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Early Blight |
By George Brown, Colorado State University Cooperative Extension, Horticulture
Early blight is a fungal disease that appears during the hot summer months.
Symptoms
Tomato leaves that have spots that begin as small, dark brown to black areas, and are
turning yellow on the lower part of ;the
plant, may indicate the
presence of early blight. Spots frequently begin on the
older, lower
leaves inside the canopy of the plant where humidity is higher. As the spot develops, a
concentric ring pattern can frequently be detected. This is the most diagnostic symptom of
the disease and is the source of the name, "target spot," that is frequently
found in popular garden books. As the disease progresses, leaves
yellow, then drop. In severe cases, all the foliage can be blighted and premature
leaf drop result. This in turn results in leaving the fruit exposed and sunscald results. Sunscalded spots
are hard and tasteless and have to be cut out before eating the fruit.
Controls
Several cultural practices that will help to reduce tomato damage in the home garden are:
- When first noticed, remove the diseased leaves to reduce infection. Do not remove so many leaves that you create conditions that are ideal for sunscald.
- Keep Plant foliage off the ground, use mulches and/or stakes.
- Space plants to allow for good air movement and dry of the leaves. You may need to do some selective pruning of tomato branches and even plants to accomplish this.
- Avoid overhead watering, which can splash the disease-causing spores from infected to uninfected tissue.
- Do not over fertilize with nitrogen (no more please!)
- Improve drainage and cultivate around plants to hasten drying at the crown. This will also help with leaf curl.
- Do not work among the plants when the leaves are wet.
- Remove severly diseased branches and plants (the fungus survives on plant debris). Destroy these by burying in the soil or your compost pile. Do not leave exposed because spores will continue to be produced.
- Next year, plant tomatoes in a different location, if possible.
In some instances, there may be a need for spraying a fungicide. Only use a fungicide if you have an accurate diagnosis and know for sure what disease is attacking the tomatoes.
If the infestation is heavy, sulfur may be used to protect new leaves from becoming infected. Follow label directions, since the use of sulfur on hot days can burn plant tissue.
Other effective and commonly available fungicides for the home gardener are copper based. When selecting a suitable copper based fungicide read the label carefully to insure that it can be used, and follow the directions on the label.
Photos: Judy Sedbrook
