Cause Phoma terrestris (syn. = Setophoma terrestris), a fungus that may survive several years in soil, and can also persist in plant debris.
Symptoms The disease may appear either in young seedlings or later in the onion's growth. Infected roots turn yellow, then pinkish, then bright pink to red and finally to dark purple as they dry and disintegrate. New roots produced thereafter become diseased and also die. Seedlings may die. Otherwise, bulbs lack normal development, are reduced in size and vigor, and have stunted tops.
- A 3- to 6-year crop rotation reduces the severity of infection; however, the fungus still will invade some plants.
- Use adequate fertilizer, irrigation, etc., to speed bulb growth.
- Partially resistant varieties of the 'Sweet Spanish' type are available. Poor rooting clones are prone to infection. 'El Capitan', 'Avalanche', 'Bullseye', 'Bravado', 'Merit', 'Durango', 'Yula', 'Armada', 'V', and 'Fiesta' were bred for resistance. 'Yellow Globe Danvers' types are resistant.
Chemical control Preplant soil fumigation effectively controls the disease on mineral soils but not on peat soils. Note Fall fumigation is more effective than spring.
- Telone C-17 at 10.8 to 30 gal/A. Caution Fumigation must be before planting, so it is important that the planting row be directly over the fumigated soil if only the planting row is treated. Variations of a few inches can cause complete loss of control. Use highest rates on peat soils. Restricted-use pesticide. 5-day reentry.
- Telone II at 9 to 12 gal/A on mineral soils, 25 gal/A on peat soils. Caution: Fumigation must be before done planting, so it is important that the planting row be directly over the fumigated soil if only the planting row is treated. Variations of a few inches can cause complete loss of control. Restricted-use pesticide. 5-day reentry.