The Septoria lycopersici fungus causes septoria leaf spot and attacks crops in the solanaceae family, including tomato, potato and chilli peppers. Leaf drop indicates that this fungus is probably infecting your garden. Know the favourable conditions for Septoria leaf spot, control and prevent a comeback the following year.
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Signs of septoria leaf spot are circular spots with a brown margin occurring on the underside of the lower leaves. Spots enlarge to a diameter of 1/4 inch. Fruiting bodies of the fungus appear in the middle of spots as dark-brown or black dots, a characteristic distinct from other tomato diseases. Eventually, infected leaves turn yellow and drop off. The fungus then infects higher leaves and flowers on the plant. Loss of foliage reduces fruit size. Without a leafy canopy, tomatoes are also vulnerable to sunburn.
Ideal conditions for septoria leaf spot are moderate temperatures, between 15.5 to 26.6 degrees C and prolonged rainy or humid conditions. In Maine, leaf spot crops up in tomato gardens in early to mid-August. During this time, gardens have lush vegetation and little air circulation, creating a humid environment among plants. In Alabama, the fungus injures plants in spring and fall when ideal conditions encourage fungal spread.
There are organic ways to stop septoria leaf spot from taking over your garden. Observe your tomato plants daily, especially when environmental conditions favour leaf spot development and spread. Control the fungus in the early stages by removing lower leaves that are infected. Remove weeds, such as nightshade and horsenettle. These plants share the same botanical family as tomato and serve as an alternative home for leaf spot. Overhead watering facilitates the spread of leaf spot, so consider drip irrigation. Attaching the plants to cages or stakes helps prevent excessive moisture within the plants.
A few steps early in the season prevent septoria leaf spot from occurring. Ensure that seeds originate from a source free of disease. Move tomato plants to a new location in the garden each year. Virginia Cooperative Extension suggests rotating tomatoes with cereals, corn or legumes. Apply fungicides to the lower leaves following formation of the first fruits. Continue the applications every week, particularly when environmental conditions favour septoria leaf spot.
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- Cornell University; Septoria Leaf Spot of Tomato (Septoria lycopersici); Thomas A. Zitter; December 1987
- Virginia Cooperative Extension; Septoria Leaf Spot of Tomato; Mary Ann Hansen
- University of Maine Cooperative Extension: Septoria Leaf Spot of Tomato
- Missouri Botanical Garden: Septoria Leaf Spot of Tomato
- Alabama Cooperative Extension System; Foliar Diseases of Tomatoes; Edward J. Sikora; November 1988
- Texas A&M University: Solanaceae--The Nightshade Family