Colletotrichum onion pathogen

The fungus is characterized by the formation of acervuli — small, disc-shaped fruiting bodies that typically possess dark, stiff setae. These structures allow for the rapid dispersal of spores under humid conditions.

The life cycle of the pathogen involves the production of conidia that spread through water splashes, contaminated tools, or infected soil. The fungus also survives as sclerotia, which can remain dormant in the field for several years.

Infection occurs through direct penetration of the host's epidermal cells or via natural openings such as stomata. The rapid colonization of tissues is facilitated by the release of cell-wall-degrading enzymes.

Temperature and humidity are critical factors for its growth, with an optimal temperature range of 20°C to 25°C and high humidity required for the pathogen to thrive and initiate disease development.

This pathogen primary affects bulb crops such as onions, shallots, leeks, and garlic. It poses a significant threat to production, causing both field-level infection and post-harvest decay.

Infected bulbs suffer from reduced shelf life and quality, often becoming unmarketable due to lesions that develop during storage and transit. This directly translates into severe economic losses for commercial growers.

In the field, the pathogen causes damping-off of seedlings and premature blighting of mature foliage. This loss of photosynthetic surface area leads to stunted bulb growth and reduced final yield.

The disease can cause significant stand loss in high-density plantings, especially if weather conditions favor the spread of spores from the soil and debris.

The ability of the fungus to survive on infected bulb tissues during storage makes it highly problematic for maintaining warehouse sanitation and prevents long-term storage of infected batches.

The primary symptom on onion bulbs is the appearance of small, yellowish or brown spots that eventually darken. These spots often develop into concentric rings of black acervuli, a hallmark of the infection.

On leaves, the disease presents as elongated, water-soaked lesions that may produce a grayish or pinkish mass of spores when humidity is sufficiently high.

Neck infection results in softening of the neck tissue and eventual collapse of the foliage. This often precedes the total decay of the bulb's outer scales.

In storage, the symptoms often manifest as large patches of dark mycelium and spores on the surface of the bulbs, which can quickly spread to adjacent healthy onions.

The inner scales of the bulb may become discolored and shriveled, showing signs of necrosis. Affected bulbs often lose their firmness and become susceptible to secondary infections by saprophytic bacteria.

Effective management begins with rigorous field sanitation, including the removal and destruction of all onion debris after harvest to reduce the primary inoculum load.

• Implement a strict crop rotation schedule, avoiding Allium crops for at least 3-4 years in infected soil.
• Use certified disease-free sets and treat seeds with appropriate fungicides before planting.
• Ensure rapid curing of bulbs after harvest to reduce moisture levels on the skin.
• Maintain optimal storage conditions with low humidity and cool temperatures to inhibit fungal growth.

Fungicide applications are recommended during the growing season, particularly when weather forecasts predict extended periods of warm, wet, and humid conditions.

Breeding and planting resistant or tolerant onion varieties remains the most sustainable approach for minimizing the impact of Colletotrichum circinans in agricultural systems.