Cause Colletotrichum gloeosporioides (teleomorph = Glomerella cingulata) incites Twister disease, also known as anthracnose or seven curls. The fungus can be seedborne and survives on debris and alternate hosts. Rain and wind can spread disease, as well as insects and irrigation water. Infection is favored by high humidity and temperatures between 73°F and 86°F.
Symptoms Curling, twisting, and chlorosis of the onion leaves occurs, roots may be stunted, and there may be an abnormal elongation of the necks. Bulbs may appear slender and can decay rapidly when stored. In greenhouse transplant production, roots of young seedlings may exhibit a dark rot with spore masses, especially when root tips protrude from the growing medium. Disease may be patchy or wide-spread.
- Bury or flail infected crop residues after harvest to promote decomposition.
- Avoid overhead water and over-watering, especially in transplant production.
Chemical control Fungicide applications are rarely required for this disease as it is not normally a problem.
- Copper products are registered for other diseases of onion and would offer limited control of twister.
- Champ Formula 2 at 1.3 pints/A. 48-hr reentry.
- Cuprofix Ultra 40D at 1.25 to 3 lb/A on 7- to 10-day intervals. 48-hr reentry.
- Kocide 2000 at 1.5 lb/A or Kocide 3000 at 0.75 to 1.5 lb/A on 7- to 10-day intervals. 48-hr reentry.
- Nu Cop 50 WP at 1 lb/A on 7- to 10-day intervals. Do not apply within 1 day of harvest. 48-hr reentry. O
- Dithane DF Rainshield at 3 lb/A is registered for other diseases of dry bulb onions only and will control Twister as well. Do not apply within 7 days of harvest. Do not apply to exposed bulbs. 24-hr reentry.
References Ebenebe, A.C. 1980. Onion Twister Disease Caused by Glomerella cingulata in Northern Nigeria. Plant Dis. 64:1030-1032.
Schwartz, H.F., and Mohan, S.K. 1995. Compendium of Onion and Garlic Diseases. St. Paul, MN: APS Press.