Rhizopus rot

Pathogen and disease type. Rhizopus oryzae is a ubiquitous soil-borne fungus belonging to the Zygomycota division. It functions primarily as a saprophyte, but under favorable conditions, it acts as a necrotrophic pathogen. It is the primary causal agent of rhizopus rot, causing rapid tissue softening and mold formation in both field crops and stored agricultural products.

Affected crops. This pathogen is known for its wide host range, infecting a variety of essential crops including rice, corn, soybeans, sunflower, and peanuts. Additionally, it is a significant post-harvest pathogen that affects various fruits and vegetables, most notably sweet potatoes, tomatoes, and melons, leading to severe economic losses during storage and shipping.

Symptoms and signs. The hallmark symptom of a Rhizopus oryzae infection is the rapid appearance of dense, cotton-like mycelium that initially appears white and turns grey or black as sporulation progresses. Small black dots, known as sporangia, are visible within the mycelial mat. In seeds, the fungus penetrates the embryo, causing black discoloration and preventing germination.

Developmental conditions and impact. The development of the fungus is favored by high humidity (often exceeding 80%) and warm temperatures, with an optimum range between 30°C and 35°C. Its impact is twofold: in the field, it reduces stand establishment by causing seed rot and seedling blight; in storage, it causes rapid decay and degradation of organic materials, often making the produce entirely inedible due to the production of mycotoxins.

Protection and control measures. Effective management of rhizopus rot relies on integrated pest management (IPM) practices:

• Ensure proper drying of seeds and grain to low moisture levels before storage.
• Use certified, fungicide-treated seeds to ensure protection during the germination phase.
• Handle harvested crops carefully to prevent mechanical injuries that provide entry points for the fungus.
• Implement strict sanitation in warehouses, including temperature control and consistent ventilation to keep humidity low.