Potential for Management of Sclerotinia Blight of Peanut (Arachis hypogaea L.) caused by Sclerotinia minor with the Biological Control Agent Coniothyrium minitans

Abstract

Sclerotinia blight of peanut (Arachis hypogaea L.), caused by Sclerotinia minor (Jagger) Kohn, is an important disease in North Carolina and Virginia. Sclerotia are the main overwintering propagules of S. minor and serve as the primary source of inoculum for Sclerotinia blight. The effectiveness of the fungal mycoparasite Coniothyrium minitans, which is capable of colonizing sclerotia of Sclerotinia spp., was evaluated in a 5-year field study and in eight short-term field studies in northeastern North Carolina. Control of Sclerotinia blight was highest when C. minitans was applied for 3 consecutive years. However, application of C. minitans for 1 or 2 years also reduced disease in the long-term study. A single application of C. minitans was less effective when applied 4 to 6 months prior to planting and sclerotia numbers were only reduced in two of the eight short-term field studies. Sclerotia used as baits placed in the long-term field study as well as the sclerotia isolated from soil were infected by C. minitans, and the number of sclerotia was reduced where C. minitans was applied. Moderate resistance in the cultivar Perry and application of the fungicide fluazinam provided adequate control of Sclerotinia blight in all plots.

The integration of C. minitans with current peanut management practices is needed for successful biological control of Sclerotinia blight. Laboratory experiments evaluated the effects of nine pesticides commonly used in peanut production on mycelial growth, conidia germination, and mycoparasitic activity of C. minitans on sclerotia of S. minor. The commercial formulations of the fungicides azoxystrobin, chlorothalonil, fluazinam, pyraclostrobin, and tebuconazole, and the herbicide flumioxazin reduced mycelial growth and conidia germination of C. minitans. Eight of nine pesticides, azoxystrobin, chlorothalonil, fluazinam, pyraclostrobin, tebuconazole, diclosulam, flumioxazin, and pendimethalin applied to soil plates reduced but did not inhibit the mycoparasitic activity of C. minitans on sclerotia of S. minor. Temperature and moisture effects on mycoparasitism were also evaluated to determine optimum conditions for infection of sclerotia of S. minor by C. minitans. Optimum temperatures for infection of sclerotia of S. minor by C. minitans ranged from 14 to 22°C and soil moisture –0.33 to –1 kPa x 10². These results indicate that C. minitans should remain active throughout most of the year in North Carolina, except during the hot summer months of June, July and August.

Soil fauna such as collembola may aid in the reduction of sclerotia through direct predation and the movement of inocula of mycoparasites from infected to noninfected sclerotia. Collembola diversity and abundance were compared in four peanut fields. The most prevalent collembola families were Isotomidae, Smithurididae, Poduridae, and Hypogastruridae, with Isotomidae isolated most frequently from all sites. Abundance and diversity of collembola increased from August to October with sampling date and location having the greatest effect on the composition of the population. Determining the abundance and diversity of collembola in the field can help increase our understanding of the soil community structure.

These studies show that C. minitans is able to persist and infect sclerotia of S. minor in peanut fields of North Carolina when applied in the fall or early winter across crop residue and incorporated into the top layers of the soil. C. minitans will not eradicate infestations by S. minor, but over time has the potential to reduce inoculum levels and ultimately decrease the incidence of the disease.