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Cotton root rot

Scientific Name
Phymatotrichopsis omnivora
Scientific Author
(Duggar) Hennebert

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Diagnostic Images

Hyphal strand of Phymatotrichopsis omnivora.
J.R. Liberato DPI&F
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Diagnostic Images (3)
Diagnostic Notes
When deep-seated sclerotia contact a descending cotton root, the fungus grows as strands toward the soil surface, entwining the root. Near the soil line, the fungus changes to a floccose growth. The upper root periderm is killed and the pathogen colonizes the inner root occluding the xylem. Foliar symptoms usually appear at time of flowering as a slight bronzing of leaves followed by a rapid wilt. Plants apparently healthy wilt and die within 2-3 days. Leaves become desiccated but do not fall from the plant. Roots are covered by fungal strands extending to the soil surface (Lyda 1978, Rush et al. 1985). Mycelial strands are most obvious on the roots 5-10 cm below the soil surface (Riggs 2001).

Atypical aboveground symptoms consisted of gradual wilting followed by leaf chlorosis and defoliation have been observed by Rush et al. (1985) during periods of low soil moisture availability.

The pathogen:

“Conidial stage (Phymatotrichum omnivorum). Hyphae forming a loose layer on the surface of the soil in areas where the root rot has killed the plants; fertile hyphae arising irregularly from the mycelium, simple or forked; conidia numerous, sessile, borne on the irregularly swollen clavate or subglobose short terminal or lateral branches of the hyphae or on the sides of the undifferentiated hyphae, pale ochraceous to gray in mass, appearing nearly colorless when separate, globose to ovoid 4—6 x 5 8 µm, smooth. Sterile mycelial stage (Ozonium omnivorum) mycelium thin, floccose to arachnoid, forming thin branching strands, the dirty ochraceous branches arising from these strands are thin, somewhat rigid and branch at right-angles from somewhat swollen nodes, 2 to 4 branches from a node, usually long, thin, tapering and acute” (Shear 1925). The morphology of strands and sclerotia has been presented by Neal et al. (1934) and Alderman & Stowell (1986).

Notes: According to Shear (1925), this pathogen was first tentatively referred to Ozonium auricomum Link. by Pammel in 1889. Shear, comparing this pathogen with the type of O. auricomum, found they were quite different and described a new species Ozonium omnivorum Shear in 1907 to accommodate the cotton root rot. At that time this pathogen was know only as a sterile mycelia. In 1916, its conidial state was described as Phymatotrichum omnivorum Duggar. Afterwards, Shear (1925) described the basidiomycete fungus Hydnum omnivorum Shear associated to plants dying from Phymatotrichum root rot that he believed to be the perfect state of P. omnivorum, although it was based on circumstancial evidence. According to Burdsall Jr & Nakasone (1978), H. omnivorum is not the perfect state of the cotton pathogen and was reclassified as Phanerochaete omnivorum (Shear) Burdsall Jr & Nakasone. Barnieki and Bloss (1969) observed clamp connections, basidia and basidiospores in a few cultures of P. omnivorum and stated Sistotrema brinkmannii (Bres.) J. Erikss. (? Trechispora brinkmannii (Bres.) D.P. Rogers & H.S. Jacks.) as the perfect state of the P. omnivorum. However, according to Dong et al. (1981) the hyphal septa ultrastructure of the two fungi suggests that they are not two stages of the same species as reported by Barnieki and Bloss (1969). Thus, so far the perfect state of P. omnivorum has not been found. Hennebert (1973) has transferred this species to the genus Phymatotrichopsis.
More Information
Specimen Contact
Dr Jose R. Liberato
Liberato, JR
20/07/2006 02:23 AEST
Last Updated
16/07/2016 01:36 AEST
Liberato, JR (2006) Cotton root rot (Phymatotrichopsis omnivora) Updated on 7/16/2016 1:36:43 AM Available online: PaDIL -
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Free for use under the Creative Commons Attribution 3.0 Australia License