GOSHEN, Ind. — I can predict with great certainty that in August and September, there will be a lot of apple and crabapple trees losing their leaves to a disease called apple scab. While there are many varieties that are resistant to the scab, there are many that are not. Spraying a fungicide, beginning at blossom time in late April, and reapplied every 7 to 14 days (dependent upon the products used) could possibly have prevented the evitable drop of the leaves.
Apple scab, like many of the diseases that affect our plants, thrives in wet, warm conditions. It should be no surprise to start seeing spots on leaves of many of our plants after all this rainy weather.
When someone asks for advice about spots on their plant leaves, it is not long until they ask what can be spray to eliminate the disease. Unfortunately, it does not usually work that way.
Most of the fungicides on the market are protectants, not cures. In simple terms, protectant fungicides, when used properly, put a protective layer on the surfaces of the plant to help prevent the disease spores from penetrating the leaf surface. The preventative products need to be in place before the visible signs of the disease are present.
Since most plant diseases thrive under wet conditions, it is important to have the protection in place just before the conditions favor the disease. Generally, that means applying the fungicide before the plant gets wet from either rainfall or irrigation. That may seem crazy, because most people fear the fungicide will be washed off the plant. However, the products have built in spreader-stickers to help them stay on the plant.
Typically, a fungicide will have language on the label saying something like “reapply every 7-10 days until the risk of disease has passed.” So, if you are trying to protect a tomato plant, and you spray on June 10, you essentially have control from June 10 until June 17. If the weather forecast is calling for rain on June 19, you will want to reapply the product on June 18, a day before it starts raining. If the weather is cooperative, you can wait until June 20 for the next application.
— Jeff Burbrink, Extension Educator, Purdue Extension Elkhart County
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