Location: Southern Horticultural ResearchTitle: Rhizoctonia web blight development on container-grown azalea in relation to time and environmental factors Author
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/23/2010
Publication Date: 7/1/2010
Citation: Copes, W.E., Scherm, H. 2010. Rhizoctonia web blight development on container-grown azalea in relation to time and environmental factors. Plant Disease. 94:891-897. Interpretive Summary: Rhizoctonia web blight is an annual problem in the southern to eastern United States on container-grown azaleas in the nursery. To correlate disease development with weather, disease progress and weather variables were measured regularly from mid-May to mid-September at three locations in 2006, 2007, and 2008. Disease onset was best predicted by day of the year, being July 18. Weather conditions provided a better explanation of slow versus fast disease progress. Slow progress occurred with minimum temperatures <59°F, maximum temperatures >95°F, or average vapor pressure deficits <2.5 hPa, and anytime after Aug. 28. Weather variables were most useful when used in a negative prognosis approach to predict disease risk as being “not low” or “not high”. This information will be useful to research and extension scientists, and is being developed into a protocol that can be used by commercial ornamental plant producers.
Technical Abstract: Rhizoctonia web blight, caused by binucleate Rhizoctonia spp., is an annual problem in the southern United States on container-grown azaleas (Rhododendron spp.) that receive daily irrigation. Disease progress was assessed weekly from mid-May to mid-September in blocks of nursery-grown plants at three locations in 2006, 2007, and 2008. Disease severity was assessed by scoring each plant (180 to 506) per block and by more detailed counts of the number of blighted leaves on subsets of 15 randomly selected plants per block. Disease onset, defined as the appearance of blighted leaves at the exterior of at least one plant, over all locations and years occurred, had a lower coefficient of variation (2.7) due to calendar date (mean DOY 200) than due to weather variables. Disease severity peaked between late August and mid-September, after which it declined due to defoliation of affected leaves and regrowth of new, uninfected leaves. Based on the relative increase in the log-transformed number of infected leaves per plant from early disease development to the peak of the disease progress curve, weekly assessment periods were classified as having rapid (=10% increase), intermediate (0 to 10%), or slow (=0%) disease progress. Three-day moving averages (MAs) of various weather variables were calculated, and lagged values (by 5 days) of the MAs were used in an attempt to predict these disease progress periods. Of the periods assessed as slow disease progress for the 2006-2007 developmental data, 89.3% (25 of 28) met at least one of the following criteria for the lagged MAs: min. temperature <20°C, max. temperature >35°C, avg. vapor pressure deficit <2.5 hPa, or DOY >240. One or more of these same criteria were met in 5 of 16 (31.2%) assessment periods with rapid disease progress, indicating that periods with rapid vs. slow disease progression could be distinguished reasonably well based on weather and late-season DOY. Results were similar for the 2008 validation data, when criteria for slow disease progress were met with 72.7% (8 of 11) and 28.6% (2 of 7) of the assessment periods classified as slow and rapid disease progress, respectively. However, weather variables were not useful in separating periods with either rapid or slow disease progress from intermediate progress. Thus, weather variables might be most useful when used in a negative prognosis approach to predict disease risk as being “not low” or “not high”.