HPIPM:Onion Bacterial Soft Rot

Taxonomy

Domain	Bacteria
Phylum	Proteobacteria
Class	Gammaproteobacteria
Order	Enterobacteriales
Family	Enterobacteriaceae
Genus	Pectobacterium
Species	Pectobacterium carotovorum

Scientific Name

Pectobacterium carotovorum ssp. carotovorum

Scientific Name Synonyms

Erwinia carotovora ssp. carotovora

Common Name

bacterial soft rot

Author: Howard F. Schwartz and David H. Gent

Identification and Life Cycle

Bacterial soft rot is commonly caused by Erwinia carotovora subsp. carotovora (synonym for Pectobacterium carotovorum subsp. carotovorum)or E. chrysanthemi (synonym for Dickeya chrysanthemi). The bacterial soft rot pathogens have very broad host ranges and can attack many vegetables including carrot, potato, cabbage, and lettuce. Infection generally requires a wound caused by heavy, wind-driven rain, hail, insects, or cut necks at harvest. Splashing water, aerosols, contaminated equipment and workers, and insects spread soft rot bacteria. Bacterial soft rot pathogens are commonly found in and easily disseminated by irrigation water. Erwinia spp. survive between onion crops in soil, crop debris, and pathogenically on other crops.

Plant Response and Damage

Bacterial soft rot symptoms often appear as a soft, watery rot of individual scales that may advance and rot the entire bulb. A foul-smelling viscous fluid oozes from the neck when infected bulbs are squeezed. In the field, the youngest leaves or the entire foliage of affected plants appear bleached and wilted. Yield losses can be significant in the field and storage.

Biological Control

No biological control strategies have been developed for bacterial soft rot.

Cultural Control

Practice a three-year or longer rotation to nonhosts such as small grains. Avoid reuse of irrigation tail water and overhead irrigation. Prevent bruising and wounding of plants and bulbs during field operations, harvest, and handling. Control onion maggots and other insects that can vector soft rot bacteria. Sever all roots during lifting to promote rapid drying of foliage and necks before topping. Cure bulbs with abundant ambient air until necks are completely dry before storing bulbs at 32 to 36ºF and 70% or less relative humidity.

Chemical Control

Copper bactericides provide some bacterial control in Colorado when applied before disease is observed. Sprays should be initiated two weeks before bulb initiation, and continued on a 5 to 10 day spray interval depending on weather conditions. Apply in a sufficient volume of water to ensure thorough coverage. Include a low rate of a non-ionic surfactant to further improve coverage.

Copper tolerant strains of the pathogens are common in the U.S. Tank mixing copper bactericides with a low rate of an EBDC fungicide such as maneb is essential for effective disease suppression. Tank-mixing coppers with zinc or iron can also enhance their activity.

Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.