Tomato stem rot

Systematic position and pathogen nature: Diplodina lycopersici (also known by its sexual stage Didymella lycopersici) is a pathogenic fungus belonging to the Ascomycota phylum. It is the primary cause of stem rot, a severe disease affecting tomatoes. The pathogen is highly virulent, impacting the vascular system and structural integrity of the plant, often leading to total crop loss if left untreated.

Diseases and host crops: This fungus primarily attacks tomato plants (Solanum lycopersicum). The disease manifests as brown, necrotic lesions on the stems, particularly at the soil line (collar rot). Furthermore, the pathogen can infect foliage and fruit, causing rot and significantly lowering the marketability and yield of the tomato harvest.

Biology and life cycle: The pathogen survives as mycelium or pycnidia in crop residues and infested soil for multiple seasons. Under favorable conditions, the pycnidia release conidia, which are splashed onto stems by rain or irrigation water. Once inside the plant tissue, the fungus colonizes the stem, expanding the lesion and producing new spores to facilitate further spread.

Conditions and impact: High relative humidity and cool to moderate temperatures are ideal for the development of Diplodina lycopersici. In greenhouses, stagnant air and excessive moisture levels greatly accelerate infection. The impact is significant: the stem lesions can girdle the plant, cutting off nutrient transport and causing wilting and eventual death of the affected specimen.

Protection and control measures: Effective management requires a combination of sanitation and chemical strategies.

• Strict sanitation by removing and burning all infested plant debris.
• Crop rotation to reduce the pathogen load in the soil.
• Ensuring proper ventilation and drainage in greenhouse environments.
• Applying recommended fungicides to prevent the spread of spores during susceptible stages.