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Mailing Address:
University of Idaho
29603 U of I Lane
Parma, ID 83660-6699

Phone: 208-722-6701

Fax: 208-722-6708

Web: uidaho.edu/cals/parma

Directions

Mailing Address:
University of Idaho
1776 Science Center Drive, Suite 205
Idaho Falls, ID 83402

Phone: 208-529-8376

Fax: 208-522-2954

Web: uidaho.edu/cals/idaho-falls

Directions

Diseases

Onions are an important part of Idaho’s agricultural economy. The Treasure Valley of southwest Idaho and eastern Oregon produces nearly 30% of the nation’s onion crop. While growing conditions are ideal and considerable effort is made to produce high quality onions, diseases can still present in the crop. Below are photos and descriptions of common onion pathogens that lead to production problems.

Bacterial leaf blight (also referred to as bacterial stalk and leaf necrosis),

Leaf symptoms (naturally infected)

Pantoea agglomerans is a bacterial pathogen that causes bulb rot and leaf blight in onions. Bacterial leaf blight can present very similarly to leaf blight caused by IYSV.

 

Pantoea agglomerans – Leaf symptoms (naturally infected)

 

Pantoea agglomerans – Leaf symptoms (naturally infected)

 

Pantoea agglomerans – Leaf symptoms (naturally infected)

Bacterial leaf blight/bacterial leaf spot,

Leaf symptoms (naturally infected)

Pseudomonas spp. causes bacterial leaf blight and leaf spot on onion leaves. These leaf spots start white and transition to a brown color.

 

Pseudomonas spp. – Leaf symptoms (naturally infected)

Center rot,

Bulb symptoms (naturally infected)

Pantoea agglomerans is a bacterial pathogen that causes bulb rot and leaf blight in onions. When present in the bulb, this bacteria typically causes the inner scales to turn a watery yellow to brown color.

 

Pantoea agglomerans – Bulb symptoms (naturally infected)

 

Pantoea agglomerans – Bulb symptoms (naturally infected)

Damping off,

Bulb symptoms (naturally infected)

Pythium intermedium causes damping off and root rots in onions. Symptoms are water-soaked lesions on the roots and lower scales. Roots and older scales on seedlings may turn black. Older plants can exhibit stunting, yellowing and wilting of leaves.

 

Pythium intermedium – Bulb symptoms (naturally infected)

Enterobacter spp.,

Leaf symptoms (naturally infected)

Enterobacter spp. can infect onion leaves as well. This presents as light brown to dark brown discoloration and soft rot, eventually leading to collapsed, necrotic leaves.

 

Enterobacter spp. – Leaf symptoms (naturally infected)

 

Enterobacter spp. – Leaf symptoms (naturally infected)

Iris Yellow Spot Virus (IYSV),

Leaf symptoms (naturally infected)

IYSV is spread by thrips. Lesions start out at thrip feeding points. These start as light and dark rings or diamond shaped lesions. These infected areas can grow into each other and girdle the leaves. This ultimately causes the leaves to collapse, brown and die.

 

Iris Yellow Spot Virus (IYSV) - Leaf symptoms (naturally infected)

Neck rot,

Bulb symptoms (inoculated)

Botrytis allii causes neck rot in onions. This occurs most obviously in storage. The disease presents initially as water-soaked, yellowed scale discoloration that moves down from the neck. Grey mold forms between the scales as they break down, later leading to black sclerotia. An associated strain, Botrytis aclada, can also cause onion neck rot. Symptoms caused by Botrytis aclada are not pictured here.

Botrytis allii – Bulb symptoms (inoculated)

Pantoea agglomerans,

Bulb skin symptoms (naturally infected)

Pantoea agglomerans is a bacterial pathogen that causes bulb rot and leaf blight in onions. When present in the bulb, this bacteria typically causes the inner scales to turn a watery yellow to brown color. Another symptom of this pathogen are water-soaked lesions.

 

Pantoea agglomerans  - Bulb skin symptoms (naturally infected)

 

Pantoea agglomerans  - Bulb skin symptoms (naturally infected)

Pythium intermedium,

Bulb symptoms (naturally infected)

Pythium intermedium causes damping off and root rots in onions. Symptoms are water-soaked lesions on the roots and lower scales. Roots and older scales on seedlings may turn black. Older plants can exhibit stunting, yellowing and wilting of leaves.

 

Pythium intermedium - Bulb symptoms (naturally infected)

Rahnella sp.,

Bulb symptoms (inoculated)

Rahnella sp. is an onion associated bacteria. It can cause soft rot in bulbs. Scales of infected bulbs begin showing symptoms of watery and/or yellow discoloration. This progresses into brown to dark brown discoloration and collapsed or shrunken scales.

 

Rahnella spp.  – Bulb symptoms (inoculated)

 

Rahnella spp.  – Bulb symptoms (inoculated)

Soft rot,

Bulb symptoms (naturally infected)

Enterobacter bulb decay forms after onions have been stored. The symptoms can appear as water-soaked scales (initial photo). Symptoms also present as light brown to dark brown scales. Enterobacter spp. can also cause scales to shrink or collapse (bottom photo).

 

Enterobacter spp. – Bulb symptoms (naturally infected)

Soft rot,

Bulb symptoms (naturally infected)

Rahnella sp. is an onion associated bacteria. It can cause soft rot in bulbs. Scales of infected bulbs begin showing symptoms of watery and/or yellow discoloration. This progresses into brown to dark brown discoloration and collapsed or shrunken scales. The very rotten bulb may be affected by multiple pathogens, not only Rahnella spp.

Stemphylium leaf blight (SLB),

Leaf symptoms (naturally infected)

Stemphylium vesicarium causes two types of onion leaf blight, brown oval lesions and yellow mottle lesions. These brown lesions can lead to the development of the yellow lesions.

 

Stemphylium vesicarium – Leaf symptoms (naturally infected)

 

Stemphylium vesicarium – Leaf symptoms (naturally infected)

Stemphylium leaf blight (SLB),

Leaf symptoms (naturally infected)

Stemphylium vesicarium causes two types of onion leaf blight, brown oval lesions and yellow mottle lesions. These brown lesions can lead to the development of the yellow lesions.

 

Stemphylium vesicarium - Leaf symptoms (naturally infected)

 

Stemphylium vesicarium - Leaf symptoms (naturally infected)

 

Stemphylium vesicarium - Leaf symptoms (naturally infected)

Stemphylium leaf blight (SLB),

Leaf symptoms (naturally infected)

Stemphylium vesicarium causes two types of onion leaf blight, brown oval lesions and yellow mottle lesions. These brown lesions can lead to the development of the yellow lesions.

 

Stemphylium vesicarium - Leaf symptoms (naturally infected)

Stemphylium leaf blight (SLB),

Leaf symptoms (naturally infected)

Stemphylium vesicarium causes two types of onion leaf blight, brown oval lesions and yellow mottle lesions. These brown lesions can lead to the development of the yellow lesions.

 

Stemphylium vesicarium - Leaf symptoms (naturally infected)

 

Stemphylium vesicarium - Leaf symptoms (naturally infected)

Stemphylium leaf blight (SLB),

Leaf symptoms (naturally infected)

Stemphylium vesicarium causes two types of onion leaf blight, brown oval lesions and yellow mottle lesions. These brown lesions can lead to the development of the yellow lesions.

 

Stemphylium vesicarium - Leaf symptoms (naturally infected)

 

Stemphylium vesicarium - Leaf symptoms (naturally infected)

Stemphylium leaf blight (SLB),

Leaf symptoms (naturally infected)

Stemphylium vesicarium causes two types of onion leaf blight, brown oval lesions and yellow mottle lesions. These brown lesions can lead to the development of the yellow lesions.

 

Stemphylium vesicarium - Leaf symptoms (naturally infected)

Storage rot/soft rot,

Bulb symptoms (naturally infected)

Pseudomonas spp. presents in the neck or outside scales initially and spreads to the inner scales. The pathogen presents as water-soaked lesions, usually with a yellowish-brown color. In some onions the lesions appear grey. Onions fertilized too much and onions with higher nitrogen content are more susceptible to Pseudomonas spp.

 

Pseudomonas spp. – Bulb symptoms (naturally infected)

 

Pseudomonas spp. – Bulb symptoms (naturally infected)

 

Pseudomonas spp. – Bulb symptoms (naturally infected)

 

Pseudomonas spp. – Bulb symptoms (naturally infected)

Contact

Mailing Address:
University of Idaho
29603 U of I Lane
Parma, ID 83660-6699

Phone: 208-722-6701

Fax: 208-722-6708

Web: uidaho.edu/cals/parma

Directions

Mailing Address:
University of Idaho
1776 Science Center Drive, Suite 205
Idaho Falls, ID 83402

Phone: 208-529-8376

Fax: 208-522-2954

Web: uidaho.edu/cals/idaho-falls

Directions