Stemphylium leaf spot

The fungus overwinters on infected crop debris and in the soil as mycelium or conidia. When conditions become favorable, the conidia are dispersed by wind, rain splashes, and irrigation water to new hosts.

Optimum development occurs under conditions of high humidity and temperatures between 20°C and 25°C. A prolonged wet period on the leaf surface is essential for the germination of spores and successful infection of the plant tissue.

Once the spore germinates, the germ tube penetrates the leaf through stomata. The fungus then colonizes the tissues, secreting enzymes and toxins that lead to cell wall degradation and tissue necrosis.

The rapid life cycle of Stemphylium lycopersici allows for several infection cycles within a single growing season. This creates a high risk of epidemic spread in high-density greenhouse environments with poor ventilation.

The disease primarily attacks tomato plants, though it can also affect peppers and eggplants. The economic impact is mainly driven by severe leaf necrosis and premature senescence of foliage.

Defoliation reduces the plant's photosynthetic capacity, leading to smaller fruit size and lower total yield. Furthermore, the loss of leaf canopy exposes fruit to sunscald, which further reduces market value and quality.

Symptoms initially appear on older, lower leaves as small, dark brown spots. As these spots expand, they typically develop a characteristic light brown or grey center with a dark margin and a yellow halo.

The lesions often become angular or circular as they enlarge. Eventually, affected leaves turn yellow and wither, causing the plant to become prematurely defoliated, which severely restricts its growth potential.

Cultural management is the first line of defense, including long-term crop rotation and rigorous sanitation of crop residues. Eliminating volunteer solanaceous plants near the field helps remove alternative hosts.

Chemical control should be implemented preventatively. Effective fungicides include those from the strobilurin and triazole classes, as well as multi-site contact fungicides. Integrated pest management practices are essential to prevent resistance buildup.