In Search of the Next DADS

Above-ground symptoms of white rot (right) compared to healthy onions (left).Above-ground symptoms of white rot (right) compared to healthy onions (left).

Continuing Research on Sclerotia Germination Stimulants for White Rot Control

 By Jeremiah Dung, Oregon State University & Robert Wilson, University of California


With promising results in past and recent trials evaluating the use of fungicides and sclerotia germination stimulants to control white rot, researchers are continuing their efforts to develop an IPM strategy that will allow growers to produce a profitable crop in infested fields.

White rot, caused by the fungus Sclerotium cepivorum, is a major disease of onion and garlic (Fig. 1). The fungus is spread by small sclerotia produced on decayed bulbs and roots (Fig. 2), and as few as one sclerotium per quart of soil can result in significant crop losses. Control of white rot is difficult because sclerotia can survive in fields for over 20 years, remaining dormant in the absence of Allium hosts.

Today, thousands of acres in California are infested with white rot. White rot is also a major problem for onion and garlic seed and bulb production in the Pacific Northwest, and the Allium industry in the western U.S. is threatened by white rot. White rot-resistant cultivars of garlic and onion are not currently available and growers lack effective control options.


Fungicide Efficacy

Several University of California (UC) experiments over the last decade evaluated the efficacy and crop safety of fungicides for white rot control in onions. The majority of the studies evaluated different fungicides, fungicide rates and fungicide application times.

The fungicide tebuconazole was the most effective active ingredient for suppression of white rot. Penthiopyrad provided similar or slightly less suppression of white rot compared to tebuconazole. Tank mixing both fungicides had a slight additive effect on white rot suppression compared to using either product individually in multiple UC trials. The most effective fungicide application method was in-furrow at planting, and results consistently showed fungicides increase onion yield and decrease the incidence and severity of white rot on harvested onion bulbs. Unfortunately, fungicides alone did not consistently suppress white rot at a level that prevents economic loss, especially in soils with high sclerotia populations.


Potential of Sclerotia Germination Stimulants

Sclerotia germination stimulants offer great potential for managing white rot. White rot is a disease limited to Allium crops because sclerotia germinate only in response to sulfur-containing compounds released from Allium roots. If these compounds can be applied to the soil in the absence of an Allium crop, the sclerotia will germinate (Fig. 3), exhaust their energy reserves without a host and die.

Soil treatments with diallyl-disulfide (DADS) were shown to reduce sclerotia populations by over 90 percent, but unfortunately the small number of remaining sclerotia was often still enough to cause unacceptable disease levels at harvest.

A commercial product containing DADS was registered for use as a sclerotia germination stimulant in 1992 but is no longer available for various reasons, including high product cost and unreliability in satisfactory disease control. Research using natural or synthetic Allium compounds as sclerotia germination stimulants has become a high priority since DADS is no longer commercially available.


Two-Prong Approach

Fungicides and sclerotia germination stimulants used individually have not reduced white rot symptoms to a point that a profitable crop (less than 15 percent bulbs showing symptoms) can consistently be produced in infested soil, especially if soil inoculum densities are high. For this reason, it was decided to test a two-prong approach using germination stimulants to reduce soil inoculum density the year before growing onions and a fungicide applied in-furrow when planting the onion crop.

Results from several years of UC experiments at Tulelake’s Intermountain Research and Extension Center were quite promising, often resulting in significant reductions in white rot symptoms on onion bulbs treated with DADS and tebuconazole compared to the non-treated control. Additionally, marketable yields were often increased to levels (24 tons per acre) that would be acceptable to many growers, especially in white rot infested ground.


Research Underway

Collaborative research projects between UC and Oregon State University (OSU) are currently underway to evaluate the efficacy and feasibility of using Allium byproducts as white rot sclerotia germination stimulants. UC’s Rob Wilson and Tom Turini and OSU’s Jeremiah Dung are working together to test garlic juice, garlic oil and other potential germination stimulants at rates much greater than previous tests to determine if high rates can provide similar efficacy compared to DADS. We are also evaluating new fungicides for their ability to suppress white rot.

Concurrently, OSU’s Michael Qian is leading the effort to analyze and screen sulfur compounds present in different Allium byproducts, identify potentially new germination stimulants and establish dose-response relationships.

Our long-term goal is to develop an IPM strategy for managing white rot in onions and garlic that will allow the production of Allium crops in fields already infested with white rot. These projects are funded by the California Garlic and Onion Research Advisory Board and the California Department of Pesticide Regulation.

Originally published in Onion World May/June 2017