Pink root rot: Roots turn pink or maroon
Pathogen: Phoma terrestris (syn. Pyrenochaeta terrestris)
Symptoms
Roots turn pink or maroon when infected. Leaf number and size are reduced and the plants are easily uprooted. Reduced bulb size.
Disease development
The fungus is generally considered ubiquitous and can survive in the soil, in diseased roots and the debris of susceptible crops for several years. The fungus can be spread through soil movement and in surface water. This disease can develop at all soil moisture levels that allow onion growth. This pathogen will attack healthy crops in warm onion growing regions. Optimum temperatures for growth of the pathogen and disease development are 24-28°C (75-82°F). Little disease will develop when temperatures drop below 16°C (60°F).
Disease management
Resistance to the pathogen varies among cultivars, thus resistant cultivars should be planted when possible. Resistance may be overcome if soil temperatures of 28°C (82°F) or higher occur. Planting so the bulk of the root growth occurs prior to reaching soil temperatures that favor disease development can minimize severe losses from this disease. Long term rotation (4-6 years) with non-host crops, such as cereals, helps reduce losses. Also, soil solarization or fumigation can help to reduce pink root and increase marketable bulbs.
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