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Conditions right for leaf spot diseases to take toll on plants

Posted: June 29, 2013 - 11:09pm

Georgia’s weather is fickle.

Spring was cool and wet, and now summer is starting hot and wet. Both weather conditions provide their challenges.

Gardeners must adapt to the changes in the weather. High humidity and rainfall make a perfect environment for plant diseases, especially leaf spot.

Most leaf spot is not a real threat to the overall health of the plant, but be aware that conditions are favorable for the disease and be proactive.

There are several basic concepts of gardening that can aid in the prevention of leaf spot. One is putting the right plant in the right site in the landscape.

If the plant does not grow and thrive, it may have been placed in the wrong environment.

Before buying new plants, know what the intended site offers and make a selection that matches. Also, certain varieties of trees and shrubs are more susceptible to disease than others. Be sure to research the plants that will be added to the landscape before the selection is made.

If plants are already in the ground, the best practice to deter leaf spot is to ensure good growing conditions. Provide adequate mulch and optimize soil fertility by following soil- test recommendations.

Gardeners need to test the soil to verify optimal conditions for plant growth. Incorporating plenty of organic matter encourages rooting and water drainage. Strong, healthy plants are more resistant to stress from weather and disease pressure.

Gardeners must also allow for good air circulation so the plant does not stay wet for long periods of time.

Plants that have very dense branches and a tight leaf canopy take longer to dry out when wet. Selective pruning can open the canopy to more sunlight. Pruning will also increase air circulation throughout the plant and reduce disease incidence.

Another consideration is the application technique of supplemental watering. Water should be directed toward the roots rather than the leaves, and watering should be completed early in the morning so that the foliage can dry quickly.

Many fungi causing leaf spot diseases will survive the winter in leaves on the ground.

Severely infected leaves that have fallen on the ground should be removed from the area and destroyed. Infection occurs primarily in the milder weather of the spring and the fall.

If plants that are susceptible to leaf spot exist in the landscape, choose a fungicide to prevent leaf spot. Once the disease is apparent, the damage has been done.

In the vegetable garden, the same principles apply. All spent plants should be removed entirely from the garden. This will reduce the chance of the soil harboring the disease from year to year. Also, follow a yearly rotation of vegetables. For example, tomatoes, peppers and Irish potatoes are in the same vegetable family. All three are susceptible to the same diseases. After a couple of years of planting only tomatoes or tomatoes and peppers, switch to cucumbers or squash in that part of the garden.

If gardeners follow these guidelines, severe outbreaks of foliar diseases will be kept to a minimum. If there is an outbreak of leaf spot, fungicides are available. Fungicides can also be applied proactively to prevent the disease. Protective fungicide applications can be applied when leaves emerge in early spring and continue at 10- to 14-day intervals throughout the growing season. Two of the most used fungicides available contain chlorothalonil or propiconazole.

They are effective on many leaf spot diseases. Be sure to read and follow the application rates and safety precautions listed on the label.

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