Rhizoctonia web blight, caused by Rhizoctonia solani and binucleate Rhizoctonia spp., is an annual problem in compact cultivars of container-grown azalea (Rhododendron spp.) in the Gulf Coast states. Increasing the space between plants is commonly recommended for suppression of the disease, but experimental evidence for the effectiveness of this cultural practice in container-grown azalea is lacking. During the summers of 2002 and 2003, disease severity was measured weekly in the inoculated center plant of plots consisting of 49 potted `Gumpo White' azalea plants growing in 3.8-L containers and having a canopy diameter of about 30 cm. Plant spacing within plots was 0, 6, 12, 18, or 24 cm, and plots were arranged in three randomized complete blocks. Evaporation, leaf wetness (LW), relative humidity (RH), and temperature were monitored in each plot. Disease severity increased steadily from mid-July to late August or early September, after which it leveled off or declined. Evaporation increased and the number of hours within the temperature range favorable for disease development (25 to 30 °C) decreased significantly with plant spacing (P < 0.05), but LW and RH were not significantly different among treatments. Plant spacing also had no significant effect on disease severity. Daily overhead irrigation and compact plant form likely contributed to the lack of effect of spacing on disease development.
Warren E. Copes and H. Scherm
Warren E. Copes and Eugene K. Blythe
In the southern and eastern United States, azalea stems cut during the spring for propagation may be infested with Rhizoctonia spp. Multiple methods were evaluated in a series of laboratory experiments for the purpose of eliminating Rhizoctonia from stem cuttings of Rhododendron L. ‘Gumpo White’ [‘Gumpo White’ (Satsuki) azalea] to prevent spread of azalea web blight during the propagation phase of nursery production. Leafless stem sections were inoculated with an isolate of binucleate Rhizoctonia anastomosis group P (AG P). Disinfestants (sodium hypochlorite, hydrogen dioxide, and quaternary ammonium chloride) or fungicides (chlorothalonil + thiophanate-methyl and flutolanil) applied at several rates (below, at, and above label rates) did not eliminate Rhizoctonia AG P from stem sections. Recovery of Rhizoctonia AG P was not reduced by submersing stem pieces in 45 °C water, but was eliminated at water temperatures of 50 °C or greater. Mortality of Rhizoctonia infesting azalea stem pieces was best explained by a cubic regression model. Mortality increased with increasing time (0, 1.5, 3, 4.5, 6, 7.5, 9, 10.5, 12, 15, 18, and 21 min) in water at 50 and 55 °C and with increasing temperatures (52, 55, 58, 61, 64, 67, and 70 °C) when stem pieces were submerged for 30 and 60 s. The duration of hot water treatment at which 99% of stem pieces were predicted to be free of Rhizoctonia was 20 min 16 s at 50 °C and 5 min 19 s at 55 °C. The average water temperature at which 99% of the stem pieces were predicted to be free of Rhizoctonia was 60.2 and 56.9 °C when stem pieces were submerged for 30 and 60 s, respectively. Only minor leaf damage occurred on terminal, leafy stem cuttings when submerged in 50 °C water after 40 min. Severe leaf damage did occur if cuttings were submerged long enough in water of 55 °C or greater. Leaf damage was predicted to exceed a proportional leaf damage value of 0.25 (indicating severe damage) when leafy stem cuttings were submerged in 55 °C water for longer than 13 min 54 s or for 30 and 60 s with water temperature greater than 57.4 and greater than 56.8 °C, respectively. Of the methods tested, submersion in hot water has the greatest potential for eliminating Rhizoctonia AG P from azalea stem cuttings. Submerging stem pieces in 50 °C water for 21 min eliminated Rhizoctonia and provided the least risk for development of severe leaf damage.
Warren E. Copes and Eugene K. Blythe
Submerging terminal leafy cuttings of Rhododendron L. ‘Gumpo White’ (‘Gumpo White’ azalea) in 50 °C water for 21 min was previously shown to eliminate binucleate Rhizoctonia species, the cause of azalea web blight, from plant tissues. Before considering commercial use of this practice, a better understanding of the rooting response and tissue sensitivity of evergreen azalea cultivars to 50 °C water was needed; therefore, the current study was conducted. Terminal cuttings of the azalea cultivars Conleb (Autumn Embers), Fashion, Formosa, Gumpo White, Hardy Gardenia, Hershey Red, Macrantha Pink, Midnight Flare, Red Ruffles, Renee Michelle, Roblel (Autumn Debutante), and Watchet were collected and submerged or not submerged in 50 °C water for 20 min before propagation in one experiment. All 12 cultivars tolerated 50 °C water for 20 min. Cuttings collected from the 12 cultivars were submerged in 50 °C water for 20, 40, 60, and 80 min in a second experiment. The cultivars varied in sensitivity when exposed to 50 °C water for 60 to 80 min resulting in differing responses in root development and final leaf count. In a third experiment, degrees of leaf damage caused by hot water submersion or by leaf removal were evaluated for the effect on root development and subsequent leaf count on rooted cuttings of ‘Gumpo White’ and ‘Roblel’. Induced incremental increases in leaf damage from hot water resulted in incremental reductions in the final leaf count and extent of root development for ‘Gumpo White’ and ‘Roblel’ while increasing percentage of leaf removal caused no reduction until 75% or greater leaf area was removed. Despite the risk imposed by submersing azalea cuttings in 50 °C water, all 12 azalea cultivars were tolerant of submersion durations long enough to eliminate binucleate Rhizoctonia species from stem and leaf tissue with only a low likelihood of sustaining detrimental damage.
Warren E. Copes, Andrew Ristvey, Patricia A. Richardson, Bruk E. Belayneh, Haibo Zhang, John Lea-Cox, and Chuanxue Hong
Triplicate water samples were collected monthly from nine waterways [eight runoff containment basins (RCBs) and one stream] on four commercial ornamental plant nurseries from February to July, and from one RCB and nursery from April to October. Four RCBs, one per nursery, were actively used as an irrigation water source. Analysis was done for 18 water quality variables, including ammonium–nitrogen (NH4 +–N), nitrate–nitrogen (NO3 −–N), ortho phosphate–phosphorus (PO4–P), total-phosphorus (T-P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), aluminum (Al), boron (B), copper (Cu), iron (Fe), manganese (Mn), zinc (Zn), pH, total alkalinity (T-Alk), electrical conductivity (EC), and sodium (Na). The degree and rate of monthly change varied considerably between RCBs. Macronutrients generally increased at most nurseries in 1–2 months after fertilizer application particularly in three RCBs (MD21, VA11, and VA12), with levels of N- and P forms exceeding preferred criteria for irrigation water by June and July in VA11 and VA12. Micronutrients fluctuated less but did vary per RCB with the most monthly change occurring in MD21. Even though pH fluctuated, pH tended to remain alkaline or neutral to acidic respective of the RCB during the entire sample period. T-Alk tended to increase over the summer. EC primarily fluctuated in RCBs with high macronutrient levels. Although levels of N- and P forms were mostly suitable by irrigation water criteria, they were frequently above U.S. Environmental Protection Agency (USEPA) nutrient criteria for promoting healthy water environments of lakes and reservoirs, and are discussed.
Warren E. Copes, Haibo Zhang, Patricia A. Richardson, Bruk E. Belayneh, Andrew Ristvey, John Lea-Cox, and Chuanxue Hong
Nine runoff containment basins (RCBs), used directly or indirectly for irrigating plants in ornamental plant nurseries, and one adjacent stream were sampled for water quality between Feb. and July 2013 in Maryland (MD), Mississippi (MS), and Virginia (VA). Triplicate water samples were taken monthly. Analysis was done for 18 water quality variables including nitrate-nitrogen (NO3 −-N) and ammonium-nitrogen (NH4 +-N), orthophosphate-phosphorus (PO4-P) and total-phosphorus (T-P), potassium, calcium, magnesium, sulfur, aluminum, boron (B), copper (Cu), iron (Fe), manganese, zinc (Zn), pH, total alkalinity (T-Alk), electrical conductivity (EC), and sodium. Additionally, 15 RCBs from 10 nurseries in Alabama (AL), Louisiana (LA), and MS were sampled in 2014 and 2016. Most prevalent correlations (P = 0.01) were between macronutrients, EC, B, Fe, and Zn, but none were prevalent across a majority of RCBs. Water quality parameter values were mostly present at low to preferred levels in all 25 waterways. Macronutrient levels were highest for a RCB that receives fertility from fertigation derived runoff. Water pH ranged from acidic to alkaline (>8). Results of this study show water quality in RCBs can be suitable for promoting plant health in ornamental plant nurseries, but also shows levels will vary between individual RCBs, therefore demonstrates need to verify water quality from individual water sources.
Yonghao Li, Mark T. Windham, Robert N. Trigiano, Donna C. Fare, James M. Spiers, and Warren E. Copes
Infection process of Puccinia hemerocallidis, the causal agent of daylily rust, and resistance responses in eight daylily cultivars, were studied macroscopically and microscopically. After germination of urediniospores, appressoria were formed at the tip of germ tubes and penetrated through stomatal openings. Intercellular hyphae aggregated and formed uredia under the infection sites, and released urediniospores after rupturing the epidermis. In highly resistant cultivars `Prairie Blue Eyes' and `Bertie Ferris', intercellular hyphal growth was restricted and uredia were not formed. No macroscopic symptoms of the disease were present on the leaf surface, although a few collapsed cells were observed microscopically. Both resistant and moderately resistant reactions were characterized by necrotic lesions with many collapsed cells under infection sites. The difference between these two reactions was that uredia and urediniospores were observed on the moderately resistant cultivar `Chicago Apache', but not on resistant cultivars, `Buttered Popcorn' and `Stella De Oro'. Sporulation was observed on both moderately susceptible and susceptible cultivars, but latent periods were delayed and the amount of urediniospore production was reduced on moderately susceptible cultivars, `Mary Todd' and `Chorus Line', compared to the susceptible cultivar `Pardon Me'. The results indicate that the hypersensitive cell death is one of the resistance responses to daylily rust, but necrotic lesions on leaf surfaces are associated with the amount of collapsed host cells. The delayed latent periods and reduced sporulation that resulted from restricted intercellular hyphal growth could represent another resistance mechanism in the daylily rust pathosystem.