Early Blight on Tomatoes

Jul 15, 2010

by Helen Halpenny, Lanark County Master Gardeners

Early blight on tomato plants is prevalent this year. It is a fungal disease that causes defoliation of the leaves. Early blight is commonly called leaf spotting. First, small circular spots appear and later they become irregular in shape and affect both the central portion and the edges of the tomato leaf. The bottom leaves of the plant are affected first and the disease works its way up the plant, sometimes completely defoliating the plant, exposing the fruit to sunscald and reducing yield. Sometimes the fruit is also affected.

The pathogen survives between crops mainly in diseased plant residues. Spores can be carried by wind. The early blight fungus also can be seed-borne in tomatoes. Its short disease cycle allows for numerous infections and can cause rapid defoliation under the right conditions. When leaves are continuously wet and temperatures are high, conditions for early blight are perfect. (This year’s weather has suited blight to a tee.) Plant susceptibility increases with age, heavy fruit load and inadequate nutrition.

There are several cultural practices that will help control early blight. First of all, practise crop rotation. Plant in different ground each year. Don’t replant an area for at least three years. Use disease-resistant transplants. Minimize plant injury and maintain plant vigor by providing ample nutrients. When watering is necessary, do it early in the morning so leaves will dry off before evening.

If you want to be pro-active, spraying with a fungicide at 7-10 day intervals may help. A homemade remedy that will help control fungal disease is a teaspoon of soda in a spray bottle of water. Spray on leaves on a dry day. It is also helpful to remove diseased leaves from the site and burn or put them in the garbage. Do not compost them.