By Beth Berlin,
U of M Extension
It appears that this year’s humid, rainy conditions have led to leaf spot disease issues in gardener’s tomato plants. Early blight (Alternaria solani) and Septoria leaf spot (Septoria lycopersici) are two fungal diseases commonly associated with tomato plants. Watching for the signs and then managing it will help ensure you still have a successful harvest.
Symptoms of these fungal diseases include the plant’s lower leaves turning yellow, then eventually turning brown, drying up, and falling off. Early blight, and Septoria leaf spot are common in Minnesota and it is not uncommon for tomatoes to have both diseases at the same time. However, Septoria leaf spot is the most common fungal disease. This disease can affect plants at any stage of plant development. Round, yellow spots develop on the lower leaves; these spots enlarge and turn brown to gray. Tiny black fruiting bodies then form in the center of the leaf spots. These fruiting bodies produce spots which cause secondary infections upwards throughout the plant. Severely infected plants may have complete defoliation, which if fruit is present, may leave them too exposed to sun and result in sunscald to the fruit.
Early blight infects tomato leaves, stems and fruit. This is seen as brown concentric rings inside the leaf; stem and fruit spots are also characteristic of early blight. Brown spots are surrounded by yellow leaf tissue. As spots enlarge, more of the leaf tissue turns yellow, and eventually brown. Fruit can also be impacted; spots on the fruit are seen as dark brown to black tissue that is sunken on the fruit and has a leathery appearance. Lesions have a concentric ring and may become covered with black fungal spores.
Both Septoria leaf spot and early blight fungus will overwinter in infected plant residue in the soil as well as on nightshade weed hosts. Both diseases can also come into the garden on contaminated transplants or seeds. Wet conditions, and water splashing from rain or watering spreads the fungal spores onto new leaves leading to more infection. Gardeners can even spread the fungal spores on their hands, gloves, or clothing.
Preventing these diseases can be a very difficult task; however with persistence it can make the diseases manageable. Here are some helpful tips:
• Destroy any infected plant material and weed frequently throughout the season. As soon as symptoms are noticed, remove any infected leaves, and dispose all material. Do not remove more than one-third of the plant’s leaves.
• Use crop rotation in your garden to keep lingering pests and disease from causing issues the following year. Rotate plants so that tomatoes are not planted where potato, peppers, or eggplants have been planted in the past three to four years.
• Water plants at the base of the plant early in the day and stake or cage plants to increase air circulation and keep leaves dry. Keeping plants off of the soil prevents the disease from spreading through water splashing on the leaves.
• Use mulch around the base of the plant to help to prevent water splash from carrying spores upward into the plant, as well as to help maintain an even supply of moisture.
• Consider applying fungicides, especially during rainy seasons when disease is more prevalent. Monitor the plants and apply the fungicide as soon as symptoms are discovered. This will not be a cure; it only prevents the spread. Always read labels carefully, ensure it is labeled for tomatoes, and apply as directed.
• Fungicides labeled for use include: Bordeaux mixture, Chlorothalonil, liquid copper, Mancozeb, and Maneb
Tomatoes are a very sensitive plant to physiological conditions, including herbicide damage, and are susceptible to a variety of diseases, especially early blight and Septoria leaf spot. Scout your garden frequently for any signs of disease and act immediately.