Susceptibility is also a factor in how successfully and rapidly the pathogen grows and reproduces in the plant. As plants age and the fruit load grows, nutritional demands are higher, and tomatoes become more susceptible to early blight. Early blight invades the older, less vigorous leaves first, and moves up the plant to younger leaves. Maintaining good crop vigor throughout the season, with adequate water and nitrogen, helps plants resist early blight. Genetic resistance also can be important.
Early varieties tend to be more susceptible.
Cultural Management Is Key The first line of defense against any disease is producing healthy plants and growing them in an environment that is as free of the pathogen as possible. The fungus can be seed-borne, so it is important to start with clean seed. Alternaria solani is so widespread that it is difficult to escape it completely, but several cultural practices can help. We know that the fungus overwinters in infected crop debris and can persist from one season to the next in the same field. However, it will not survive in the soil during a rotation of 3 years or more into a non-host crop.