Monday, November 30, 2009

Biological Control and White Mold of Soybean

With the wide-spread reports of White mold (or Sclerotinia stem rot, SSR) this year in Wisconsin and across the region, we have been fielding many questions about control options. In particular, many of these questions have been about Contans WG. In this blog, Angie Peltier (Postdoctoral Research Associate in Plant Pathology) and I try to provide information that will help you understand more what biological control for white mold entails.

What is Contans WG?

Contans WG (SipcamAdvan; Durham, NC) is a commercial biocontrol agent and is a proprietary powder formulation that contains the fungus Coniothyrium minitans. Contans WG has been labeled for use in both conventional and organic soybean.

C. minitans was first described in California in 1947, and it is now known to have a world-wide distribution. The host range of C. minitans includes important plant pathogens such as Sclerotinia sclerotiorum, S. trifoliorum, S. minor, and some strains of Botrytis cinerea, B. fabae, and Sclerotium cepivorum (Turner and Tribe, 1976).

How does it work?

The fungus that causes white mold (Sclerotinia sclerotiorum) produces long-lived survival structures called sclerotia that many say resemble rat droppings. Sclerotia are important in the life cycle of Sclerotinia, allowing the fungus to survive in the soil until conditions are favorable for the disease cycle to begin: Upon canopy closure and during periods of cool and wet weather, sclerotia germinate to produce mushroom-like fruiting structures called apothecia. Apothecia produce ascospores that are wind-disseminated. If during a period of leaf wetness ascospores land on dying soybean flower tissue, they can use this food source to gain entry into susceptible soybean plants and cause disease. Many apothecia can emerge from one sclerotium, making each sclerotium an important inoculum source.

When a water suspension of Contans WG is incorporated into the soil, viable spores of C. minitans that come into direct contact with sclerotia, germinate, and then gain entry into the sclerotium by a chemical etching process, eventually causing the disintegration of the sclerotium. Degraded sclerotia cannot produce apothecia, and therefore, these sclerotia produce no inoculum to initiate infection of soybean.

What factors need to be considered for use?

Application timing. Application timing can influence efficacy. While the Contans WG label does not specifically indicate optimal application timing, the manufacturer’s “Directions for Use” (Prophyta, Germany) suggests a minimum of 3 months between application and when SSR disease is likely to develop, to allow adequate time for colonization of sclerotia. Some research suggests that there is a lag time between application and disease suppression. Important consideration: in fields with a high sclerotia load in the soil, enough sclerotia may survive to still cause a substantial level of disease.

Tillage. TheDirections for Use” states that after soil application, the product “should be incorporated as thoroughly and uniformly as possible to a depth of 5 to 20 cm”. An important consideration is also to avoid additional tillage after incorporation, as tillage can bring uncolonized sclerotia to the soil surface.

Some unanswered research questions:

Much of the published data has focused on both aerial and soil application of C. minitans (not Contans WG). Efficacy of C. minitans application has been shown through the reduction of disease in field trials for other crops susceptible to S. sclerotiorum, including dry and snap bean, sunflower and oilseed rape. To date, no research has been published on the use of Contans WG or C. minitans to control white mold in soybean. Therefore, further field research is needed to understand the efficacy of C. minitans in soybean.

In addition, the label directions state that the product must be incorporated into the soil. We are currently examining how efficacy of Contans WG may be impacted under no-till situations.

Lastly, although this issue has not yet been adequately researched, there is some indication that soil type may affect parasitism of sclerotia by C. minitans, which may reduce efficacy in some production situations.