June 30, 2011 at 8:57 a.m.
About the time those first tomatoes are beginning to form, a fungal disease called septoria leaf spot begins to infect the lower leaves of tomato plants. Check the lower leaves for circular spots about 1/8" in diameter with a light center and dark margins. Spores produced in these lesions quickly spread up wet plants infecting all green tissue. The best control for septoria leaf spot is to pinch off and dispose infected leaves when they appear in mid to late summer and hope for dry weather.
If you planted an early variety you may be able to harvest some fruit before it becomes a problem.
Mulching to keep the soil from splashing onto the plants can help.
The cool spring is ideal for verticillium wilt. This is a cool season fungal disease that attaches to tomatoes, potatoes, eggplants, peppers, strawberries and raspberries. Symptoms first appear on the oldest leaves, which wilt and show marginal yellowing. These areas eventually turn brown and the leaves fall off. A clue that it is verticillum wilt is that the leaves often turn yellow and dry up without any evident wilting beforehand.
Fusarium wilt, on the other hand, is a warm season fungal disease that only attacks tomatoes. As with verticillium wilt, the lower leaves of tomato plants will turn yellow. However, the yellowing is often confined to one side or the other of the midrib. Eventually the entire plant wilts, yellows and dies.
If black spots develop on the bottom of the tomatoes and when you open them they are rotten, it could be blossom end rot. Blossom end rot is not a disease but caused by fluctuating moisture levels. A few cells die on the flower end of the tomato opposite where it is attached to the stem. Then the spot becomes infected with fungal organisms. The brown or blackish spot enlarges until the tomato is picked.
Try to water deeply and regularly. Mulch your plants with straw or grass clippings to help reduce the fluctuation. Don't overuse the fertilizer as it can cause the plants to grow rapidly, creating more demand for water.
Another potential problem this summer could be early blight. The symptoms are that the leaves and stems develop large dark spots with each spot having many narrow concentric rings. Similar dark sunken blotches also appear on the fruit usually beginning at the end of the stem. The only thing that seems to work is to remove the infected leaves or the entire plant to control spreading. There may be some home remedies that work or fungicides that may help control the disease. If they work and are safe, go for it.
If you get through the summer without these diseases you still may have to deal with late blight. This is the same blight that caused the Irish Potato famine in the 1840's. Although the late blight can occur at any time during the growing season it is more likely to be seen in late summer or early autumn. Under favorable conditions, which are cool, rainy weather, this disease can spread quite rapidly and can destroy plants in a few days. Fruits infected by late blight show grey-green water soaked spots. A white cottony fungus develops on the fruits during wet conditions. The best control is to remove infected area or the entire plant and keep the plants dry with good airflow. Again, if you can find a good, safe, fungicide, use it.
The outlook for raising tomatoes is not as bleak as I have indicated. If you planted disease resistant plants, keep your tomatoes clean and if we have a decent late summer and fall, we all may have nice tomatoes this fall.
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