Stephen M. Marek1, Tim Samuels1, Ian Moncrief1, Simone Macmil2, Fares Najar2, Bruce Roe2, Carolyn Young3, Srinivasa Rao Uppalapati3, Kiran S. Mysore3, and Richard Dixon3. (1) Oklahoma State University, 127 Noble Research Center, Entomology and Plant Pathology, Stillwater, OK 74078-3033, (2) University of Oklahoma, 101 David L. Boren Blvd., Room 2107, Stephenson Research and Technology Center, Norman, OK 73019, (3) Samuel Roberts Noble Foundation, Inc., 2510 Sam Noble Pky., Ardmore, OK 73401
Phymatotrichum or cotton root rot is caused by the soilborne fungus Phymatotrichopsis omnivora (syn. Phymatotrichum omnivorum) and occurs throughout the alkaline calcareous soils of the southwestern U.S. and northern Mexico. This important disease of cotton has been extensively studied for over a century, but no consistently effective control has been developed. While cultural practices, such as crop rotation and later planting dates, have been used with varying success, the ideal management strategy would be to develop resistant varieties. However, no reliable resistance has been discovered among cotton (Gossypium spp.) germplasm accessions. Therefore, novel multidisciplinary approaches are now being used to study the causal fungus and its interaction with another important host, alfalfa (Medicago sativa), and the model legume, barrel medic (M. truncatula). Using multigene phylogenies, the confused taxonomy of P. omnivora has been resolved to the Ascomycota (Class Pezizomycetes, “operculate cup fungi”). A collection of ~100 isolates of P. omnivora is being used to delineate the biodiversity and population structure of the fungus thoughout its range in the U.S. Also, the entire genome of an isolate of P. omnivora from alfalfa has been completely sequenced. Preliminary sequence data indicate the fungus possesses multiple copies of its genome in a single isolate. Gene expression studies of infected M. truncatula have revealed a rapid wound response and increased lignin biosynthesis. Towards resistant crop varieties, several genotypes of M. truncatula, out of hundreds tested, have displayed resistance to the fungus in replicated greenhouse and growth chamber assays. These promising findings and resources hopefully will be transferrable to cotton, providing alternative strategies for managing this currently recalcitrant disease.
