Location: Southern Horticultural Research
Title: Fungicide timing rules to prevent azalea web blight damage Author
Submitted to: Mississippi Nursery and Landscape Association
Publication Type: Trade Journal
Publication Acceptance Date: April 22, 2013
Publication Date: May 10, 2013
Citation: Copes, W.E. 2013. Fungicide timing rules to prevent azalea web blight damage. Mississippi Nursery and Landscape Association. Spring:9-11. Technical Abstract: This article provides directions for timing fungicide applications to control Rhizoctonia web blight. Research has shown that many azalea cultivars are infested with the web blight pathogen (binucleate Rhizoctonia). The fungus lives 12 months of the year on azaleas, yet does not harm the plant most of the year. Web blight symptoms develop during the summer, but even then only some of the infected plants will develop severe damage. Weather conditions can promote or slow down how fast web blight symptoms develop. Web blight development starts inside the canopy about mid-June as temperatures increase. Irrigation provides enough moisture for slow to moderate disease development. Rapid web blight development is favored by three or more daily rain events within 7 days when temperatures are mildly hot (such as early- to mid-July). This seems intuitive because the effect (rapid blight development) occurs within days of the cause (afternoon rains). Moderate summer temperatures (in the 90’s for only several hours in the early to late afternoon) and cloudy conditions favor a steady, moderate web blight development that is not easily noticed; web blight is further favored when combined with morning or late afternoon rain, even light rain. The typical hot and sunny summer day temporarily slows web blight development. Rhizoctonia is adaptable because it can take advantage of the moderate conditions even when these days occur occasionally over several weeks. This is not intuitive, because the fungus grows unnoticed over weeks before severe damage noticeably appears. Spraying on calendar dates is the simplest rule to control web blight. Treat cultivars that are more susceptible to web blight on a different schedule than cultivars that are less susceptible (instructions stated in article). Scouting is an additional tool that allows you to check the current status of blight severity and adjust for yearly weather differences. Use the calendar schedule as a base and scouting to adjust timing only by a week or two earlier or later (instructions stated in article). Weather awareness can help you decide when to spray, but is more difficult to keep track of then scouting. Scouting is a physical check on current disease levels. Weather is used to anticipate risk of severe disease outbreak.