HPIPM:Onion Xanthomonas Leaf Blight

Taxonomy

Domain	Bacteria
Phylum	Proteobacteria
Class	Gammaproteobacteria
Order	Xanthomonadales
Family	Xanthomonadaceae
Genus	Xanthomonas
Species	Xanthomonas axonopodis

Scientific Name

Xanthomonas axonopodis pv. allii

Common Name

Xanthomonas leaf blight of onion

Author: Howard F. Schwartz and David H. Gent

Identification and Life Cycle

Xanthomonas leaf blight (also called onion bacterial blight or bacterial leaf blight), is caused by the bacterium Xanthomonas axonopodis pv. allii. The disease was first detected in the continental U.S. in Colorado in 1994, but also has been reported from Texas, California, Hawaii, Georgia and several other tropical and sub-tropical onion production regions of the world. X. axonopodis pv. allii is also pathogenic to other allium crops such as garlic, leek, chive, and Welsh onion. Strains of X. axonopodis pv. allii from Barbados are reportedly pathogenic to several pulse crops, including soybean and snap bean. However, strains from other onion producing regions of the world are nonpathogenic or only weakly virulent to pulse crops.

Disease occurs when bacteria are blown or splashed onto leaves, where they multiply to form large populations (greater than 10,000 colony forming units per leaf) in the presence of dew or other moisture, and infect through natural openings or wounds. Leaf abrasion by wind and windblown sand may favor infection. In Colorado, disease is associated with moderate to high temperatures and rainfall at bulb initiation and continuing through bulb development. Early season weather conditions do not appear to be significantly associated with disease appearance or severity in Colorado, but may be important when short-day onion varieties are grown under tropical or sub-tropical conditions. The pathogen can be disseminated between fields by irrigation water and contaminated workers and equipment. The pathogen survives between onion crops epiphytically and pathogenically on weeds, volunteer onion, and leguminous hosts such as alfalfa, in crop debris, and on contaminated seed.

Plant Response and Damage

Xanthomonas leaf blight symptoms begin as small, chlorotic spots or lens-shaped lesions with water-soaked margins. Lesions enlarge into long chlorotic streaks and large greasy, water-soaked areas that are most prominent on the flattened side of older leaves. Tip death and blighting of leaves reduces the plants photosynthetic area, resulting in a reduction in bulb size. In some onion production regions, plants may also be killed before bulb initiation. Yield losses vary depending on the time of infection, but range from 20 to 100% under disease-favorable conditions. A bulb rot does not occur.

Biological Control

Bacteriophage mixtures specific to the Xanthomonas leaf blight pathogen have been developed and appear promising for disease control, but must be applied at least twice weekly to be effective.

Cultural Control

Plant only pathogen-free seed. Hot water seed treatments may reduce seed contamination, but can also reduce germination. Practice a three-year or longer crop rotation to non-hosts such as small grains. Avoid rotations with pulse crops such as lentil, soybean, and dry bean, as these crops may serve as a reservoir of the Xanthomonas leaf blight pathogen. Deeply bury crop residues after harvest to reduce pathogen survival. Eliminate weeds and volunteer onion in and around fields. If possible, avoid overhead irrigation and reuse of irrigation tail water. Drip irrigation may reduce disease severity compared to furrow irrigation. Apply adequate but not excessive nitrogen fertilizer, especially post-bulbing or after storm-damage. Avoid working in fields when the foliage is wet, which can easily spread bacteria throughout the field. Plant wider rows in the direction of prevailing winds to increase air movement in the crop canopy and decrease periods of leaf wetness. Varieties vary widely in their reaction to Xanthomonas leaf blight, and highly susceptible varieties should be avoided. White and red market class varieties tend to be less susceptible or more tolerant.

Chemical Control

Copper bactericides provide effective disease 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, with a low rate of a non-ionic surfactant, to ensure thorough coverage. Disease forecast models have been developed in Colorado that may improve spray timing and effectiveness.

Copper tolerant strains of the pathogen have not been detected in the U.S., but may appear with continued reliance upon copper bactericides. If copper tolerant strains are present, tank mixing with a low rate of an EBDC fungicide such as maneb can enhance 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.