By D. H. Gent and C. M. Ocamb
Cause Several bacteria and a yeast produce bulb rots in onion. These include bacterial soft rots caused by Pectobacterium carotovorum subsp. carotovorum (syn. Erwinia carotovora subsp. carotovora) and Erwinia chrysanthemi (syn. = Dickeya chrysanthemi), slippery skin caused by Burkholderia gladioli pv. alliicola (syn. = Pseudomonas gladioli pv. alliicola), sour skin caused by Burkholderia cepacia (syn. = Pseudomonas cepacia), Enterobacter bulb decay caused by Enterobacter cloaca, and yeast soft rot caused by Kluyveromyces marxianus var. marxianus. All these organisms are thought to overwinter in plant debris in soil or cull piles. All can enter the onion through wounds in the neck or bulb. Onion maggot activity can introduce and spread soft rot in the bulb. Warm, rainy weather, heavy irrigation, or flooded fields favor the diseases.
Symptoms Soft rot symptoms include fleshy scale tissues become water soaked and pale yellow to light brown, then become soft as the rot progresses. The entire bulb may break down, and a watery, foul-smelling viscous liquid may ooze from the neck when squeezed. If infected in the field, leaves will wilt and turn white.
Foliar symptoms of slippery and sour skin include a canker phase in which the bacteria generally cause a necrotic bleaching and collapse of one or two leaves. The pathogens progress down the leaf into the bulb, where they cause a pale yellow to light brown decay or breakdown. However, there may be no exterior symptom initially with slippery skin except a softened neck. When cut, one or two inner fleshy scales are seen to be soft and have a water-soaked appearance. Rot progresses from the top of the scales down without spreading to adjacent scales. The onion core may pop out when the base of the bulb is pressed. The entire internal bulb may rot, dry out, and shrivel.
Sour skin symptoms are similar to slippery skin, but affected scales are yellowish to light brown. Other Allium spp. may be affected. Enterobacter bulb decay symptoms are generally absent in the field, but appear after one to three months in storage. Internal scales have a light to dark brown discoloration and decay; externally bulbs appear healthy. The incidence of affected bulbs is generally less than 5%.
Yeast soft rot is similar to both soft rot and slippery skin.
- Practice a 3-year or longer crop rotation. However, rotations of corn may increase populations of B. cepacia in the soil, and are not advised.
- Avoid reuse of irrigation water and overhead irrigation.
- Allow tops to mature and dry fully before harvesting.
- Reduce mechanical injuries during cultivation and weeding.
- Avoid bruising bulbs during harvest and handling.
- Promptly and thoroughly cure onions before storing.
- Store at 32°F and less than 70% humidity.
- Some control can be obtained by using water from wells rather than from ditches.
- LifeGard WG (Group P6) at 1 to 4.5 oz/A on 7- to 14-day intervals for activating plant resistance. Refer to label for appropriate rate per application volume. 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.