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Identifying and Controlling Early Blight In Tomatoes

A tomato plant infected with early blight.

What is it?

Early blight is a fungal disease caused by fungi in the Alternaria family that occurs on tomatoes throughout North America. Early blight can affect seedlings but is generally observed on older plants and is especially severe on plants of poor vigor. Plants infected with the fungus can display collar rust on the stems, infected older leaves, and fruits that crack at the stem. Infection on leaves is the most common symptom. Early blight spores survive on old plant debris or in the soil. Spores are spread by wind and rain, but occasionally, flea beetles transmit this disease. Fungal spores enter a host through wounds in the plant cuticle. Spores thrive in moist, warm temperatures (80–90 degrees F) and can persist in partially decomposed garden waste for at least a year.

Symptoms of Early Blight

Early Blight of Tomato
Concentric rings, and yellowing of lower foliage is a typical sign of early blight.

The appearance of circular or irregular dark spots on the lower, more mature leaves is one of the first symptoms of infection. Eventually, the spots enlarge into a series of concentric rings surrounded by a yellow area. The entire leaf may be killed and will drop off the plant. Early blight can result in extensive defoliation, exposing fruit to sunscald and reducing yields. This disease typically progresses from the base of the plant, upward.

Control Strategies

Prune Determinate Tomatoes | Purdue University Vegetable Crops Hotline
Notice the pruning of lower shoots off the stem. This ensures no contact with the ground, greatly decreasing the chances of early blight development.
  1. Plant resistant varieties. Varieties such as ‘Celebrity’, ‘Rutgers’, ‘Mountain Pride’,’Big Beef’ and others have some tolerance to early blight. These varieties will require a less intensive management program than susceptible varieties.
  2. Maintain plant vigor. Stressed plants are more susceptible to early blight. Prune lowest leaves up stalk 6-8 inches once the plant begins to mature, and do not let lower leaves touch the ground. Water the plants regularly, but don’t fertilize until the plants are well-established and in full blossom. Do not mulch until the soil is warm.
  3. Do a thorough cleanup of the garden in the fall. Remove plant debris or till it into the soil. Pull weeds that compete for light, water, and nutrients, especially nightshade, horse nettle, and other weeds in the tomato family.
  4. Rotate crops. Practice a 2- or 3-year crop rotation. Avoid planting eggplant or potatoes where tomatoes were last planted.
  5. Avoid activity when plants are wet. Confine staking and picking to times when foliage is dry. Disease is more readily spread when plant foliage is wet.
  6. Protect clean foliage with a fungicide. Effective fungicides containing chlorothalonil (sold as Bravo or Daconil), or mancozeb work well. Apply at fruit set and reapply every 7–14 days.

Conclusion

Early blight is one of the most common and detrimental diseases we deal with in tomatoes. However if you follow the control stratiegies outlined above you will be well on your way to keeping it at bay and producing healthy plants and fruit.