Early blight in tomato plants — Let’s Garden

This week, we delve into the challenges of nurturing our favorite garden fruiting vegetable: the tomato. A few weeks ago, we analyzed the pros and cons of pruning, that is, shaping our tomato plants so that they grow and produce the maximum number of fruit and the size we desire. Today, we take a close look at a common and destructive foliar disease of tomatoes: early blight. 

I hear many tomato growers complain that their young plants were growing well until their lower leaves suddenly began to show signs of yellowing and discoloration, spreading upward, eventually affecting the entire plant. Death typically followed.

The culprit is most likely early blight. This blight is caused primarily by the soil-borne fungus Alternaria linariae, formerly known as A. solani. Lesions first develop on lower leaves as small, brownish-black spots that expand to ¹⁄₄ – ¹⁄₂-inch in diameter with characteristic concentric rings in the darkened area. The area surrounding the lesions may become yellow, eventually discoloring the entire leaf.  In later stages, lesions may appear in the upper leaves and defoliation may occur in the lower part of the plant, leaving the fruit susceptible to sunscald. Fruit may become infected with brown spots in the green or red stage, sometimes covering the entire fruit.  

Early blight is encouraged by warm temperatures (75 to 84 degrees F) and moderate to heavy rainfall. The fungus can survive between seasons on crop debris in the soil. Volunteer tomato plants also can be a primary source of the fungus. Spores of A. linariae are primarily spread by wind and water splash. 

To fight early blight, begin with healthy transplants. There are currently no completely resistant tomato varieties, only those that possess some degree of resistance or are somewhat tolerant of the disease. These include ‘Mountain Merit’, ‘Mountain Magic’, ‘Defiant’, ‘Legend’, ‘Iron Lady’, ‘Mountain Supreme’, ‘Jasper’, ‘Juliet’, ‘Verona’, ‘Old Brooks’, and ‘Stellar.’

Second, keep plants growing vigorously. Maintaining adequate fertilization can reduce susceptibility to early blight. 

Next, minimize lower leaf wetness with drip irrigation and mulching. Mulching your plants immediately after transplanting will minimize splashing of water from the soil to the undersides of lower leaves following rains and overhead irrigation. 

Providing good air circulation is another preventative measure. This can be accomplished by proper plant spacing. 

Pruning the bottom-most leaves as the plant grows may also help. These leaves are usually more infected than the upper parts of the plant.

Crop rotation is yet another strategy. Avoid planting tomatoes for 2-3 years in the same area in which Irish potatoes and eggplant have been grown. 

Lastly, apply preventative fungicides prior to the onset of warm, wet weather. There are many fungicides that may be helpful. Consult your county Cooperative Extension agent or yours truly for additional advice.

Tim Lewis is a Georgia Green Industry Association Certified Plant Professional, gardening writer, former Perry High School horticulture instructor, and former horticulturalist at Henderson Village and Houston Springs. He and his wife, Susan, own and operate Lewis Farms Nursery located on Hwy 26 two miles east of Elko, where he was born and raised. He can be reached at (478)954-1507 or timlewis1@windstream.net.