Timing of fungicides in relation to calendar date, weather, and disease thresholds to control Rhizoctonia web blight on container-grown azalea

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. Fungicides are the only practical control method, but a guideline for timing of fungicides is not available. Typically, producers apply fungicides in July based on past experiences such as when frequent afternoon rains are forecast, or after severe blight symptoms develop. The objective of the study was to evaluate fungicide timing criteria, including a calendar schedule, and disease incidence and decision-based weather thresholds. Experiments with a randomized complete block design were performed at a site in AL and MS for three years. For 2009, 2010, treatments included a non-treated control and five treatments where fungicide timing was based on a calendar schedule, three disease threshold criteria and a rain frequency criterion. In 2011, treatments included a non-treated control, a calendar schedule, and two disease threshold criteria. The calendar schedule was the only timing treatment that resulted in significantly less blight incidence than the control in all experiments. Fungicide application based on the lowest disease threshold (16–30 blight leaves per plant quadrant) and the rain frequency criteria (>3 rain events of >6.3 mm rain within the previous seven days) suppressed blight development equal to the calendar schedule in three experiments and all other criteria provided equal disease suppression in at least one experiment. While not tested in combination, the recommendation is to schedule fungicide applications based on calendar-dates (the most consistently reliable criterion), and use the disease threshold of >16 blighted leaves per plant quadrant (a moderately reliable criterion) to adjust timing within 1–2 weeks of the calendar-date in response to year-to-year differences in disease pressure, which can vary due to weather variables such as rainfall frequency (a moderately reliable criterion).