Thanatephorus cucumeris (Cotton)

Scientific Name Thanatephorus cucumeris (Frank) Donk [teleomorph], Rhizoctonia solani [anamorph]
Synonyms Sclerotium irregulare [anamorph] Miyake, Rhizoctonia aderholdii [anamorph] Kolosch, Rhizoctonia microsclerotia [anamorph], Hypochnus solani [teleomorph] Prillieux & Delacroix, Hypochnus aderholdii [teleomorph] Kolosh., Corticium vagum [teleomorph] Berk. & Curt., Hypochnus cucumeris [teleomorph] Frank, Corticium areolatum [teleomorph], Hypochnus filamentosus [teleomorph] Pat., Corticium praticola [teleomorph] Kotila, Thanatephorus praticola [teleomorph] (Kotila) Flentje, Moniliopsis solani (Kühn) R. T. Moore, Moniliopsis aderholdii Ruhland, Ceratobasidium filamentosum (Pat.) Olive, Botryobasidium solani
Common Names English: Seedling disease, wirestem, boll rot; German: Wurzeltöterkrankheit, Weißhosigkeit, Blattfleckenkrankheit an Baumwolle; Spanish: Mustia hilachosa; French: Rhizoctone commun, Rhizoctone brun
Description Thanatephorus cucumeris is a basidiomyceteous fungus, the anamorph Rhizoctonia solani belongs to the Agonomycetales which do not produce conidia. R. solani is considered to be an aggregate species including different (host-specific) entities which may be differentiated by anastomosis grouping (AG); R. solani AG-4 and AG-13 cause diseases on cotton.

Biology

Damage

As part of a complex of pathogens including also species of the genera Pythium, Thielaviopsis and Fusarium, T. cucumeris can attack the seed before or at germination. The pathogen(s) causing seedling disease result(s) in pre- and post-emergence damping off. Early infection causes seed decay and pre- and post-emergence damping-off. Symptoms include seed decay and seedling decay before emergence, girdling of the emerged seedling stems, and root rot. The disease is associated with a soft, watery rot.

Later infection affects the stem cortex and may result in stem canker, wirestem and eyespot. Infected seedlings that emerge are pale, stunted, slower growing, and may die within a few days. Infections due to T. cucumeris are characterized by reddish brown, sunken lesions at or below ground level. Lesions enlarge, girdle the stem, and cause shrivelling (wirestems). Seedling disease results in uneven, slow-growing stands; in some years, replanting is necessary.

Cotton boll rot is also caused by a complex of soil-borne pathogens including Ascochyta gossypii, Glomerella gossypii, Fusarium spp., Botryosphaeria rhodina, Phytophthora spp., and T. cucumeris. Symptoms may vary depending on the organisms involved. First symptoms are small water-soaked spots on the outside of boll capsules or bracts. With abundant moisture, the spots enlarge and become sunken and brown or reddish brown. In later stages they may cover the entire boll. Final symptoms may be absent but complete destruction of seed and fibre will occur; dry bolls usually split open, exposing darkened cotton.

Lifecycle

T. cucumeris forms hyaline, multinucleate hyphae (Ø 8 - 12 µm) with a prominent dolipore septum. Young hyphae typically branch at 90 ° angles, with constrictions at the point of origin of hyphal branches. The formation of sclerotia, often on the plant surface, is stimulated by a sudden change in temperature or by brief flooding. They are irregularly hemispherical, 1 - 6 mm in diameter, but may coalesce to form larger structures. The sclerotia are extremely resistant and may remain infective for up to 21 months in dry soil. Survival in soil is influenced by soil factors and by soil micro-organisms. The fungus spreads by soil movement with water, and on contaminated tools and plant parts. Basidiospores are only rarely produced and are considered to be of minor importance in epidemiology.

Cause

The fungus is present in most soils, surviving as actively growing mycelium, resting mycelium or sclerotia, and has been also detected on cotton seeds.

Occurrence

Additional Crop Information

T. cucumeris has a very wide host range; even a specific anastomosis group can infect and damage plants from several families. AG-4 is regularly associated with Chenopodiaceae, Cruciferae, Pinaceae, Leguminosae. Malvaceae, and Solanaceae.

Agricultural Importance

In the USA crop losses to seedling disease was estimated at 3.1 % for the period 1995 to 1999. Loss data indicate little variability in disease impact over the last decades placing this complex as second most important disease of cotton only to nematodes. In other regions, losses may be significantly higher.

Control

Integrated Crop Management

Seedling disease occurs more frequently under cool, wet conditions and seems to be more prevalent on sandy, low-organic-matter soils. Other factors like planting too deep, poor seed bed conditions, compacted soil, nematode or insect infestations, and misapplication of soil herbicides may increase the problem. Seedling diseases tend to be more severe in reduced tillage situations and when beds are absent. Damage from boll rots is generally much greater during wet growing seasons. Rain and high relative humidity during ripening favor boll rot. As some pathogens enter the boll through wounds insect feeding on the boll should be controlled. The excessive use of nitrogen fertilizer which promotes rank growth should be avoided.

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