When it rains, plant diseases flourish

Jeff Burbrink

Rain and plant diseases tend to go hand in hand in Indiana. Just like the mildew that can grow in your shower, moisture, including high humidity, is essential to the development of most of our plant diseases.

With all the rain we’ve had for the past months, it’s a given we are going to see a lot of plant disease in 2019. Leaf spots, root rots, blights, malformations and cankers are signs that fungi, molds, bacteria, or viruses are hard at work on plants. Often these maladies use wounds, such as breakage from wind or hail or pruning to enter plant tissue.

Nearly everyone I speak to about plant disease asks which fungicide to use. Unfortunately, most of the fungicides on the market for both farmers and homeowners are far better at preventing a disease than curing an existing problem.

Since fungicides are preventatives, the products need to be applied before signs of the disease are present. For example, apple scab is a common disease that loves moisture and humid conditions. Growers are told to apply fungicide when the trees are showing just a tiny bit of green leaf, and continue those sprays every seven to 10 days until the conditions are no longer optimum for scab infection. In our climate, those cover sprays may well into July for apple scab.

Thorough coverage of the plant is very important when using fungicides. Most fungicides do not move into the tissues of the plant or circulate through the plant. Instead, the fungicides coat the surface of the leaves, putting a barrier on those leaves that the offending fungus cannot penetrate. Poor coverage of the leaf surface means a higher chance the disease will penetrate the plants leaves.

There are some systemic fungicides on the market now that do penetrate the leaf surface and give some internal protection. However, these products for the most part do not move more than a few millimeter once inside the plant. Thorough coverage, therefore, is still very important if you are going through the trouble of applying a product.

Fungicide labels often use words like “reapply every seven to 10 days” or every 10 to 14 days, as long as conditions are encouraging for disease. This reapplication of product is not a scheme to part you with your money. The fungicides do break down over time, so this reapplication is important.

Rarely does a single application of a fungicide ever fix a problem. In fact, if the product you are using has a seven-to-10-day window, and it’s not supposed to rain on day 7 or 8, you could wait until day 9 to reapply. You do not have to worry about the product washing off when it rains. The manufacturers put “stickers” in their products to make sure the product stays in place for at least the minimum number of days.

Fungicides also cannot control everything. Some are good at fungi, others do better with bacterial infections. Some products work well on one fungus, but not another. And no product on the market does much to control viruses. We still rely on sanitation (removal of the sick plant) when it comes to viral diseases and many of the soil borne diseases.

The take-away message about fungicides is that timing, thorough coverage and persistence are important to use them successfully. And, if the plant is already infected, you probably will not see results if you spray it after the fact.

Jeff Burbrink is extension educator at Purdue Extension Elkhart County. He can be reached at 574-533-0554 or jburbrink@purdue.edu.

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