Monitoring of fungicide resistance service for blackberry and strawberry farms

Clemson University is offering a service to determine resistance to a series of fungicides on your farm. See article below.

First results of 2013 monitoring for fungicide resistance in the gray mold pathogen Botrytis cinerea

Dolores Fernández-Ortuño and Guido Schnabel, Clemson University

Gray mold caused by the fungus Botrytis cinerea is probably the most economically important disease of strawberry worldwide. Infection of strawberry flowers is caused by spores and results in fruit decay. Fruit infections begin as small, firm, light brown lesions that enlarge quickly and fruit become covered with a gray fuzzy mass of spores followed by a soft rot. Gray mold disease can easily spread during periods of rainy and cool weather, heavy dews, or high relative humidity.

            Chemical control of gray mold is essential to prevent fruit decay before and after harvest but resistance in B. cinerea to key fungicides is emerging. Therefore we started an extension program at Clemson University, which provides farm-specific fungicide resistance profiles. In our laboratory, fungicide sensitivity assays are performed that allow the distinction of sensitive from resistant isolates. During the 2011/2012 growing season we collected gray mold from commercial fields in Arkansas, Florida, Georgia, Kansas, Maryland, North Carolina, South Carolina, and Virginia. From each field we collected spores from ten berries (called a sample) and confirmed that the gray mold fungus was resistant to multiple chemical classes. The majority of the samples analyzed contained fungus that was resistant to Topsin M and Pristine. Half of the samples had a significant portion of fungus that was resistant to Scala and Elevate fungicides. Resistance to Rovral and Switch was rare (Figure 1).

This season we are providing the same service to strawberry growers and so far, we received samples from Georgia, Maryland, North Carolina, South Carolina, and Virginia. Fungicide resistance is still present and virtually all samples are resistant to Topsin M. In contrast to last year, however, resistance to Pristine has not been found that often. Only every 5^th sample on average is resistant to Pristine. Rovral and Switch remain to be great options against gray mold disease but remember that Rovral is restricted to one application prior to bloom (Figure 1).

            If you are are a strawberry or blackberry grower and you are interested in getting your farm-specific resistance profile to identify ineffective fungicides, send us around 40 dead strawberry (or blackberry) flowers or collect spores from newly infected, decaying fruit with a cotton swab. We need about 10 to 15 of those swabs (each from a different fruit and each fruit from plants far enough apart to represent an acre or so). Make sure that you only collect the fungus spores, do not touch the fruit (Figure 2). Mail the flowers or the swabs to Guido Schnabel, Clemson University, 114 Long Hall, Clemson, SC 29634 and tell us the origin of the sample, your name, and e-mail so that we can send you the report electronically. Upon receipt, we need about 3 (for cotton swabs) to 5 (for flowers) working days to get a report to you outlining farm specific gray mold management recommendations. 

Figure 1. Percent of samples with resistance to fungicide (red) collected from 8 states.

Figure 2. How to collect spores from strawberries affected with gray mold using cotton swabs.