Answer: Wood chip mulch will not harm your plants directly, but microorganisms will use up available nitrogen in the soil trying to decompose the high-carbon wood chips. Any mulch, such as a thick layer of grass clippings, paper, etc., that will protect your plants from soil splash should suffice. If early blight (Alternaria solani) is the problem, then crop rotation is the first best defense. Don't grow tomatoes in the same plot year after year, or where potatoes have been grown the previous year, as the fungus is present in the soil (this is where the mulch comes in!). Potatoes can pass early blight to tomatoes, so separate them in your garden. Some varieties of tomato are more resistant than others, though none are completely immune. Late blight (Pytophthera infestans) is not carried in the soil, but the spores are wind-borne from plants in the environment. High humidity and moisture on plant leaves encourages fungus growth. Again, potatoes and tomatoes should be separated in the garden. Pruning and training the tomatoes to ensure good air circulation around the leaves to encourage drying is a useful preventive measure. Some say that misting the plants with compost tea is helpful as well. Currently there are no resistant varieties.
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