Root and hypocotyl rot

Initial symptoms appear as dark, water-soaked lesions on the hypocotyl of young seedlings. As the disease progresses, these lesions turn brown or dark brown and may eventually girdle the stem entirely.

The root system of infected plants becomes brown, with observable decay of secondary and tertiary roots. In severe cases, the taproot rots, leading to a total failure of nutrient and water uptake.

Above-ground, plants exhibit wilting, particularly during the hottest parts of the day, even when soil moisture is adequate. Leaves begin to yellow from the bottom up and eventually desiccate.

In nursery settings, the pathogen causes seedling collapse (damping-off). The base of the stem becomes soft and necrotic, causing the seedling to topple over under its own weight.

Microscopic inspection of the necrotic tissue often reveals fungal growth, which may appear whitish or slightly pinkish depending on the moisture level of the environment.

The disease is caused by the fungus Plectosphaerella melonis, which belongs to the class Ascomycetes. This pathogen is highly virulent towards plants in the Cucurbitaceae family.

The fungal mycelium actively penetrates root and hypocotyl tissues, degrading cell walls through the secretion of lytic enzymes. The pathogen can persist in the soil for long periods as chlamydospores.

The fungus affects a broad spectrum of crops, including watermelon, cucumber, melon, zucchini, and pumpkin. Cases of infection have also been reported in pepper, tomato, and lettuce.

Inside the plant, the infection spreads through the xylem vessels, blocking the transport of water and nutrients to the aerial parts, leading to metabolic failure.

The biology of the pathogen allows it to parasitize both weak and relatively healthy plants, provided that environmental conditions favor its colonization and reproduction.

Infection is favored by high soil and air humidity, commonly associated with over-irrigation or poor drainage management within greenhouses.

The optimal temperature for the growth of Plectosphaerella melonis ranges from +18°C to +25°C. Rapid temperature fluctuations under high humidity conditions often trigger disease outbreaks.

Crop debris left in the soil from previous seasons serves as the primary reservoir for infection. The pathogen spreads easily via irrigation water, contaminated substrates, or unsterilized tools.

High planting density and poor ventilation in greenhouses increase the likelihood of disease spread, as soil surface moisture persists for longer periods.

Physical injury to roots during transplanting or cultivation provides entry points for fungal spores, allowing for rapid colonization of host tissue.

The economic impact of the disease stems from a sharp reduction in plant density, directly affecting the total yield of the cultivated area.

Infected plants produce fewer and smaller fruits, which are often deformed and have poor flavor profiles due to compromised nutritional uptake.

The disease causes significant financial losses in greenhouse operations, requiring expenditures on chemical treatments, soil replacement, or expensive fumigation.

The infection is characterized by high spread rates, making it nearly impossible to contain local outbreaks without removing infected plants entirely.

In cases of severe early-season infection, crop losses can reach 80–100%, rendering the cultivation of cucurbits in infested soils economically unviable.

The foundation of effective management is a strict crop rotation program, ensuring cucurbits are not planted in the same area for at least 4–5 years.

The use of healthy, treated seeds and the selection of resistant hybrids significantly mitigate the risk of pathogen introduction in the early stages of development.

• Regular removal and destruction of crop residues after the harvest season.
• Disinfection of soil substrates, greenhouses, and gardening tools with appropriate fungicidal agents.
• Maintaining optimal soil moisture and aeration to prevent waterlogging.

Biological fungicides based on Trichoderma spp. can help suppress pathogen development in the rhizosphere by establishing a competitive environment.

If initial symptoms are detected, targeted application of approved chemical treatments as a soil drench may be necessary to halt the spread of the infection to adjacent healthy plants.