Rhizoctonia Web Blight Development on Azalea in Relation to Leaf Wetness Duration in the Glasshouse

Moisture variables have not been a consistent predictor of Rhizoctonia web blight development on container-grown azalea. A vapour pressure deficit <2.5 hPa was the only moisture variable attributed to slow web blight development in one study, yet in another study, frequent rainfall provided a moderately successful decision criterion for applying fungicide. To characterize web blight development in response to leaf wetness, plants were inoculated with two isolates of binucleate Rhizoctonia AG-U and maintained in a glasshouse in open-topped, clear plastic chambers with 0-, 4-, 8-, 12-, 16- and 20-h daily cycles of 20–30 s mist at 30-min intervals under day and night temperatures of 29 and 22°C, respectively. Leaf wetness duration closely matched misting cycle duration. Disease incidence was measured per chamber as a mean of the number of blighted leaves per total leaves per stem. A mixed model procedure was used to compare area under the disease progress curves (AUDPC) over 4–6 weeks in experiments performed in 2008 to 2010. Isolate response to mist cycle durations was not different (P = 0.4283) in 2008, but was different in 2009 (P = 0.0010) and 2010 (P < 0.0001) due to one isolate becoming less aggressive over time. AUDPC was consistently higher on azaleas under 16- and 20-h mist cycles, which formed a higher disease group not significantly different from each other. AUDPC under 0-, 4-, and 8-h mist cycles mostly formed a lower disease group, while ranking for a 12-h mist cycle varied across experiments from the higher, intermediate, or lower AUDPC groups. Current data demonstrate an empirical relationship between long daily leaf wetness durations and development of severe web blight symptoms within a temperature range considered favourable for Rhizoctonia web blight development. Additional studies would be required to model Rhizoctonia web blight development under natural temperature fluctuations.