HPIPM:Onion Bacterial Streak Bulb Rot

Taxonomy

Domain	Bacteria
Phylum	Proteobacteria
Class	Gammaproteobacteria
Order	Pseudomonadales
Family	Pseudomonadaceae
Genus	Pseudomonas

Scientific Name

Pseudomonas viridiflava

Common Name

bacterial leaf streak and bulb rot

Author: Howard F. Schwartz and David H. Gent

Identification and Life Cycle

Bacterial streak and rot is caused by the bacterium Pseudomonas viridiflava. The bacterium efficiently colonizes the leaf surfaces of several weeds, crops, and onion. During cool, wet weather the bacteria infect leaves and progress downward into the bulb where they cause a decay of inner scales. The pathogen survives between onion crops epiphytically and pathogenically on weeds and other crops.

Plant Response and Damage

Bacterial streak symptoms initially appear as water-soaked, oval lesions and streaks on leaves. Leaf streaks become darkened with age; infected leaves later wither and collapse. Bulb rot symptoms are generally confined to inner scales, and appear as a reddish-brown discoloration and decay. Rotted scales are firmer than those caused by other bacteria, but infected bulbs can be colonized by other pathogens and secondary pathogens that can rot the bulb entirely. Yield losses of 20 to 80% have been reported in Georgia, but the disease occurs infrequently in Colorado and the High Plains.

Biological Control

Biological control strategies have not been developed for bacterial streak and bulb rot.

Cultural Control

Practice a three-year or longer crop rotation to non-hosts such as small grains. Avoid rotations with pulse crops such as alfalfa, soybean, and dry bean, as these crops may serve as an alternate host of the bacterial leaf streak 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. Harvest and store onions only after thorough curing in the field and the packaging shed. Avoid wounding or bruising bulbs during harvest and storage operations. Store bulbs at 32 to 36ºF with a relative humidity of 70% or less.

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 or when disease is first observed, 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.

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

Product List for Bacterial Streak and Bulb Rot:

Pesticide	Product per Acre	Application Frequency (days)	Remarks
Copper-based Fungicides—not all formulation listed
Champ DP	1.33 lb	7-10 days
Cuprofix	2.5-6 lb	7-10 days	Can be phytotoxic
Cuprofix MZ Disperss	3.5-5 lb	3-7 days	Maximum of 78.9 pounds per season; 7 day PHI
Kocide 2000	1.5 lb	7-10 days	Can cause phytotoxicity to leaves
Kocide 3000	0.75 lb	7-10 days	Can cause phytotoxicity to leaves
ManKocide	2.5	3-7 days	Maximum of 160 pounds per season; 7 day PHI
Nordox	2-4 lb	7-10 days
NuCop 50WP	2.0 lb	7-10 days
Top Cop with S	2-3 qt	7-10 days

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.