Pink root of onion

Taxonomic classification and pathogen type: Pyrenochaeta terrestris is a soil-borne fungal pathogen belonging to the phylum Ascomycota. It is recognized as the primary agent responsible for pink root rot in onions. The fungus exists in the soil as a saprophyte on plant debris and produces pycnidia, which serve as survival structures under unfavorable environmental conditions.

Diseases and susceptible crops: The pathogen primarily affects all varieties of Allium cepa (onions), garlic, and shallots. It can also colonize the root systems of various monocotyledons, including corn and cereal grasses. The hallmark symptom is the transition of infected roots from white to pink or reddish-purple, leading to root decay and eventual necrosis.

Biology and life cycle: The life cycle involves the production of asexual spores (conidia) within pycnidia formed on dead or dying tissues. These spores are disseminated through irrigation water and movement of infested soil particles. Once in contact with susceptible host roots, the fungus penetrates the epidermis, colonizing cortical tissues and disrupting nutrient and water uptake.

Environmental conditions and impact: Disease development is heavily influenced by soil temperatures, with optimal growth occurring between 24°C and 28°C. Poorly drained, compacted, or moisture-stressed soils exacerbate the infection process. The economic impact is severe, resulting in stunted plant growth, reduced bulb size, and significant yield losses, often exacerbated by secondary rot organisms.

Management and control: Integrated pest management strategies are crucial, starting with a crop rotation cycle that excludes Allium species for at least 5 years. Improving soil drainage and structure is essential to limit disease spread. Chemical control, including soil fumigation or the application of fungicides, can provide short-term suppression, though the use of resistant commercial onion hybrids remains the most sustainable approach.