By David Graper
Early Septoria leaf spot on a tomato plant in mid-June. Mitchell, S.D.
Septoria leaf spot is beginning to show up in some gardens. It was spotted in Mitchell last week on just a few plants. Areas of the state that have had warm weather and lots of rain are probably going to be the first places where this common fungal disease is going to show up.
Symptoms
The disease wil cause small, dark, somewhat angular spots to develop on the leaves, usually the lower leaves first. After a few days, the leaves will turn yellow and eventually fall off. Spores are produced during periods of wet weather that can be spread by overhead irrigation or rain. Infections can spread upwards in the plant quite quickly. Tomato cultivars that are particularly susceptible may become mostly defoliated by mid-summer with spots also appearing on the fruit.
Management
If Septoria leaf spot has been a problem in your garden, try planting resistant tomato varieties, or at least plant several different varieties to have a better chance of at least getting some good tomatoes later in the season. Rotate planting locations in the garden so that you do not plant tomatoes or potatoes in the same spot less than three years apart. Spread out your tomato plants so that there is good air circulation around each plant. That will help the foliage to dry out more quickly and reduce the chances of having an environment conducive to the disease. Use mulches to cover the soil and reduce or slow the spread of fungal spores that are present on diseased foliage from last year, getting splashed up into the plant. If possible, use drip tubes or ooze hoses for irrigation, rather than overhead sprinkling to water the garden. If you see signs of the disease, start using a protective fungicide that contains chlorothalonil on a regular basis, as directed on the label. Picking off the first leaves that get the disease can also help control early spread of the disease.