The challenges of disease control during rainy spells

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.   

Extended periods of wet weather spell feast for fungal plant pathogens, since they are highly dependent on moisture for spore dispersal and plant infection. While dry spells earlier this spring might have threatened to create a “famine” year for fungi, the tables have indeed turned. Repeated or continuous wetting of infected tissues over several days is particularly conducive to spore production as it allows thorough wetting of infected canes or other overwintering plant parts and promotes spore release. In addition, heavy rains assist rain-splash-dispersed pathogens in getting the spores to susceptible plant tissues. Furthermore, extended wetness periods (12-48 hours) provide ample moisture for spore germination and infection of plant tissues. Diseases in small fruit crops that are promoted by warm wet weather include Phomopsis diseases; black rot, downy mildew, and anthracnose of grapes; leaf spot, spur blight, and anthracnose of raspberries; common leaf spot, Phomopsis leaf blight, scorch, and fruit rots in strawberry; and rusts in raspberries and blueberries. While powdery mildew generally thrives under warm-dry conditions, it does need rainfall in the spring and early summer to release ascospores from overwintered cleistothecia. So, rainfall at this time will increase powdery mildew disease risk later this season.

The challenge is to apply sprays before rainfall events – with as much rain as we’ve had it is likely that most protectant fungicides have been washed off. A study by Xu et al. (2008) showed that when Captan was applied to apple leaves, Captan loss was primarily due to wash-off by rain. In fact, as little as one mm of rain washed off about 50 percent of Captan. Subsequent rainfall did not result in much more loss of the fungicide. The results may be explained by the fact that most of the Captan on fruit/leaf surfaces following an application can be washed off easily, but the remaining deposit is more tenacious. This has to be taken into account and the application rate may be adjusted accordingly. During periods like these, especially when followed or accompanied by windy conditions, it is very difficult to get the fungicides on at the right time, e.g., before an infection. This may be further complicated by fields being flooded preventing access with sprayers. Systemic fungicides should be used to get: 1) better coverage, 2) better rain-fastness, and 3) kick-back (curative) activity. They generally provide better disease control during or after extended rainy periods. Products are usually rain-fast within a couple of hours of drying, although longer drying periods may be better. The table (pdf format) shows which fungicides for small fruit crops are systemic or have systemic components. If relying on post-infection activity, use them at the highest labeled rate for the crop. Do consider that even systemic fungicides work better when thorough coverage is strived for by increasing spray volume and spraying every row or every other row. The pre-harvest interval and re-entry interval should also be considered as we are at or approaching.  

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