By C. M. Ocamb and D. H. Gent
Cause Fusarium oxysporum f. sp. cepae is a soilborne fungus that can affect onion and other Allium spp. The fungus survives a long time in soil. Infection often is associated with pink root, maggots, or other insect injury.
Symptoms Bulbs may become infected at any point during their time in the field. Yellowing and browning of leaves begins at the tip and moves down. Plants exhibit weak growth and may wilt. Red-brown rot appears where roots are attached to the basal plate. Rot and discoloration usually affect the entire base and up into the bulb scales; affected tissue appears brown and watery when bulbs are cut open. Sometimes, a white moldy growth develops on the stem plate or between affected scales. Bulbs may appear normal at harvest, but rot may progress in storage.
- Plant resistant varieties. In a Malheur County, OR test, these onion varieties had less basal rot: 'Golden Cascade', 'Cima', 'Oro Grande', 'Valient', and 'Cashe'. Other onion varieties described as tolerant but not tested in Oregon are 'Bronze Reserve', 'North Star', 'Sassy Brassy', and 'Sentinel'.
- Use 4-year or longer crop rotations.
- Protect plants from insect, fertilizer, or other injury.
- Storing bulbs at 39°F minimizes postharvest losses.
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 done 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 rate 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 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.
Biological control Efficacy unknown in the Pacific Northwest.
- Amplitude at 3 to 4 quarts/A as a soil drench on 10- to 21-day intervals. Preharvest interval is 0 days. 4-hr reentry. O
- Prestop at 1.4 to 14 oz/10 gal water (0.1% to 1.0% suspension) as a soil drench or incorporation into growing medium. 0-hr reentry. O
- Stargus at 6 to 8 fl oz per 1,000 ft row as an in-furrow treatment, or 3 to 4 quarts/A as a soil drench (drip or chemigation) on 10- to 21-day intervals. Preharvest interval is 0 days. 4-hr reentry. O
Reference Schwartz, H.F., and Mohan, S.K. 1995. Compendium of Onion and Garlic Diseases. St. Paul, MN: APS Press.