The challenges of disease control during rainy spells
Extended periods of rainy weather spell “feast” for fungal plant pathogens, since they are highly dependent on moisture for their growth and infection.
While relatively dry, cool conditions in early spring may have delayed pathogen development, recent warm, wet conditions mean that fungal and bacterial diseases are getting a jump start. This even applies to powdery mildews, which prefer warm dry conditions during the growing season, but need sufficient moisture to release the first spores of the season. Repeated or continuous wetting of infected tissues over periods of several days will aid spore production as it allows thorough wetting of infected woody plant parts and promotes fungal spore development. In addition, rain assists rain splash-dispersed pathogens in splashing the spores to susceptible plant tissues. Furthermore, extended wetness periods (24 to 48 or more hours) provide ample moisture for spore germination and infection of plant tissues. Wet periods during bloom are especially conducive to twig and blossom blights, and fruit rot pathogens can also get their start in blossom remnants.
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 millimeter (1/25 inch) of rain washed off about 50 percent of the captan. Subsequent rainfall did not result in much additional loss of the fungicide. This shows that a portion of the captan following an application can be washed off easily, but that the remaining residue is more tenacious. Our studies in grapes and blueberries have shown that 0.1 inch of rain may wash off 20-25 percent of protectant fungicides such as Ziram and Penncozeb, but it takes 1 to 2 inches of rain to remove enough residues to reduce disease control efficacy. However, when the residue is a week old and partially degraded by UV light or microbial activity, 0.5 inches of rain may be sufficient to drop below efficacious levels.
Our research has also shown that a heavy rain event tends to wash off more fungicide residue than multiple light rain events. 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 more strongly 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 and provide better coverage to start with. In addition, sticker-extenders may increase retention of fungicides during rain events (see graphs with Nu-Film P).
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 coverage and disease control than protectant materials during or after extended rainy periods. Systemic fungicides should be used alone or in the 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, especially those that have affinity for the wax layer, may be rainfast within a few hours after application, but longer drying periods may improve absorption into the plant. When you have a short window to apply the material, you may decide to choose a fungicide that becomes rainfast in one to two hours (this is often listed on the label).
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 tractor speed, and by spraying every row if possible and adding an adjuvant if recommended on the label. Surprisingly, even systemic fungicides suffer from wash-off by rain, but less so than protectant materials. At least they tend to be more rainfast under light amounts of rain, but eventually they will also wash off. 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.