Sclerotium rolfsii (Southern stem rot of peanut)

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Basidiomycota
Subphylum	Agaricomycotina
Class	Agaricomycetes
Subclass	Agaricomycetidae
Order	Agaricales
Family	Amylocorticiaceae
Genus	Agroathelia

Scientific Name

Agroathelia rolfsii

Scientific Name Synonyms

Sclerotium rolfsii

Athelia rolfsii

Common Name

southern blight

Author: Barbara Shew, North Carolina State University, Alan Henn, Mississippi State University

Scientific Name Synonym

Athelia rolfsii (Curzi) C.C. Tu & Kimbr. [Teleomorph]

Pathogen

Sclerotium rolfsii produces abundant white mycelium on infected plants and in culture. Advancing mycelium and colonies often grow in a distinctive fan-shaped pattern and the coarse hyphal strands may have a somewhat ropy appearance. In culture, mycelium appears smooth at first, but some cultures may develop aerial mycelia that cover all or part of the culture after a few days. The fungus produces at least two types of hypae, large diameter (5 to 9 µm) main branch hyphae and smaller diameter (2 to 4 µm) branch hyphae. Cells are hyaline with thin cell walls and sparse cross walls. Main branch hyphae may have clamp connections on each side of the septum (Figure 1). In agar plate culture, sclerotia are not formed until the mycelium covers the plate. In vitro or in vivo, sclerotia begin as small tufts of white mycelium that form spherical sclerotia 0.5 to 1.5 mm in diameter (Figure 2). Sclerotia darken as they mature, becoming tan to dark brown in color (Figure 3). Young sclerotia often exude droplets of clear to pale yellowish fluids. Mature sclerotia are hard, slightly pitted, and have a distinct rind. Although most sclerotia are spherical, some are slightly flattened or coalesce with others to form an irregular sclerotium. S. rolfsii does not form asexual fruiting structures or spores.

The teliomorph of S. rolfsii (Athelia rolfsii) is rarely observed on peanut or in culture. A. rolfsii produces basidia on an exposed hymenium and basidia produce four haploid basidiospores. The appressed hymenium develops in small, thin, irregular patches. The clavate basidia are 4 to 6 µm x 7 to 14 µm; basidiospores are hyline, 1.0 to 5 µm x 5 to 12 µm.

Hosts, Signs, and Symptoms

Sclerotium rolfsii causes disease on a wide variety of plants, including field, vegetable, fruit, and ornamental crops. It is a major pathogen of peanut wherever the crop is grown.

Signs of infection are common and include the presence of coarse white strands of mycelium growing in a fan-shaped pattern on lower stems, leaf litter, or soil (Figure -). Strands of coarse white mycelium may also be visible on the surface of pods and thick rhizomorph-like strands are sometimes seen on below-ground portions of the plant (Figure –). Occasionally, mycelium takes on a tufted appearance or forms sheaths around the main stem (Figure 4). In dry weather, strands or fans of mycelium are less apparent; instead, white to pale buff mycelium may be appressed to lower stems, looking like small patches of paint (Figure –). Later, tan to brown round sclerotia 0.5 to 1.5 mm in diameter may be present on the mycelium, lesions, or soil, or on litter near infected plants. The signs of S. rolfsii are diagnostic of southern stem rot, but damage can occur even when signs are not evident.

Early symptoms of southern stem rot include wilting or yellowing of individual branches or the main stem (Figure –). Stem lesions usually are formed near the crown of the plant, usually starting at points of contact with the soil or previously infected stems. Lesions are the color of a brown paper bag and may have a canker-like appearance. As the disease progresses, lesions may extend into the crown of the plant, killing it. Colonized stems and crowns are dark brown, brittle, and shredded (Figure 5). Brown, shredded pegs may rot without producing a pod, or pegs may separate from pods that have already formed, causing them to shed. Distinct lesions may be formed on pods during early stages of infection, but later pods are completely consumed with a dry brown rot (Figure –). Rotted pods are thin and brittle and seeds may be stained, rotted, or absent. Pod shedding and rot is a major cause of yield loss.

Ecology

S. rolfsii is a necrotrophic soil borne plant pathogen, killing plant tissues in advance of colonization by production of oxalic acid and cell-wall degrading enzymes. It survives as sclerotia in soil and as mycelium in crop debris. Sclerotia are known to survive several years in the absence of a host. The extremely broad host range of S. rolfsii also contributes to long-term survival between peanut crops. S. rolfsii thrives in highly aerobic environments and thus survives best near the soil surface. The light-textured, slightly acid soils favored for peanut production also are very favorable for growth and survival of S. rolfsii. Temperatures of 27 to 35°C are optimal for growth and sclerotia production. Germination of sclerotia is stimulated by the presence of volatile organic compounds generated from decay of leaf litter and other plant materials. The role of moisture in the initiation and development of stem rot epidemics is not clear, but disease generally is more severe in wetter climates and in irrigated fields than under dryer conditions. Disease often is first evident when the crop canopy begins to close (45 to 60 days after planting in the United States). Under favorable conditions, disease spreads by plant-to-plant contact and infection. Spread is promoted by typical production practices, in which stems of adjacent peanut plants are intermingled due to close within-row plant spacing. Infected plants thus tend to be found contiguously within rows.

Geographic Distribution

Sclerotium rolfsii is cosmopolitan in warm climates and is found in peanut production areas throughout the world. Southern stem rot is most serious in warm, humid locations and seasons.

Management

Cultural

S. rolfsii does not attack small grains, corn, sorghum, and some other grass species. Rotations of two years or more to a non-host crop helps to prevent build-up of inoculum and disease problems in peanut. Longer rotations are necessary to reduce disease once high levels of inoculum are established. Rotations with soybean, tobacco, melons, and vegetables should be avoided. Weed control must be maintained during rotations to prevent increase on susceptible species, which includes hundreds of dicots and species in several families of monocots.

Complete resistance to southern stem rot is not known in cultivated peanut. However, a few cultivars with good partial resistance are available and are very useful for disease management. Highly susceptible cultivars should be avoided, particularly in regions where disease is severe, in fields with a strong history of disease, and in irrigated fields.

Excess canopy development and over-irrigation promote disease development and should be avoided. Cultivation should be avoided during the season because it can promote disease by uncovering buried sclerotia, improving aeration, and by throwing infested soil onto plants. Deep plowing can be used to bury debris and sclerotia of S. rolfsii, but this is seldom practiced due to cost and erosion concerns.

Chemical

Several fungicides have good to excellent activity against southern stem rot. Some products are applied specifically to control S. rolfsii, but more commonly fungicides that control both stem rot and foliar diseases are used. Depending on the severity of disease in an area, the first fungicide application is made 45 to 75 days after planting, with applications at 45 to 60 days being most common. At least one additional application of a fungicide with activity against stem rot is recommended 14 to 28 days after the first application. In areas with severe stem rot problems, up to four applications at 2 to 4 week intervals may be needed. Foliar fungicide sprays must reach the lower stems and crowns of plants where S. rolfsii is most active. This can be accomplished by use of fungicides that redistribute readily on the plant surface; the use of surfactants or fungicide mixtures that cause fungicides to adhere to leaves should be avoided. Other tactics include application in high volumes of water, or spraying plants at night when the plant’s leaves are folded, exposing stems and crowns. Application of fungicides in-furrow or in banded sprays within 3 to 4 weeks after emergence also appears to enhance stem rot control.

Diagnostic procedures

S. rolfsii is easily diagnosed in humid weather from the abundant signs of the pathogen. Under dry conditions, signs may not be apparent on the above-ground portion of the plant. In those cases, signs may be present below ground on pods and pegs. When signs are absent, incubating symptomatic stems, pods, or seeds in a moist chamber for 24 to 48 hours usually results in abundant vegetative growth of the fungus and later, production of sclerotia (Figure 6). Incubation is most successful if the symptomatic tissue is placed in direct contact with a moistened paper towel. The fungus can readily be isolated on a variety of agar media with isolation and culture on PDA being the most common. Cultures grow rapidly and should be transferred before the advancing colony margin reaches the edge of a Petri plate. Cultures also can be obtained from surface-sterilized sclerotia. In spite of its vigorous growth, S. rolfsii frequently dies or becomes contaminated with parasites such as Trichoderma spp. in culture. Isolates are best maintained as dried sclerotia or as mycelium on dried colonized grain.

Occasionally, the peanut diseases caused by S. rolfsii and Sclerotinia minor can be confused since both fungi are stem pathogens and both produce visible white mycelium. However, the water-soaking, bleaching, and extensive stem shredding symptoms typical of Sclerotinia blight are not seen with S. rolfsii infections. Sclerotia are diagnostic if present; sclerotia of S. minor are irregular and black, compared to the spherical tan to brown sclerotia of S. rolfsii. These fungi are readily distinguished after incubation in a moist chamber or in culture on agar media, particularly once sclerotia are produced.

Stem lesions can also be confused with those caused by R. solani if signs are absent. S. rolfsii and R. solani can be distinguished by microscopic examination hyphae present in infected tissue, by production of signs after incubation in a moist chamber, or by culturing on agar media.

Acknowledgements

• National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30708 as part of "Diagnostic Image Series Development for Supporting IPM in the Southern Region" (USDA-NIFA-RIPM-003351)