Abrupt weather changes can lead to plant diseases. Here’s how to help those tomatoes

Early spring rainfall has been both a blessing and a curse for gardeners. Not so much a blessing was the abrupt change from cool and rainy to hot and dry. Unfortunately, the rain and humidity can facilitate an outbreak of leaf diseases. Tomatoes are susceptible to diseases. If left unchecked, disease can significantly reduce the savory harvest we eagerly anticipate each summer.

Septoria leaf spot and early blight, two common foliar diseases, cause problems in area gardens. These diseases escalate in damp foliage environments, and once the infections start, they are difficult to control. Symptoms begin on the lower leaves of the plant, and as summer progresses the plant defoliates from the bottom up. Infected leaves develop brown to black spots, or blotches that eventually yellow and finally turn 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. There are practices to help you manage them, thus reducing the need for chemical treatments.

Successful reduction of disease spread begins at planting. Since the disease progression accelerates when the foliage is damp, the goal is to implement cultural practices that allow the foliage to dry quickly. Tomatoes thrive best in full sun locations with good air movement. This combination reduces moisture on the foliage and the chance of infection. It also helps dry the foliage when the spores land on the leaves, reducing the potential for damage. Overcrowded plants are more susceptible to disease, so increased spacing between the plants is recommended.

Another good practice is to remove the lower foliage as the plant begins to grow. The initial disease development begins at the base of the plant and originates 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 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 air flow and circulation, as well.

Application of mulch around the plant with straw, grass clippings, landscape fabric or some other material will reduce fungal spores from splashing on the leaves, which reduces the chance for disease. Proper irrigation may be your best cultural defense. If 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, allow the plant adequate time to thoroughly dry before nightfall.

Fungicide applications may still be necessary in many gardens. Ideally, the earlier you can start these treatments, the more effective in keeping the diseases constrained. Products labeled for home gardeners include chlorothalonil, mancozeb or copper-based fungicides for organic gardeners. Always read and follow the labeled instructions for use.

Be on the lookout for these diseases and address them early for best outcomes, as it just isn’t summer without delicious homegrown tomatoes.

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