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

Abstract

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.

Weather-Based Forecasting of Rhizoctonia Web Blight Development on Container-Grown Azalea

“…Model N-W was the only model that weighted leaf wetness risk higher than the temperature risk. The risk classifications of 16 or more hours of leaf wetness and/or 18 or more hours of temperature between 20 and 30°C when daily maximum temperature was less than 35°C are indicative of conditions that are not regularly met yet are con sidered favorable for binucleate Rhizoctonia infection and myceli um growth, respectively (4,8). The rain risk of greater than 6.7 mm rain above the maximum daily irrigation is representative of com mon rainfall amounts that are not uniquely explanatory of patho logical functions.…”

“…The rate and degree o f web blight development differed between wet, moderate, and dry years, and weather variables o f tem pera ture, rainfall, and duration o f leaf wetness do influence disease development and control (4)(5)(6)(7)(8). Rhizoctonia web blight on azalea behaves like a polycyclic disease (7,8,14).…”

“…Further investigation found that web blight control was obtained in two o f three field trials when the first fungicide application o f the season was applied within 2 days after a 7-day period with three or more rainfalls greater than 0.63 cm (5). The im portance of moisture was further demonstrated in a controlled environmental study (4), when m oderate to severe web blight symptoms developed in response to 16 h or more durations of daily leaf wetness. Despite the irregular relevance o f weather variables, the information was collectively used to define conducive weather variables.…”

“…Temporal studies on Rhizoctonia web blight on azalea in south ern M ississippi have reported several different weather conditions that are conducive for disease development (4)(5)(6)(7)(8). Rhizoctonia web blight development on azalea was initially correlated with air tem peratures between 25 and 30°C for 6 to 8 h per day, relative humid ity >95% for >8 h per day, and/or leaf wetness for >6 h per day (7).…”