Many gardeners may start to see spots appearing on the lower leaves of their tomato plants. The two common fungal diseases of tomatoes, early blight (Alternaria solani) and Septoria leaf spot (Septoria lycopersici), typically show up every summer in early July.
Early blight symptoms appear as small brown leaf spots with target-shaped concentric rings and are often surrounded by a yellow margin. As the season progresses, these spots enlarge and can coalesce.
The pathogen can attack leaves, fruit and stems and when the weather is favorable (doesn’t get much more favorable for diseases than the last few weeks!), extensive defoliation can occur. When this happens, yields can be affected, and the remaining tomato fruits are more susceptible to sunburn.
Septoria leaf spot appears as smaller dark leaf spots up to 1/8-inch with gray centers. The pathogen can infect petioles, stems and foliage, but not the fruit.
Both leaf spot pathogens overwinter on dead infected tomato refuse. Spores of the fungus splash up to the lower leaves and cause infections when the weather is wet or the plants have been watered with a sprinkler.
These leaf spots will produce more spores that will advance up the plant with each rain. Depending on the weather, the plants may be inundated with the disease as summer progresses, or the pathogen may move up the plant very slowly or not at all in dry years.
To avoid these two tomato pathogens, the majority of commercial growers have moved their tomato production into high tunnels where they are protected from rain and the plants are not prone to infection.
To manage the diseases, rotate tomato growing areas in the garden and clean up debris or till under at the end of the season to reduce the overwintering inoculum or spores. Provide adequate spacing between plants, and mulch in summer to prevent rain from splashing up. Avoid overhead watering. Prune suckers and lower branches to improve air circulation.
Once the disease appears, gardeners can use organic or conventional protectant fungicides on a weekly basis to protect new foliage from becoming infected. There also may be cultivars that have some resistance to these pathogens.
by Ann Hazelrigg, Extension Plant Pathologist, UVM