As I am writing this, the water is gushing down.  Needless to say, we have experienced significant rainfall as of late. When it hasn’t been raining, it has been cloudy and humid. This is “great” fungal disease weather – especially for downy mildew and black rot. With the rain we have been experiencing, contact chemicals are being washed off the vines and grape clusters, so we have to tighten our spray schedules substantially.  After a rain such as I hear outside my home at the moment, probably 50% of a contact fungicide (e.g. captan or mancozeb) would have been washed off (at a minimum). However, the plants are not completely naked, so don’t panic, but you do need to reapply fungicide relatively soon. When it is raining like this (excessive), shorten the spray interval to 7-10 days, as opposed to 10-14 days we would recommend under dry conditions. Tank-mix application of a systemic and a contact would be good as well.  If infections have occurred, the systemic may have some kickback activity (killing out the fungus from recent infections), but this is variable. This would also be a really good time to incorporate the most effective fungicides for downy mildew.

The information below comes from Annemiek Childers at Michigan State, and I have sent this out in various ways in the past. However, if you have not read it recently, it is a well-written article about fungicide spray residues under wet conditions. The one warning when reading this is that you can’t legally shorten the minimum spray interval as dictated by the label. Also, some spreader-stickers can cause plant damage, and some fungicides have these already (no additional additives required). Generally, non-ionic surfactants are ok, but some tank mixes can be toxic. Test a strip to make sure you don’t cause damage.

Posted on May 24, 2011 by Annemiek Schilder, Michigan State University Extension, Department of Plant Pathology

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 cool temperatures may have slowed down fungal development to some extent, the wet conditions are very conducive to disease development. Repeated or continuous wetting of infected tissues over several days will aid spore production as it allows thorough wetting of infected canes or other overwintering plant parts and promotes spore release. In addition, rains assist rainsplash – dispersing pathogens by splashing the spores to susceptible plant tissues. Furthermore, extended wetness periods (12 to 48 hours) provide ample moisture for spore germination and infection of plant tissues.

Diseases in small fruit crops that are promoted by wet weather include Phomopsis and Botrytis diseases; black rot, downy mildew, and anthracnose of grapes; cane and leaf diseases of raspberries; leaf spots and fruit rots in strawberries; and rusts in raspberries and blueberries. While powdery mildew prefers warm, dry conditions, it does need rainfall in the spring and early summer to release ascospores from overwintered cleistothecia. Therefore, rainy springs will increase powdery mildew disease risk as well, particularly if rains are followed by warm dry weather which will promote successful establishment of the first colonies.

The challenge is to apply sprays before rainfall events. With as much rain as we’ve had it is difficult to keep the plants covered with fungicide. In addition, with rapid plant growth, new growth may not be covered or the fungicide residue is too diluted to be effective. 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. As little as 1 mm (1/25 inch) of rain washed off about 50 percent of the Captan. Subsequent rainfall did not result in much more loss of the fungicide. This shows that much of the Captan on fruit or leaf surfaces following an application can be washed off easily, but that the remaining residue is more tenacious and more tightly bound to the plant surface.

Our studies in grapes have shown that 0.1 inch of rain may wash off 20 to 25 percent of protectant fungicides such as Ziram and Penncozeb, but it takes 1 to 2 inches of rain to detect a significant reduction in disease control activity. This suggests that there is usually sufficient amount of active ingredients left after light rain events. But to achieve good to excellent control, one has to reapply the fungicide after a major rain event or when significant plant growth has occurred. And even protectant fungicides require some time to bind to the plant surface and it is advisable not to spray them within a few hours before rain. A spreader sticker may help the fungicide adhere to the plant surface.

During rainy periods, especially when followed or accompanied by windy conditions, it is difficult to get the fungicides on at the right time, e.g., before an infection period. This may be further complicated by fields being flooded, preventing access with spray equipment. Systemic fungicides generally provide better disease control than protectant materials during or after extended rainy periods. Systemic fungicides should be used alone or in a tank-mix with protectant fungicides to get better coverage, improved rainfastness, post-infection (curative) activity, and a broader spectrum of disease control.

The degree of post-infection activity varies by fungicide. Systemic fungicides may be rainfast within a few hours after application, but longer drying periods (e.g., 24 hours if possible) may be better for improved absorption into the plant. When relying on post-infection activity, use fungicides at the highest labeled rate for the crop and make sure coverage is optimized by adjusting nozzles, spray volume, and speed; and by spraying every row if possible.

Surprisingly, even systemic fungicides suffer from wash-off by rain, but less so than protectant materials. Remember that systemic fungicides may also be diluted inside plant tissues due to rapid plant growth and may need to be reapplied sooner during warm periods that promote rapid plant growth.