Tomato blight is a term used for several tomato diseases that can be devastating to home gardeners and commercial growers. The fungus known as Alternaria solani causes early blight, and the algal-like organism Phytophthora infestans results in late blight. Late blight is one of the more famous plant diseases because it is also the causal agent of potato blight. It was responsible for the Irish potato famine of the mid-1800s.
Late blight of tomatoes is a worldwide and highly contagious disease that can spread rapidly and destroy whole fields within a short time. It prefers wet weather and moderate temperatures. This pathogen can tolerate high temperatures, as long as the nights are cool.
The symptoms of this type of tomato blight begin as tiny brown spots on the leaves. These lesions grow to become the size of a nickel, and are typically sunken, and either brown, or dark-green in color. The stems generally have brown lesions that develop white fungal growth when there is a lot of moisture. The fruit also develop brown, firm spots.
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Fortunately, tomato seeds do not carry this pathogen. This organism also will not survive on tomato cages and stakes. Late blight will be killed by freezing temperatures, since it requires a live host. There is no cure for this disease, only prevention. Varieties of tomato that exhibit some resistance or tolerance to this disease do exist, and should be planted whenever possible.
Both home gardeners and commercial growers should be vigilant about inspecting their tomato plants for signs of this disease. It is likely to manifest first in wet areas. Plants that show signs of the disease should be pulled and destroyed, not composted. Surrounding plants should be sprayed immediately with a systemic fungicide, and then treated regularly with a protectant fungicide. These products work better if applied more frequently at lower dosages, rather than less frequently at higher dosages.
The other prominent form of tomato blight is early blight, which is usually present wherever tomatoes are grown. This fungal disease is generally one of the greatest problems for home gardeners growing tomatoes. Uncontrolled, it can destroy most of the leaves on the plants, greatly reducing yields.
The early symptoms are small, dark lesions on older foliage. One difference from late blight is that these lesions are surrounded by a yellow layer. As they get larger, they may develop a 'bulls-eye' appearance. In late summer, there can be so many lesions that the plants lose their leaves. The fruit can also become affected, showing the concentric rings. Up to 50% of the immature fruit may be lost, and usually falls off the plant.
It is recommended to start early with attempts to control this tomato blight. Alternaria solani survives in soil, infected plant debris, related host weeds, and tomato seeds. New varieties are available that produce some tolerance or resistance to early blight. Gardeners should still use a combination of strategies to control this disease.
Growing tomato plants in a different location can minimize the chance of infection. Staking the plants is important, since it minimizes contact with the soil. Mulching is also important because it reduces the chances of infected soil splashing up onto the plants themselves. Experts recommend treating transplants with fungicides as soon as possible after they are set out. This treatment should be continued at seven to ten day intervals during the growing season.