White spots on tomato plants are most often caused by fungal diseases, which can quickly wipe out otherwise healthy tomato crops. To minimise tomato problems, choose certified, disease-resistant plants and place them in the ground when daytime temperatures are at least 18.3 degrees Celsius. Follow good cultural practices, since preventing diseases is almost always more effective than treating them.
When the soil lacks adequate phosphorus, the tomato plant develops white, wilted leaves in response to the deficiency. If a soil test indicates a lack of phosphorus, apply a fertiliser that contains this nutrient. Phosphorus may be present in the soil, but tomato plants are unable to use it under cold conditions. Don't plant tomatoes too early in the spring and lay a plastic mulch on the soil to warm it.
Septoria Leaf Spot
Septoria leaf spot is a fungal disease characterised by white or grey spots on leaves. The spots are surrounded by a black or brown ring. Remove diseased foliage and pick up any infected leaves on the ground, since the fungus overwinters in the soil. Treat severely infected plants with a sulphur powder. To minimise infections in the future, rotate crops so tomatoes aren't planted in the same location. Use drip irrigation to keep the leaves dry and space the tomatoes at least 3 feet apart to provide adequate air circulation.
The fungal disease late blight most often occurs in late August, during a prolonged period of rainy, cool weather. Symptoms include large water-soaked green and black blotches on the upper leaves and a white, fuzzy growth on the leaves' undersides. Control late blight using the same methods as prescribed for septoria leaf spot -- good watering and sanitation practices and crop rotation.
Powdery mildew is more common in humid greenhouses than in garden settings. This fungal disease is characterised by white patches on the upper surfaces of the leaves that later develop brown lesions. Treat this disease by applying fungicides. Run fans in the greenhouse to increase air circulation and space plants adequately. Avoid overhead irrigation to keep the leaves dry.