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

Authors: Copes, Warren; HAGAN, AUSTIN - Auburn University; OLIVE, JOHN - Auburn University

Submitted to: Crop Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/16/2012
Publication Date: 10/1/2012
Citation: Copes, W.E., Hagan, A., Olive, J. 2012. Timing of fungicides in relation to calendar date, weather, and disease thresholds to control Rhizoctonia web blight on container-grown azalea. Crop Protection . 42:273-280.

Interpretive Summary: Fungicides can effectively control Rhizoctonia web blight on azalea, but reliable guidelines about when to spray haven’t been available. Previous research identified weather conditions that promote rapid blight, but web blight progresses over too wide of weather conditions for simple predictions that could warn commercial producers when fungicide should be applied. In this study fungicide timing criteria, including a calendar schedule and decision-based weather and disease incidence thresholds, were evaluated over three years at locations in southern Alabama and southern Mississippi. A calendar-based fungicide timing schedule (a spray in early July and a spray in the first of August) provided the most dependable control. A simple scouting method of counting dead leaves with in a plant (spray when 16 and 30 diseased leaves are counted) provided control in many of the trials, and could be used in conjunction with the calendar schedule to adjust for the year-to-year differences that do occur. The information provides commercial fungicide timing guidelines for azalea producers and will benefit university extension personnel, private agricultural consultants, and commercial ornamental plant nursery 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. 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 this study is to evaluate fungicide timing criteria, including a calendar schedule, and decision-based weather and disease incidence thresholds. Experiments with a randomized complete block design were performed at a site in AL and MS for three years. For 2009 and 2010, treatments included a non-treated control, and five treatments where fungicide timing was based on a calendar schedule, a rain frequency criterion, and three disease threshold criteria. 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. The lowest disease threshold (16 to 30 blight leaves per plant quadrant) and the rain frequency criteria (>3 rain events of > 6.3 mm rain within the previous 7 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), but adjust timing within 2 weeks of the calendar-date in response to year-to-year differences in disease pressure using the > 16 blighted leaves per plant quadrant threshold (the disease threshold that exhibited moderate specificity) with consideration of rain frequency.