Onion

by Sherrie Smith and Jason Pavel

Onion White Rot – Sclerotium cepivorum

Photo by Sherrie Smith, University of Arkansas Cooperative Extension

Typically, onions are set out in February and March in Arkansas. Although easy to grow in most locations, on ground with a history of White Rot losses may reach 100%. All members of the Allium family are susceptible to White Rot, caused by Sclerotium cepivorum, including chives, shallots, leeks, onion, and garlic. Crop losses can be severe in fields with a history of the disease.

Infected plants are usually stunted with yellowed foliage. White fluffy mycelial growth on the stem plate extends around the base of the bulb, moving up the bulb and inward through the storage leaves, causing a soft rot. Small, black, poppy seed-sized sclerotia form in the dying tissues. The sclerotia can remain dormant in the soil for up to 15 years until the roots of host plants begin to grow nearby. Sclerotia then germinate, and the mycelia typically grow up to several inches through the soil to attack the roots and bulb of the plant. However, sclerotia have been known to cause bulb decay when located as deep as 12 inches below the bulbs. Sclerotia can be spread throughout a planting area by flood water, equipment, or on plant material. This is a very difficult disease to control. Fungicides provide only marginal control when inoculum levels are high, and conditions are conducive for disease  development.  

Rovral  75WG  and Folicur 3.6F are labeled for use in commercial fields. Wider spacing between plants can slow the spread of White rot. Homeowners with small plots may consider replacing the soil altogether. Soil solarization may have some benefits. The area to be solarized should be raked clean, thoroughly wetted, and clear plastic placed over the area. The plastic should be left in place for 4-6 weeks. Warm season flooding of the soil has been found to greatly reduce the number of sclerotia as this is a cool season pathogen. Boots and tools should be cleaned to prevent accidently moving the pathogen to new areas. Gardeners who grow onions in infected soils have less infection generally by planting seed instead of onion sets. This is because the seedlings have a smaller root mass, thus fewer chemical signals, at the time temperatures are optimal for disease development.

Onion White Rot – Sclerotium cepivorum

Photo by Sherrie Smith, University of Arkansas Cooperative Extension

Take Aways

• Wider plant spacing
• Soil solarization
• Clean tools, footwear, and machinery
• Plant seed instead of sets in fields with a history of infection

This work is supported by the Crop Protection and Pest Management Program [grant no. 2017-70006- 27279/project accession no. 1013890] from the USDA National Institute of Food and Agriculture.