Gardeners that grow tomatoes in Minnesota are often quite familiar with fungal leaf blights. High humidity, frequent rains and heavy dew can all favor fungal leaf blight. All three environmental conditions are common in Minnesota and it is difficult to grow tomatoes without encountering fungal leaf blights.
There are two very common fungal diseases of tomato that occur in Minnesota; Septoria leaf spot, caused by Septoria lycopersici, and early blight, caused by Alternaria solani. These two fungi thrive in very similar conditions, and it is not uncommon for tomatoes to have both diseases at the same time. Both fungi can come into the garden on contaminated transplants or seeds. They then survive in leaf debris from year to year. Rain and irrigation splash fungal spores up onto new leaves from the soil and plant debris below. New leaf spots soon produce spores. These spores are then splashed onto higher leaves with rain and irrigation. Both diseases progress in this fashion until every leaf is infected. Infected leaves first have spots, than turn yellow and finally turn brown.
Septoria leaf spot infects leaves but not fruit of the tomato. Leaf spots are small round spots with a dark brown to purplish border and a light gray center. Tiny black dots (the spore producing structures) can be seen in the center of the spot with a hand lens. Leaves may yellow, but most damage is done by leaf loss due to the infection. Fruit may suffer from sunscald or other rots because they are not sheltered or shaded by the tomato leaves.
Early Blight infects tomato leaves, stems and fruit. Brown concentric rings inside of leaf, stem and fruit spots are characteristic of early blight. Brown spots are surrounded by bright yellow leaf tissue. As spots grow bigger, more of the leaf tissue turns yellow, then brown, resulting in a completely blighted plant. Fruit spots are dark brown to black, sunken and leathery. Ridges of concentric rings can be seen within the fruit spot.
Preventing tomato leaf blights altogether may be impossible once the fungi are established in the garden. However, use of the cultural control practices below can delay the appearance of the disease and reduce the number of leaf spots on the plant. Often this is enough to allow the tomato plant to produce a good crop. Remember, the goal is to grow tasty tomatoes, not to have a pretty looking plant.
- Plant tomatoes where no tomatoes, potatoes, peppers, or eggplants have been for the past 3-4 years. In small yards consider moving tomatoes to pots for a year or two if tomato leaf blights occur regularly in the garden.
- Keep tomato leaves as dry as possible.
- Space plants so that air flows between plants even when they are full grown.
- Stake or cage plants.
- Use drip irrigation or soaker hose to water plants.
- If using sprinkler irrigation, water in the morning so leaves dry quickly in the sun.
- Mulch all exposed soil with plastic or organic mulch.
- Examine the lower leaves once a week. Pinch off lower leaves with leaf spots. Never remove more than 1/3 rd of the plants leaves.
- At the end of the season, remove or bury infected tomato plants to reduce the amount of fungi that survive to the following season.