Answer: I agree with you - it is way too early for spider mite damage. Spider mites like hot, dusty conditions - normally found in late summer. When they feed (which is on the undersides of the leaves) they leave a stippling effect on the foliage, not yellow blotches. I think instead of insect pests, your plants are showing symptoms of disease. It's hard for me to diagnose without actually inspecting the plants, but two problems common to tomatoes come to mind:
Early blight is caused by Alternaria solani. The fungus causes disease in tomato, potato, eggplant, horse nettle, and black nightshade. It is very destructive in temperate humid climates. Although the disease is called "early" blight, it can occur on the plant at all stages of development. Early blight can cause a decrease in fruit quantity and quality.
Leaf spots are circular, up to 1/2" in diameter, and dark to light brown. They have a distinct pattern of concentric rings, like a target. Spots may occur singly or in large numbers on the leaf. The leaf may turn yellow, then brown, and fall off. Older leaves are usually affected before the disease works up the plant. In a severe infection, the entire plant may be defoliated. Defoliation may cause the fruit to become sun scalded.
The fruit is usually affected at the stem end. One or more firm, depressed rot spots appear on either the green or ripe fruit. The spots usually have distinct concentric rings; they may appear leathery and may be covered with a velvety mass of black spores.
Prevention. Use disease-free seed and transplants. Resistant varieties are available. Avoid planting near potatoes or other tomatoes. Avoid injury to the plant, especially during transplanting. Long crop rotations are important. High soil fertility reduces the severity of this disease, as does maintaining plant vigor. Limit leaf wetness by watering early in the day and by watering at the base of the plant. Control weeds, especially horse nettle and black nightshade. Eradicate volunteer tomato plants. Remove or destroy debris immediately after harvest, or at least plow under deeply. If disease is severe enough to warrant chemical control, select one of the following fungicides: maneb, mancozeb, chlorothalonil, or fixed copper. Follow the directions on the label.
Septoria Leaf Spot
Septoria leaf spot is caused by Septoria lycopersici. It is one of the most destructive diseases of tomato foliage. The fungus can cause disease on tomato, potato, eggplant, petunia, nightshade weeds, horse nettle, and ground cherry. Defoliation of the plant leads to a decreased yield.
Symptoms. Symptoms of this disease can be found on the leaves and other green parts of the tomato plant. Small, round, yellowish spots appear on the leaves, usually after the first fruit sets. Symptoms generally start on the lower leaves first. The spots later become brown or grayish and may have a yellowish or dark border. On susceptible tomato varieties, leaf spots may be up to 1/8" in diameter; but they are smaller on resistant varieties. Dark specks or dots appear in the center of the spots on susceptible varieties. These are usually fewer or absent on resistant varieties. Heavily infected leaves fall off, normally starting at the bottom of the plant and progressing upward. Fruit infection is rare.
Disease Identification. Leaf spots are small and may have dark specks in them; no spots on fruit.
Similar Diseases. Similar diseases exhibit somewhat different symptoms. Early blight leaf spots are generally larger (1/2"), have a target appearance, and may have a light-colored ring around each spot. Early blight causes large target-like spots near the stem of green or ripe fruit. Leaf spots caused by bacterial speck and bacterial spot are dark brown to black. They may have a yellow ring around them. These diseases cause dark specks or spots on the fruit.
Prevention. Use disease-free seeds and transplants. Resistant varieties are available and should be used whenever possible. A crop rotation of 1 to 3 years, controls weed hosts and , therefore, decreases the level of disease. Planting early may help reduce disease, since the pathogen prefers warm temperatures. Avoid working in plantings when leaves are wet. Stake plants to keep foliage dry and off the ground. Avoid overhead irrigation. Immediately after harvest, remove all debris from the field and bury it. As an alternative, use a rotary `brush-hog' mower and plow under debris. These techniques prevent the fungus from moving to other hosts to overwinter on them. Repeated fungicide applications will keep the disease in check.
Hope this information helps you determine the problem with your tomato plants.
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