White rot is caused by the soil-borne fungus Sclerotium cepivorum. White rot has not been identified in Colorado, and likely would not be of concern to onion production because spring and summer temperatures are not favorable to the pathogen. White rot is favored by cool weather (below 68°F). Only Allium spp. such as onion, leek, and shallot are attacked. Once the pathogen is introduced into a field, it is very difficult to grow onions successfully. Sclerotia allow the pathogen to survive for more than 15 years in soil until susceptible host root exudates stimulate them to germinate. Sclerotia are spread within and between fields by cultivation equipment, irrigation water, on infected transplants, grazing animals, and movement of infested soil.
White rot appears on isolated plants, usually at bulb formation. However, young plants also may be affected. Symptoms become obvious as older leaves yellow, wilt, and fall to the ground. A semi-watery soft rot gradually destroys the bulb and roots. The pathogen can continue to develop and spread between bulbs in storage. Affected plant parts are covered with a thick white mycelial mat bearing numerous small (0.03 inch diameter) black sclerotia.
Biological controls have not been developed for white rot.
Practice crop rotations of at least five years. Only high quality transplants free from the white rot pathogen should be planted. Select fields with adequate and promote thorough drainage. Avoid over-wintering onion production. Thoroughly wash equipment to prevent moving contaminated soil into new fields.
Planting time treatments can reduce white rot losses, but white rot occurs so infrequently in the High Plains that chemical controls are not necessary.
Product List for White Rot:
Pesticide |
Rate per acre |
Application Frequency (days) |
Remarks |
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Azoxystrobin |
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Amistar |
2-4 oz |
7-14 days |
Do not make more than 2 consecutive applications before rotating to a fungicide with a different mode of action; 0 day PHI |
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Thiophanate-Methyl |
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Topsin M 70W |
0.33-0.5 lb/1000 ft |
In furrow drench at planting |
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2,6-Dichloro-4-nitroaniline |
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Botran 75W |
5.33 lb |
One application per season |
Do not rotate with spinach |
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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. |
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Categories: Onion, Disease, White Rot
Date: 04/01/2007