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Solutions and Signs for Common Tomato Diseases

By Horticulturalist-in-Residence, Dr. Ann Hazlerigg

The fungus that cause Early Blight overwinters in tomato leaves from the previous year. Rotating beds is the easiest way to prevent this fungal disease.
The fungus that causes Early Blight overwinters in tomato leaves from the previous year. Rotating beds are the easiest way to prevent this fungal disease.

This summer, the farm-to-table garden within the King’s Garden will feature Brandywine and Purple Cherokee tomatoes. Although it has been a cool spring and many of our warmer season vegetables are still waiting in the greenhouse, let’s review some common problems in tomatoes that we encounter in the King’s Garden.

Septoria Leaf Spot (shown here) and Early Blight strike lower leaves on tomato plants first, as they're closer to the spores coming up from dead tomato leaves on the ground.
Septoria Leaf Spot (shown here) and Early Blight strike lower leaves on tomato plants first, as they’re closer to the spores coming up from dead tomato leaves on the ground.

Tomatoes should be planted when soil temperatures become warmer and the danger of frost is over. Fungal leafspots are the main problem in tomatoes with the two most common being early blight and septoria leaf spot. We see these to some extent every year, usually near the end of June, with rainy years resulting in more disease. The diseases look a little different (early blight leafspots have a bulls-eye appearance and septoria leafspots are usually smaller with gray centers) but basically both work the same way. The fungus overwinters on diseased tomato leaves left in the garden from last year. When the weather becomes warmer, the spores are splashed onto the lower tomato leaves. Leafspots develop, then produce more spores that are windblown and water splashed to the upper leaves of the plant. With each rain event, the disease works its way up the plant. Fortunately, there are some easy cultural practices we can do early in the season that will help minimize the damage from these diseases.

Late blight blows in on the wind & rain from warmer climates further south. It can, but doesn’t always hit tomatoes in the North Country.

An easy place to start is rotating tomato beds to new locations each year, keeping new plants away from infected tomato leaves from the previous year. Using mulch around the base of the plants can help provide a barrier to spores that could splash onto the lower leaves. Avoiding overhead watering in the garden will help reduce leaf wetness which reduces infection. Staking tomatoes improves air circulation and drying. Keeping up the fertility of the plants along with good regular watering allows the plant to better withstand the disease, sort of like us, and eating healthy foods and getting plenty of sleep! Cultivars may vary on their susceptibility to these leaf spots and there have been some developed in the last few years that are resistant to these leafspot diseases. The last resort is the use of a fungicide. This provides a barrier that prevents the spore from gaining access but since tomatoes grow fast and the fungicides break down in sun and rain, the material would need to be applied every 7-10 days depending on the weather.

Because Late Blight comes from spores carried on the rain, the upper leaves of tomato plants will be effected first. This disease can kill back tomato plants rapidly and must be treated with fungicide.

A third tomato disease called late blight is very aggressive and was the cause of the great potato famine in Ireland in the 1840s. This fungal-like pathogen does not overwinter in upstate New York or Vermont but blows in from southern states. Depending on weather patterns, the pathogen ‘leapfrogs’ its way up to the North Country on storm fronts. If storms do not come up from the south during the growing season, we may not see the disease at all, which was the case in 2018 and 2019. The late blight pathogen starts in the upper part of the plant since the windblown spores rain down onto the upper leaves during storm events. The pathogen causes a larger water-soaked leafspot and can wipe out planting in about a week if the weather is cool and wet. For this disease, fungicides are the only control measure. Typically, when this disease is nearby, extension specialists will alert growers and gardeners to start protecting their plants with fungicides.

Click here for more information and pictures of these tomato diseases.