Incidence of Phytophthora and Pythium Infection and the Relation to Cultural Conditions in Commercial Blueberry Fields

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  • 1 U.S. Department of Agriculture, Agricultural Research Service, Horticultural Crops Research Unit, 3420 NW Orchard Avenue, Corvallis, OR 97330
  • 2 Oregon State University, North Willamette Research and Extension Center, Aurora, OR 97002

Fifty-five commercial blueberry (Vaccinium spp.) fields were sampled in northwest Oregon in 2001 to determine the incidence of Phytophthora and Pythium root rot pathogens and identify cultural factors that increase the probability of developing infection. Phytophthora was detected in 24% and Pythium was detected in 85% of the fields sampled. The only species of Phytophthora identified in the study was P. cinnamomi. Root infection by P. cinnamomi was significantly related to cultivar with incidence observed more frequently than expected in ‘Duke’ and ‘Bluecrop’. Both blueberry cultivars are two of the most popular grown in the region, representing 42% of the fields in this survey and ≈46% of the total area planted in Oregon. Two other cultivars found infected by P. cinnamomi were ‘Rubel’ and ‘Briggitta Blue’, together accounting for an additional 24% of the fields surveyed. Phytophthora was not detected in fields planted with ‘Berkeley’, ‘Bluejay’, ‘Bluetta’, ‘Darrow’, ‘Earliblue’, ‘Elliott’, and ‘Powderblue’, each of which represented only 2% to 7% of the fields surveyed. Pythium spp. were identified to genus only, but one or more species of Pythium was found in all 11 cultivars included in the survey. Occurrence of either Phytophthora or Pythium was unrelated to soil type, planting age, or cultural practices such as bed type, cover crop, mulch, irrigation system, fertilizer application, fungicide use, or the source of plant material used in the fields. Overall, most fields with Phytophthora or Pythium remained largely symptomless under good soil drainage conditions and had similar levels of vigor as those without the pathogens.

Abstract

Fifty-five commercial blueberry (Vaccinium spp.) fields were sampled in northwest Oregon in 2001 to determine the incidence of Phytophthora and Pythium root rot pathogens and identify cultural factors that increase the probability of developing infection. Phytophthora was detected in 24% and Pythium was detected in 85% of the fields sampled. The only species of Phytophthora identified in the study was P. cinnamomi. Root infection by P. cinnamomi was significantly related to cultivar with incidence observed more frequently than expected in ‘Duke’ and ‘Bluecrop’. Both blueberry cultivars are two of the most popular grown in the region, representing 42% of the fields in this survey and ≈46% of the total area planted in Oregon. Two other cultivars found infected by P. cinnamomi were ‘Rubel’ and ‘Briggitta Blue’, together accounting for an additional 24% of the fields surveyed. Phytophthora was not detected in fields planted with ‘Berkeley’, ‘Bluejay’, ‘Bluetta’, ‘Darrow’, ‘Earliblue’, ‘Elliott’, and ‘Powderblue’, each of which represented only 2% to 7% of the fields surveyed. Pythium spp. were identified to genus only, but one or more species of Pythium was found in all 11 cultivars included in the survey. Occurrence of either Phytophthora or Pythium was unrelated to soil type, planting age, or cultural practices such as bed type, cover crop, mulch, irrigation system, fertilizer application, fungicide use, or the source of plant material used in the fields. Overall, most fields with Phytophthora or Pythium remained largely symptomless under good soil drainage conditions and had similar levels of vigor as those without the pathogens.

Root rot is a major disease in blueberry. Characteristic symptoms include discolored and necrotic roots, stunted growth, pale yellow to reddish leaves, marginal leaf necrosis, premature defoliation, and, in some cases, plant death (Cline and Schilder, 2006). Young plants are usually most susceptible to root rot, although severe instances can develop in mature plants located in soils with poor drainage (Sterne, 1982). The causal organism most commonly associated with the disease is Phytophthora cinnamomi Rand. (Caruso and Ramsdell, 1995), a widespread soil pathogen with a large host range first reported in northern highbush blueberry (Vaccinium corymbosum L.) in 1961 (Raniere, 1961). Since then, P. cinnamomi has been documented in many blueberry plantings throughout the United States, including in Arkansas (Sterne, 1982), Florida (Lyrene and Crocker, 1991), Maryland (Draper et al., 1971), Mississippi (Smith, 2002), New Jersey (Royle and Hickman, 1963), and North Carolina (Clayton and Haasis, 1964; Milholland and Galletta, 1967).

Pythium spp. also cause root rot in many plants, including members of the Ericaceae family (which includes blueberry) such as azalea and rhododendron (Rhododendron spp.; Coyier and Roane, 1986), but typically have not been associated with the disease in blueberry (but see Brannen and NeSmith, 2006). Like Phytophthora, Pythium spp. are “water molds” that readily infect and spread in wet, poorly drained soils (Duniway, 1979); however, they usually only infect young, succulent feeder roots and therefore often lack the ability to kill the host (Hendrix and Campbell, 1973). Thus, if Pythium spp. occur on blueberry, their effects may be more subtle than Phytophthora. Plants infected by Pythium may simply lack vigor, producing less growth than a noninfected plant. General declines in plant growth resulting from Pythium spp. have been documented in other perennial fruit crops (e.g., Hendrix et al., 1966; Mazzola et al., 2002; Spies et al., 2006). Severity of root rot by Pythium spp. was significantly reduced by applications of mefenoxam, fosetyl-Al, and phosphonate fungicides on a new planting of southern highbush blueberry (V. corymbosum hybrid ‘Millenium’) in Georgia (Brannen and NeSmith, 2006).

The objective of this study was to determine the incidence of Phytophthora and Pythium in commercial fields of blueberry in Oregon and to identify any factors that increase the probability of developing infection. Oregon currently has 1780 ha of blueberry, most of which is planted in the northwest part of the state, and produces 16,150 t of the fruit annually (U.S. Department of Agriculture, 2007a). Although root rot is prevalent in the region, no information is available on the distribution and severity of the disease.

Materials and Methods

Soil cores (2.5 cm diameter × 0.30 m deep) were collected in Aug. 2001 from 55 commercial blueberry fields located in northwest Oregon. Field size averaged ≈12 ha and ranged from 2 to 50 ha. Each field was sampled within rows, ≈1 m from the base of a plant, collecting three cores at five representative locations (at least 50 m apart). Weed and grass roots were carefully avoided during sampling. Each core was separated by depth into 0- to 0.15-m and 0.15- to 0.30-m sections. Roots were removed from each section by washing, surfaced-sterilized in sodium hypochlorite (2%) for 5 min, rinsed, and placed into cups filled with 100 mL of dH2O. Leaf discs (5 mm diameter) of Camellia sasanqua Thunb. were floated on the surface of each cup as bait for collecting zoospores (Linderman and Zeitoun, 1977). After a minimum of 24 to 48 h, leaf discs (five per cup) were direct-plated onto petri dishes filled with either P5ARP agar (Kannwischer and Mitchell, 1978), which is selective for members of the family Pythiaceae, or P5ARP agar amended with 25 ppm hymexazol (P5ARP + H; Tachigaren, 70% a.i.; Saankyo Co., Tokyo), which is selective for Phytophthora spp. (Masago et al., 1977). The isolation dishes were incubated in the dark at 20 °C and then examined daily for pythiaceous colony growth for at least 7 d. Pythium isolates were identified to genus only, but Phytophthora isolates were identified to species. Species identification was done at 100 to 400× magnification and was based on morphological characteristics of sexual and asexual structures observed in the isolates (Stamps et al., 1990). Roots were oven-dried (65 °C) after baiting and weighed to determine total dry biomass of each sample.

Planting age, cultivar(s), bed type, cover crop, mulching, irrigation method, fertilizer application, types of pesticides and fungicides applied, and source of planting material in each field were noted during sampling or obtained from the growers. Fields were also rated for vigor with a relative rating scale of 1 to 5 in which 1 = no growth, severely stunted; 2 = poor growth, low yield (less than 5 t·ha−1); 3 = moderate growth and yield (5 to 10 t·ha−1); 4 = good growth, high yield (10 to 15 t·ha−1); or 5 = excellent growth and yield (greater than 15 t·ha−1); young fields (less than 4 to 5 years old) were evaluated primarily on growth, whereas mature fields were evaluated on production. Further details on soil characteristics, root distribution, and mycorrhizal status of the fields were reported by Scagel and Yang (2005). Field soil types were obtained from soil survey maps (U.S. Department of Agriculture, 2007b).

Weather data for the region was obtained from a U.S. Bureau of Reclamation AgriMet weather station located in Forest Grove, OR (45°33′N, 123°05′W, and 55 m elevation). The weather station was centrally located among the surveyed fields, and data were collected hourly from 1 Sept. 1991 to 31 Aug. 2001.

Associations between Phytophthora and Pythium infection and field characteristics were analyzed using χ2 test of independence. Characteristics were grouped accordingly to meet minimum sample size requirements for the test (Good et al., 1977) using categories presented by Scagel and Yang (2005). Adjusted residuals were calculated to determine significant differences between observed and expected values at P ≤ 0.05. The percentage of infected root samples and root dry weight at different depths in fields with Phytophthora and Pythium spp. were analyzed by analysis of variance using PROC GLM in SAS (SAS Institute, Cary, NC).

Results and Discussion

Typical weather conditions in northwest Oregon are summarized in Table 1. Mean air temperatures are moderate, ranging between 10 and 19 °C during the growing season (April to October). Most precipitation occurs during winter and spring months, whereas summers are dry and sunny, with relative humidity averaging ≈70% and usually declining to less than 40% during the daytime. Soil conditions are often quite favorable to Phytophthora and Pythium infection, especially in spring and early summer when soils are regularly saturated from rain or irrigation and the temperatures range from 9 to 27 °C. Sporangia of P. cinnamomi are produced in large numbers when the soil is just below saturation, and infection typically occurs when soil temperatures are at 15 to 28 °C (Kuhlman, 1964; Zentmeyer and Marshall, 1959) and is optimum at ≈21 to 26 °C (Strik et al., 1993). Many other Phytophthora and Pythium spp. have similar soil moisture and temperature requirements (Duniway, 1979; Hendrix and Campbell, 1973).

Table 1.

Monthly weather conditions and maximum and minimum daily soil temperature in Forest Grove, OR.z

Table 1.

There was considerable variation in the cultural characteristics of the fields surveyed, including differences in plant age, cultivar, soil type, bed type, cover crop, mulching practices, irrigation system, fertilizer application, fungicide use, and the source of plant material. Plantings ranged in age from 1 to 50 years and consisted of 10 different cultivars of northern highbush blueberry, including ‘Berkeley’, ‘Bluecrop’, ‘Bluejay’, ‘Bluetta’, ‘Briggitta Blue’, ‘Darrow’, ‘Duke’, ‘Earliblue’, ‘Elliott’, and ‘Rubel’, and one cultivar of rabbiteye blueberry (V. ashei Reade), ‘Powderblue’. Altogether, these 11 cultivars represent ≈90% of the total area of blueberry planted in Oregon (Yang, 2002). The most common cultivars sampled in the study were ‘Duke’, ‘Bluecrop’, and ‘Rubel’, which accounted for 56% of all the fields surveyed (Table 2). Approximately 7% (n = 4) of the fields consisted of a mix of two or more cultivars, three fields of which the cultivars were unknown.

Table 2.

Distribution of cultivars in 55 commercial blueberry fields sampled for Phytophthora and Pythium in northwest Oregon.z

Table 2.

Soil types included Latourell and Quatama loam in 11%; Aloha, Cornelius, Kinton, Huberly, Saum, and Willamette silt loam in 65%; and Cazadero, Chehalis, Jory, and Nekia silty clay loam in 24% of the fields surveyed. Fields had either flat (74%) or raised (26%) planting beds and most had grass alleyways (89%) maintained between the beds. Mulch, which usually consisted of Douglas fir (Pseudotsuga menziesii Franco) sawdust applied on the soil surface along the length of the planting bed, was used in 58% of the fields. Most fields were irrigated by overhead sprinklers (96%) with only 4% irrigated by drip. Fertilizer applications ranged from 45 to 500 kg·ha−1 of nitrogen (N; usually applied as ammonium sulfate) per year, although the majority of growers applied between 110 and 170 (33%) or 170 to 225 (33%) kg·ha−1 of N per year. Only 18% of the fields sampled reported use of fosetyl-Al (Aliette, Bayer CropScience, Research Triangle Park, NC) or mefenoxam (Ridomil Gold, Syngenta Crop Protection, Greensboro, NC) fungicides for prevention and control of root rot, whereas the remaining 82% reported no use of any fungicide for root rot. Plants in 89% of the fields sampled came from one of two commercial nurseries located in Oregon.

Phytophthora was detected in 24% of the fields sampled, including in fields of ‘Bluecrop’, ‘Brigitta Blue’, ‘Duke’, and ‘Rubel’ (Table 2). It was not detected, however, in ‘Berkeley’, ‘Bluejay’, ‘Bluetta’, ‘Darrow’, ‘Earliblue’, ‘Elliott’, and ‘Powderblue’, which together comprised 27% of the fields sampled. The only species of Phytophthora identified from the infected samples was P. cinnamomi. Infection by P. cinnamomi was significantly related to cultivar (χ2 = 6.384; degree of freedom = 1; P < 0.05), with infection observed more frequently than expected in ‘Bluecrop’ and ‘Duke’ and less frequently than expected in ‘Earliblue’. Draper et al. (1971) reported high resistance to P. cinnamomi in Me-US 32 [later released as ‘Patriot’ (Hepler and Draper, 1976)], a seedling from a cross of ‘US 3’ and ‘Earliblue’, but later determined that ‘Earliblue’ was susceptible and probably not the source of the resistance (Draper et al., 1972). High resistance to P. cinnamomi has also been reported in rabbiteye blueberry, whereas ‘Bluecrop’ and ‘Bluetta’ have been described as very susceptible (Draper et al., 1972; Erb et al., 1987).

Phytophthora infection was not related to any of the other cultural characteristics, including soil type and the application of fungicides, or to field vigor. Any association of P. cinnamomi with low vigor was more likely related to localized conditions within an infected field. It was noticed during sampling, for example, that weak growth usually occurred in low areas. Low-lying areas often remain saturated after rain or irrigation, leading to problems with root rot (de Silva et al., 1999). Unfortunately, we were unable to confirm such an association because topographical characteristics of each sample location were not recorded. Lack of any relationship between P. cinnamomi and field age suggests that growers may be successfully controlling the fungus either directly through use of fumigants and fungicides or indirectly with cultural practices (both of which, as mentioned previously, are not supported by our data) or that pathogenicity of the disease is limited in the region by climate (e.g., dry weather during the summer months) and site conditions (e.g., good drainage, high organic matter content, low pH).

Pythium was detected in 85% of the fields sampled and was found in every cultivar included in the survey (Table 2). Unlike P. cinnamomi, occurrence of Pythium spp. was not related to cultivar. The percentage of fields with Pythium infection in each cultivar was simply a function of the number of fields sampled. Pythium infection was also not related to field characteristics or to field vigor nor was it related to the presence of P. cinnamomi. The genus of Pythium spp. found varied considerably based on their characteristics in culture, but presumably each had some capacity to infect and damage blueberry roots. Impacts of Pythium spp., however, appear minimal, having no noticeable effect on productivity of the crop. Again, like with Phytophthora, if any association exists between Pythium spp. and low vigor, it may only occur in areas of the field most conducive to infection. Propagule densities of Phytophthora spp. have been found to be spatially correlated to areas with heavier irrigation in vegetable fields (Ristaino et al., 1992).

Within fields with detectable levels of Phytophthora or Pythium, P. cinnamomi was found at 43% and Pythium spp. were found at 60% of the sample locations. In both cases, there was a tendency for infection to occur more often at 0.15- to 0.30-m than at 0- to 0.15-m depth (P = 0.0859 and 0.0729 for Phytophthora and Pythium, respectively; Fig. 1A), which may have been simply the result of significantly more root biomass at the lower depth (P = 0.0219 and 0.0447 for Phytophthora and Pythium, respectively; Fig. 1B). However, because soil temperatures in summer usually decrease with soil depth, and soil moisture tends to increase, conditions may have been more favorable for infection at 0.15 to 0.30 m.

Fig. 1.
Fig. 1.

(A) Percentage of infected root samples and (B) root dry weight at 0- to 0.15-m and 0.15- to 0.30-m depth in fields with Phytophthora and Pythium. Each bar represents the mean of 13 fields with Phytophthora and 47 fields with Pythium and errors bars represent 1 se.

Citation: HortScience horts 43, 1; 10.21273/HORTSCI.43.1.260

Although blueberry root rot in a given region will undoubtedly vary both seasonally and annually, the present study clearly indicates that infection by P. cinnamomi and Pythium spp. associated with the disease is a fairly common occurrence under commercial production in Oregon. Root rot infection was associated more often than expected with certain cultivars, suggesting that cultivar selection may be a useful tool to avoid problems with root rot in situations (e.g., heavy soils, poor drainage) in which potential for developing the disease is high. Compared with Phytophthora, Pythium was much more abundant in the survey. The common occurrence of Pythium spp. warrants further assessment of their importance as potential pathogens in blueberry.

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Contributor Notes

We thank A. Davis for technical assistance and the Oregon Blueberry Commission for financial support.

Mention of trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that also may be suitable.

To whom reprint requests should be addressed; e-mail brylad@onid.orst.edu.

  • View in gallery

    (A) Percentage of infected root samples and (B) root dry weight at 0- to 0.15-m and 0.15- to 0.30-m depth in fields with Phytophthora and Pythium. Each bar represents the mean of 13 fields with Phytophthora and 47 fields with Pythium and errors bars represent 1 se.

  • Brannen, P.M. and S. NeSmith. 2006. Fungicidal control of Pythium root rot of blueberry in high-density bark bed planting, 2005. F & N Tests 61:SMF016.

  • Caruso, F.L. & Ramsdell, D.C. 1995 Compendium of blueberry and cranberry diseases APS Press St. Paul, MN

    • Export Citation
  • Clayton, C.N. & Haasis, G.A. 1964 Blueberry root rot caused by Phytophthora cinnamomi in North Carolina Plant Dis. Rptr. 48 460 461

  • Cline, W.O. & Schilder, A. 2006 Identification and control of blueberry diseases 115 138 Childers N.F. & Lyrene P.M. Blueberries. For growers, gardeners, promoters Dr. Norman F. Childers Hort. Publ Gainesville, FL

    • Search Google Scholar
    • Export Citation
  • Coyier, D.L. & Roane, M.K. 1986 Compendium of rhododendron and azalea diseases APS Press St. Paul, MN

    • Export Citation
  • de Silva, A., Patterson, K., Rothrock, C. & McNew, R. 1999 Phytophthora root rot of blueberry increases with frequency of flooding HortScience 34 693 695

    • Search Google Scholar
    • Export Citation
  • Draper, A.D., Mircetich, S.M. & Scott, D.H. 1971 Vaccinium clones resistant to Phytophthora cinnamomi HortScience 6 167 169

  • Draper, A.D., Stretch, A.W. & Scott, D.H. 1972 Two tetraploid sources of resistance for breeding blueberries resistant to Phytophthora cinnamomi Rand HortScience 7 266 268

    • Search Google Scholar
    • Export Citation
  • Duniway, J.M. 1979 Water relations of water molds Ann. Rev. Phytopathol. 17 431 460

  • Erb, W.A., Moore, J.N. & Sterne, R.E. 1987 Response of blueberry cultivars to inoculation with Phytophthora cinnamomi Rands zoospores HortScience 22 298 300

    • Search Google Scholar
    • Export Citation
  • Good, I.J., Grover, T.N. & Mitchell, G.J. 1977 Exact distribution for chi-squared and for the likelihood-ratio statistic for equiprobable multinomial distribution J. Amer. Stat. Assoc. 65 267 283

    • Search Google Scholar
    • Export Citation
  • Hendrix F.F. Jr & Campbell, W.A. 1973 Pythiums as plant pathogens Ann. Rev. Phytopathology 11 77 98

  • Hendrix F.F. Jr, Powell, W.M. & Owen, J.H. 1966 Relation of root necrosis caused by Pythium species to peach tree decline Phytopathology 56 1229 1232

  • Hepler, R.E. & Draper, A.D. 1976 ‘Patriot’ blueberry HortScience 11 272

  • Kannwischer, M.E. & Mitchell, D.J. 1978 The influence of a fungicide on the epidemiology of black shank of tobacco Phytopathology 68 1760 1765

  • Kuhlman, E.G. 1964 Survival and pathogenicity of Phytophthora cinnamomi in several western Oregon soils For. Sci. 10 151 158

  • Linderman, R.G. & Zeitoun, F. 1977 Phytophthora cinnamomi causing root rot and wilt of nursery-grown native western azalea and salal Plant Dis. Rep. 61 1045 1048

    • Search Google Scholar
    • Export Citation
  • Lyrene, P.M. & Crocker, T.E. 1991 Commercial blueberry production in Florida Fla. Coop. Ext. Ser. Handbook SP 179, Univ. Fla Gainesville

    • Export Citation
  • Masago, H., Yoshikawa, M., Fukada, M. & Nakanishi, N. 1977 Selective inhibition of Pythium spp. on a medium for direct isolation of Phytophthora spp. from soils and plants Phytopathology 67 425 428

    • Search Google Scholar
    • Export Citation
  • Mazzola, M., Reganold, J.P. & Levesque, C.A. 2002 Frequency, virulence, and metalaxyl sensitivity of Pythium spp. isolated from apple roots under conventional and organic production systems Plant Dis. 86 669 675

    • Search Google Scholar
    • Export Citation
  • Milholland, R.D. & Galletta, G.J. 1967 Relative susceptibility of blueberry cultivars to Phytophthora cinnamomi Plant Dis. Rptr. 51 998 1001

  • Raniere, L.C. 1961 Observations on new or unusual diseases of highbush blueberry Plant Dis. Rptr. 45 844

  • Ristaino, J.B., Hord, M.J. & Gumpertz, M.L. 1992 Population densities of Phytophthora capsici in field soils in relation to drip irrigation, rainfall, and disease incidence Plant Dis. 76 1017 1024

    • Search Google Scholar
    • Export Citation
  • Royle, D.J. & Hickman, C.J. 1963 Phytophthora cinnamomi on highbush blueberry Plant Dis. Rptr. 47 266 268

  • Scagel, C.F. & Yang, W.Q. 2005 Cultural variation and mycorrhizal status of blueberry plants in NW Oregon commercial production fields J. Fruit Sci. 5 85 111

    • Search Google Scholar
    • Export Citation
  • Smith, B.J. 2002 Susceptibility of southern highbush blueberry cultivars to phytophthora root rot Acta Hort. 574 75 79

  • Spies, C.F.J., Mazzola, M. & Mcleod, A. 2006 Characterization of Pythium and Phytophthora species associated with grapevine decline in South Africa 62 44th Congress of the Southern African Society for Plant Pathology, Programme & Abstracts

    • Export Citation
  • Stamps, D.J., Waterhouse, G.M., Newhook, F.J. & Hall, G.S. 1990 Revised tabular key to the species of Phytophthora 162 CAB International Mycological Institute Mycol

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