4047 ha.

  • Fig. 2.

    Open, containerized blueberry propagation system using impact sprinklers in Bacon County, GA, USA. Note the standing water and potential for pathogen introduction from soil and water. The inset shows the number of cuttings set per pot; individual plants will be repotted for additional growth and sales.

  • Fig. 3.

    Closed, containerized blueberry propagation system (shade-house) in Pierce County, GA, USA, with cuttings produced in flats located on benches. Bench systems allow for better sanitation between propagation runs, and groundwater/soil is less likely to be a source of pathogen inoculum.

  • Fig. 4.

    Closed, bark bed-based blueberry propagation system in Bacon County, GA, USA. Bark bed systems can be constructed outside or inside of greenhouses. They are difficult to sanitize if bark is removed and replaced, and because of the cost of bark, growers are often tempted to simply reuse the bark for the next round of propagation.

  • Fig. 5.

    Fungicide usage in surveyed blueberry propagation nurseries in southern Georgia, USA, in 2007 and 2022.

 

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