As the vegetable growing season continues, we know that diseases will start to appear on vegetables, so we must practice constant vigilance to assure that we can control them when they are first noticed.
Tomatoes can get two destructive soil fungus diseases — Verticillium and Fusarium wilt — that result in the fungi plugging the plant’s vascular system, causing wilting and plant death. These diseases are soilborne so it’s best to obtain tomato varieties that carry genetic resistance, as there is no control after the plant is affected.
Leaf-spot fungus diseases that attack tomatoes are early blight, late blight and Septoria leaf spot. These fungi spread by spores, which can be called seeds of the fungus. The spores are airborne and land on plant leaf surfaces. The spore then penetrates the leaf and starts feeding on internal structures of the leaf. Once this occurs, most fungicides cannot control the disease.
Applying fungicides on a preventive basis will provide a barrier on the leaf, so that when a fungus spore lands, it is killed before it gets a chance to penetrate. This is why it is very important to scout your fields so that you can learn to recognize the early signs of the disease. For example, early blight forms large “bull’s eye” spots on the leaves, Septoria forms smaller black spots with no bull’s eye pattern that is usually more noticeable after a heavy rain, and late blight produces many grayish water-soaked spots.
Southern blight is a destructive soil fungus disease that can quickly kill tomato plants. This fungus can remain in the soil from overwintering dead tomato plant stems. These overwintering “mustard seed” structures, called sclerotia, can attack newly planted tomatoes in the field. They invade the plant stem by girdling it and also producing a white cottony mass of spores which can be found above the stem girdle. The only practical way to control Southern blight is to move to a field that does not have the disease, or to turn the upper layer of the soil by plowing. Burying the sclerotia will deprive them of oxygen and will kill them. Wait at least 12 months before replanting an infected field with tomatoes.
A physiological disorder that commonly occurs on tomatoes is called leaf roll. Uneven soil moisture or the lack of soil moisture can cause tomato leaf margins to roll upward and meet at the center of the leaf. The leaves roll upward in order to conserve moisture as is seen in corn. The control for this one is to have even soil moisture throughout the season by watering deeply down to a six-inch depth. When the surface of the soil is dry, take a long handle screwdriver and push through the soil until it becomes hard to push. Then pull it up and measure how deep the soil is wet. Then water until you reach a six-inch wetted depth of soil and note how long it takes to achieve that depth. You will then know how long it takes to water your soil to maintain a six-inch depth when you do the “screwdriver test”.
For pumpkin and winter squash (acorn, butternut, spaghetti) growers, two troublesome fungus diseases start to infect plant leaves as the fruit is starting to mature. Powdery mildew shows up on the upper leaf surfaces as a white powdery growth that can quickly engulf the leaf. Downy mildew starts out as yellow spots that turn brown on the upper leaf surface and then produce gray spots on the lower leaf surface. These two fungus diseases can cause tremendous leaf damage, which results in the reduction of photosynthesis taking place, which inhibits fruit maturation.
When the vines and leaves shrivel up, this exposes the immature fruit to the sun, which causes sunscald on the fruit and leads to premature breakdown, caused by secondary fungi which grow on the fruit tissue, after the sunscald has wounded the fruit.
For summer squash (zucchini, yellow squash, and patty-pan squash) growers, be on the lookout for Choanephora rot (also known as blossom end rot or wet rot). This starts out after the female flower has been pollinated and fertilized. As the tiny immature fruit enlarges, the fungus attacks in the high moisture and humid area at the base of the plant. The fungal stalks are gray with black spores produced at the tips of the fungal stalks. The symptom looks like a man’s beard. Fungicides generally are not recommended to control Choanephora rot. As air temperatures increase and the base of the plants become less humid, the disease will disappear on its own, with later fruits being unaffected. Choanephora can also affect okra, which can cause early forming pods on the plant to rot. Again, warmer weather usually controls this disease.
There is no substitute for field scouting to learn how to recognize these symptoms. Once you learn them, you can apply the proper control methods. Obtain a copy of the agricultural chemical manual that is published by your state Cooperative Extension Service, located in the county where you reside. These are usually updated every year. You can also obtain color photos of various vegetable diseases from your local county Extension office for free or for a nominal charge.