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In wet summer weather, watch out for plant disease

The forecast for continued wet weather this summer seems to be coming true. I recently contacted Sara May, coordinator of the Penn State Plant Disease Clinic, who said that continued wet weather would certainly increase the number of disease samples that will come through the clinic from both commercial growers and home gardeners. Last year my Norway maple tree lost all its leaves due to anthracnose Discula (Gloeosporium) and never achieved its beautiful yellow fall color. This past week, I recorded 2.5 inches of rain in the rain gauge so we are definitely receiving plenty of rain and the continued wet conditions are certainly conducive to promoting diseases. Problems caused by excessive rainfall can persist well after the rainy period ends.

The upper parts of plants really don’t mind the rain all that much, as they shed the water off, although we do normally see an increase in the number of foliar, or leaf, diseases during periods of wet weather. This is because the spores of most fungi that infect leaves need a thin film of water to sprout and cause an infection. That is why I have always encouraged watering in the early morning so that the leaves have a chance to dry out before entering the night period or use drip irrigation so as not to wet the leaf surface in the first place. The more leaves stay dry, the less opportunity exists for fungal infections to occur. The more the leaves stay wet, as in periods of frequent rains, the more likely fungal spores will land on the leaves, sprout and cause infections. Planting shrubs that are prone to foliar diseases, such as roses, in locations where they receive morning sun that will dry the dew early will also help control fungal diseases.

Although the upper plant parts can deal with rainy periods pretty well, the roots are where most problems occur. Excessively wet soil combined with high temperatures can create stressful, and potentially destructive, conditions for the roots of bedding plants, perennials, vegetables and shrubs. After it rains, the soil is saturated, and pore spaces in the soil that usually hold air are filled with water. Normally, gravity quickly pulls the excess water out of the upper soil, restoring the proper balance. We help this out by planting flowers, vegetables and shrubs on raised beds. But when rain showers come frequently as they have recently, the soil tends to stay wet over an extended period of time. Plant roots need oxygen to be healthy, and when the pore spaces are filled with water for a long period, the roots begin to suffer and do not work properly.

An unhealthy root system leads to a sick plant. These wet conditions also encourage fungus organisms that live in the soil to attack the roots or crown of a plant and cause it to rot. These disease organisms can cause wilting, scorched or brown leaves, leaf drop and dieback and can even kill plants. Once infection occurs, little can be done to help a plant. Plants with succulent stems such as impatiens and begonias, those that like cooler temperatures such as geraniums and dianthus, and those that prefer drier, well-drained soils such as Indian hawthorns, azaleas and agapanthus, are particularly susceptible.

One final point to remember is that sandy soils drain quickly and do not have the water holding capacity of the clay soil so we add organic matter to a sandy soil to encourage better water holding capacity. In a heavy clay soil, organic matter can improve the soil structure and increase the ability of the soil to drain better. A loamy soil is in between a clay soil and a sandy soil. My soil over the mountain is a shale soil that drains well and with the addition of organic matter is an excellent garden soil.

One final trivia point is that 1 acre inch of water is 27,154 gallons, so you can figure what 2.5 inches of rain in the gauge equaled in gallons of water on an acre of land.

Bill Lamont is a professor emeritus in the department of plant science at Penn State and can be reached by email at wlamont@psu.edu.