Homegrown tomatoes are a fresh treat that Central Kansas gardeners greatly look forward to each season. This is why it can be disheartening when tomato plants begin looking ragged during the summer due to two common leaf diseases. So what are gardeners to do?
Two common leaf diseases
Septoria leaf spot and early blight are the most frequent diseases found on tomato plants in mid to late summer and both are characterized by brown spots (lesions) on the leaves that lead to leaf yellowing.
Septoria leaf spot usually appears earlier in the season than early blight and produces small dark spots. Spots made by early blight are much larger and often have a distorted “target” pattern of concentric circles.
Heavily infected leaves eventually turn yellow and drop. Older tomato leaves are more susceptible than younger ones, so these diseases often start at the bottom of the plant and work up.
Take steps to minimize disease
Both septoria leaf spot and early blight can overwinter on the soil and on leftover debris or weeds. This is why caging, or staking plants is key to minimizing disease. Plants should be kept upright and off of the soil.
Caging also provides better air circulation to help keep foliage dry and reduce the opportunity for infection. It is also important to space tomato plants out from each other as best as possible since tightly spaced plants are also more susceptible to problems.
Drip irrigation is another helpful tool when growing tomatoes since it places water directly at the root system which avoids getting the leaves wet (which can lead to more disease pressure).
Mulching around tomato plants is highly recommended since it helps prevent water from splashing the soil and debris and carrying disease spores to the plant. Wheat straw, untreated grass clipping, or old shredded leaves are just a few options for providing mulch.
Pruning off and removing any heavily infected lower leaves and branches is also recommended to slow the spread of these diseases. As plants mature, disease infected tomato leaves and branches below the fruit level can be removed from the plant and disposed of without slowing down production.
In situations where these diseases have been a problem in the past, rotation is also a good strategy. It is too late for that now, but keep it in mind for next year. Many gardens are too small to make large rotation practical but if you have room, rotate the location of the tomatoes each year to an area that has not had tomatoes or related crops (peppers, potatoes, eggplant) for several years.
If rotation is not feasible, fungicides are often helpful if applied in a timely manner. Be sure to cover both upper and lower leaf surfaces, and reapply fungicide if rainfall removes it. Plants usually become susceptible to disease when the tomato fruit is about the size of a walnut.
Chlorothalonil is a good choice for fruiting plants because it has a 0-day waiting period, meaning that fruit can be harvested once the spray is dry. Chlorothalonil can be found in numerous products including Fertilome Broad-Spectrum Landscape and Garden Fungicide, Ortho Garden Disease Control, GardenTech Daconil and others. Be sure to start protecting plants when the disease is first seen. It is virtually impossible to control this disease on heavily infected plants.
For a thorough read on Kansas tomato diseases and disorders check out our K-State Research and Extension Publication called Tomato Leaf and Fruit Diseases and Disorders