KC Gardens

Seeing spots on your tomato plants? Here’s to prevent the leaf disease from spreading

Tomato plants can be susceptible to foliar leaf diseases. The key to preventing the disease is to help the plant stay as dry as possible.
Tomato plants can be susceptible to foliar leaf diseases. The key to preventing the disease is to help the plant stay as dry as possible. K-State Extension

Ample rainfall this year has been a blessing and a curse for gardeners. It has reduced the need for watering, but the rain and humidity bring with it an outbreak of foliar leaf disease.

Tomatoes, our most popular vegetable, can be susceptible to diseases that, if left unchecked, will significantly reduce the harvest.

Septoria leaf spot and early blight, two common foliar leaf diseases, cause problems in area gardens. These diseases favor damp foliage. Once the infection starts, it is difficult to control.

Symptoms begin on the lower leaves of the plant and as summer progresses, defoliates from the bottom up. Infected leaves develop brown to black spots or blotches, eventually yellowing and finally a dry brown. The loss of foliage reduces the plant’s ability to manufacture food to feed the developing fruits.

Researchers are working tirelessly to breed tomato varieties that are disease resistant but have yet to achieve this breakthrough.

Combating and preventing the spread of these diseases is difficult, but there are practices to help manage them, thus reducing the need for chemical treatments.

Reducing the spread starts at planting. Because the diseases favor damp foliage, the goal is to help the foliage dry quickly.

Tomatoes thrive best in full sun locations with good air movement. This combination reduces both moisture on the foliage and the infection period. It will help to dry the foliage from the spores landing on the leaves.

Overcrowded plants are more susceptible to disease, so increased spacing between the plants is recommended.

Another recommended practice is to remove the lower foliage as the plant begins to grow. The initial disease development starts at the base of the plant from spores splashed onto the lower leaves from rain or irrigation. As the plant matures, remove the leaves up to the first or second cluster of fruit.

Remove any of the suckers or side shoots that develop below this point. Suckering the plant, as it is called, will not reduce yields. Instead, it opens up the plant for increased airflow with the elimination of a few of the side branches.

Caging the plants to get them up off the ground helps improve the airflow as well. Mulching around the plant with straw, grass clippings, landscape fabric or some other material also reduces splashing on the leaves and keeps the diseases in check.

Proper irrigation may be your best defense. If at all possible, water the plant at the base either by flooding the soil around the plant or using drip irrigation. If you must water the plant with overhead sprinklers, avoid doing so in the evening so the plant has time to dry thoroughly before nightfall.

Fungicide applications may still be needed in many gardens. Ideally, these treatments would start early on. Several products for home gardeners include these actives: chlorothalonil, mancozeb or, for organic gardeners, copper-based fungicides. Always read and follow labeled instructions for use.

Be on the lookout for these diseases. It just isn’t summer without homegrown tomatoes.

Dennis Patton is a horticulture agent with Kansas State University Research and Extension. Got a question for him or other university extension experts? Email them to garden.help@jocogov.org.

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