Pink root rot of onion

Setophoma terrestris (formerly identified as Pyrenochaeta terrestris) is a soil-borne fungal pathogen responsible for the devastating plant disease known as pink root rot. Classified within the kingdom Fungi and phylum Ascomycota, this pathogen primarily attacks the root systems of Allium species, with onions and garlic being the most economically impacted crops. It can also survive on the roots of various cereal grasses without causing visible severe symptoms.

The characteristic symptom of the infection is the pinkish discoloration of the root system, which progresses to dark red, purple, or even black as the disease advances. These infected roots eventually disintegrate, lose their turgidity, and die off. Because the root system is severely compromised, the above-ground portion of the plant exhibits symptoms of water and nutrient stress, such as leaf tip dieback, chlorosis, stunted growth, and premature senescence, significantly reducing bulb size and quality.

The life cycle of the fungus is highly dependent on environmental conditions. As a soil-inhabiting saprophyte, Setophoma terrestris can persist in the soil for several years on infested crop debris. It is a warm-weather pathogen; infection and colonization of onion roots are most aggressive when soil temperatures reach between 24°C and 28°C. Higher soil temperatures significantly exacerbate the disease's impact on onion production.

Epidemiological spread is facilitated by contaminated soil movement, infested planting stock, and irrigation runoff. The fungus enters the roots through wounds or direct penetration of the epidermal cells. The overall harmfulness of the disease is magnified when host plants are already stressed by unfavorable soil conditions, excessive moisture, or nutrient deficiencies, which allows the pathogen to establish itself more effectively in the root tissues.

Managing pink root rot requires an integrated approach due to the pathogen's ability to survive in the soil for extended periods. Key control strategies include:

• Selection of onion hybrids with verified resistance to Setophoma terrestris.
• Implementation of long crop rotations, avoiding Allium crops for at least 4 to 5 years.
• Maintenance of optimal soil health and balanced fertilization to reduce plant stress.
• Use of solarization or soil fumigation in high-value production systems to reduce pathogen inoculum.
• Strict sanitation measures to remove and destroy infected plant debris at the end of the season.