Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
11/13/00
Contacts:
Chris Feeley, Extension Plant Pathology, (515) 294-0581,
cfeeley@iastate.edu
Elaine Edwards, Extension Communication Systems, (515)
294-5168, eedwards@iastate.edu
Yard and Garden Column for the Week Beginning Nov. 17
The Fight Against the Blights
By Chris Feeley
Extension Plant Pathologist
Iowa State University
We have all heard it said before, hindsight is 20/20. Now that the gardening season is over we can take a look at some of last summer's problems and take steps to avoid them next year.
In Iowa, tomatoes seem to be the favorite plant. Every spring we venture out looking for our favorite tomato seeds or tomato plants for our garden. Browsing through the numerous selections on the market you may have noticed the letters V, F, N and sometimes T. These letters indicate that the tomato is resistant to Verticillium wilt (V), Fusarium wilt (F), nematodes (N) or the tobacco mosaic virus (T). When you buy disease-resistant tomatoes, you typically can avoid dealing with these problems.
After your tomatoes were planted in the ground and well on their way to a bumper harvest, they struck -- the dreaded and common tomato blights. Small dark spots appeared on the lower leaves after warm wet weather. Eventually the black spots became numerous, and the leaves turned yellow and started withering. This problem worsened as early summer turned into midsummer.
Why did the blights hit your tomato plants so hard when you selected disease-resistant plants? The answer is simple. Tomato plants have not been bred to be resistant to the blights, unlike the wilt fungi (V and F), nematodes or viruses. In Iowa, two common blights tend to be problematic, Septoria leaf spot and early blight.
Septoria leaf spot, caused by the fungus Septoria lycopersici, is probably the most common tomato disease in Iowa. The symptoms first appear as small water-soaked spots that develop into larger spots measuring about 1/8 inch in diameter. The spots have grayish white centers and a dark outer ring. With closer inspection you may be able to see the fungal fruiting bodies, which appear as small dark specks in the center of the lesions. The lower leaves become yellow and wither, and the disease spreads upward, damaging more and more leaves.
The fungus Alternaria solani causes early blight. Like Septoria, it attacks the lower leaves first and then moves upward to damage additional leaves. Brown-to-black spots measuring up to 1/2 inch in diameter can be found on the lower leaves. These spots develop into velvety lesions with concentric rings that can occur on the leaves, stems and sometimes the fruit. Like Septoria leaf spot, early blight can cause severe damage to the plant.
Several steps can be taken to help alleviate the risk of tomato blights to next spring's planting. This fall, remove as much of the dead plant material as possible and till under the remaining debris. Before adding the dead plant debris to your mulch pile, keep in mind that this debris should not be used as mulch around next season's tomato crops. If possible, plan to rotate your tomatoes to a different part of the garden and make sure to purchase healthy and disease-resistant plants (V, F, N and T) in the spring.
Healthy plants are better able to resist diseases, so keep your plants healthy by watering and fertilizing as needed. When watering your tomato plants, remember to water them early in the day, avoid overhead watering systems, and mulch around them to help maintain the soil moisture. In addition, leave adequate space between your plants to improve the air circulation. This practice allows the leaves to dry and discourages fungal growth. These simple steps can help limit the blights, but you may need to visit your local garden store for labeled fungicides if the problem continues. Taking these few steps in the fall and in the spring can help get you on your way to a great tomato crop.
ml: isugarden
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