July 29, 2010 at 8:43 a.m.
There are three ways that help control tomato disease and I have already covered using disease-resistant plants.
Using good cultural practices is the second way of control. I received a call the other day from a gardener concerned that his tomato leaves were turning yellow from the bottom. The first question I asked was if there was bare ground beneath the plants? Since there was, I told him to pinch off the yellow stems and put dry straw beneath the plant. This keeps mud from splashing on the leaves, which causes many diseases. The mulch also keeps the roots moist and cool and the leaves dry. Another good practice is water in the early part of the day and always water on the ground.
The third way to control disease is by using a fungicide. Fungicides will control the spread of disease but will not eliminate it. They should be applied according to the product labels. When you select a spray be sure you are spraying for the right disease.
Weather conditions have everything to do with the success of raising tomatoes. Hot, humid, moist weather invites early blight. It is a disease that is caused by a fungus that attacks leaves, fruit, and stems of the tomatoes. It starts on the lower leaves as dark brown or black spots with dark rings in the center that make it look like a target. These spots start small but can be one-half inch in diameter. Spots can grow together to cover the leaf causing it to dry up and fall off the plant. Stem and fruit infections are also black often with dark growth rings inside. Fruit spots usually start where the stem connects to the fruit and can grow to cover the top half of the fruit. Early blight has been shown to be more severe when the plant has been stressed by drought. So it is important that tomatoes are watered on a regular basis during a dry spell.
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 late blight can occur at anytime during the growing season it is more likely to be seen in late summer or early autumn. This disease can spread rapidly with cool rainy weather 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 fruit during wet conditions. The best control is to remove the infected area or the entire plant, and keep the plants dry with good air flow. Again, if you can find a good, safe, fungicide, use it.
I could write a couple more articles, if I included all the things that could go wrong raising tomatoes. One problem that is more of a nuisance unless you sell tomatoes, are growth cracks. They result from extremely rapid growth brought on by periods of abundant rain and high temperatures, especially when such weather conditions follow drought. Since I don't sell tomatoes it is more important to have a smaller well rounded tomato. I have better control of growth cracks if I plant an early or mid season tomato variety.