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It sounds like your tomatoIt sounds like your tomato plant might have powdery mildew, which shows up as yellow patches on leaves, but also a bit of the characteristic powdery white growth. This interferes with the vigor of the plant and fruit size, but should not kill it, unless severe (it will kill individual leaves). To avoid, water the plant only on the bottom, as wet leaves tend to encourage the disease. Also, provide good air circulation and sunlight. To treat, consult a local garden nursery for various products, organic and inorganic depending on your preference. A common preventative involves formulas containing sulfur; horticultural oil is a common treatment if the disease is already there. Also, remove affected leaves (only remove up to a third of the leaves on the plant, though). Clean tools before and after pruning.