Diversity, Virulence, and 2,4-Diacetylphloroglucinol Sensitivity of <italic>Gaeumannomyces graminis</italic> var. <italic>tritici</italic> Isolates from Washington State

Kwak, Y.-S., Bakker, P. A. H. M., Glandorf, D. C. M., Rice, J. T., Paulitz, T. C., and Weller, D. M. 2009. Diversity, virulence, and 2,4-diacetylphloroglucinol sensitivity of Gaeumannomyces graminis var. tritici isolates from Washington State. Phytopathology 99:472-479. We determined whether isolates of the take-all pathogen Gaeumannomyces graminis var. tritici become less sensitive to 2,4-diacetylphloroglucinol (2,4-DAPG) during wheat monoculture as a result of exposure to the antibiotic over multiple growing seasons. Isolates of G. graminis var. tritici were baited from roots of native grasses collected from noncropped fields and from roots of wheat from fields with different cropping histories near Lind, Ritzville, Pullman, and Almota, WA. Isolates were characterized by using morphological traits, G. graminis variety-specific polymerase chain reaction and pathogenicity tests. The sensitivity of G. graminis var. tritici isolates to 2,4-DAPG was determined by measuring radial growth of each isolate. The 90% effective dose value was 3.1 to 4.4 μg ml–1 for 2,4-DAPG-sensitive isolates, 4.5 to 6.1 μg ml–1 for moderately sensitive isolates, and 6.2 to 11.1 μg ml–1 for less sensitive isolates. Sensitivity of G. graminis var. tritici isolates to 2,4-DAPG was normally distributed in all fields and was not correlated with geographic origin or cropping history of the field. There was no correlation between virulence on wheat and geographical origin, or virulence and sensitivity to 2,4-DAPG. These results indicate that G. graminis var. tritici does not become less sensitive to 2,4-DAPG during extended wheat monoculture. Additional keywords: biological control, Pseudomonas fluorescens, take-all. Take-all, caused by Gaeumannomyces graminis (Sacc.) von Arx & Olivier var. tritici Walker (G. graminis var. tritici), generally develops at a soil pH of 5.5 to 8.5 and where wheat is grown under moist conditions (7,9,19,22,29). As a result, take-all is common throughout the world and is considered the most important root disease of wheat (7,10,22). G. graminis var. tritici survives saprophytically as mycelium in dead roots, crowns, and tiller bases, the inoculum source for the next crop (9,19,29). Primary infection of roots of seedlings occurs with the growth of dark runner hyphae on the root surface. Hyaline hyphae penetrate into the cortex and colonize the vascular tissue, causing characteristic black lesions. Runner hyphae continue to grow over the root surface to other roots and upward to the crown and stem bases. Early infection of the plant ultimately causes yellowing of lower leaves, stunting, and premature death of plants in patches. Wheat is highly susceptible to take-all, but other members of the family Poaceae (i.e., barley, rye, and triticale) are also infected (9,19). Crop rotation and tillage are effective controls of take-all; however, the trends are toward less tillage and growing two or three wheat crops before a break, and these practices exacerbate the disease. Take-all is also controlled by take-all decline (TAD), the spontaneous decrease in take-all incidence and severity induced by continuous wheat or barley monoculture after a severe outbreak of the disease (12,22,39). In the Pacific Northwest of the United States and in The Netherlands, TAD develops because of the build-up of populations of 2,4-diacetylphloroglucinol (2,4DAPG)-producing Pseudomonas fluorescens (phlD) and production of the antibiotic in the rhizosphere during wheat and barley monoculture (32,33,38,39). The 2,4-DAPG biosynthetic locus includes the operons phlACBDE and phlF that function in synthesis, export, and regulation (2). 2,4-DAPG accumulation in the wheat rhizosphere is proportional to the population density of phlD P. fluorescens at densities above 10 colony forming units (CFU) g of root, and 2,4-DAPG concentration per phlD population unit is constant (0.62 ng/10 CFU) (31). Roots of wheat grown in a Quincy TAD soil supported 4.3 × 10 CFU of phlD isolates and 19.1 ng of 2,4-DAPG g of root (31). Among all soilborne pathogens common in Pacific Northwest wheat fields, G. graminis var. tritici is the most sensitive to 2,4DAPG (1,24). Mazzola et al. (24) screened G. graminis var. tritici isolates from seven countries and seven states and showed differential sensitivity to 2,4-DAPG. Isolates that were not inhibited by 2,4-DAPG at 3 μg/ml were not suppressed by phlD P. fluorescens strain Q2-87 on wheat roots, whereas isolates sensitive to this concentration were controlled. The genetic variation and diversity known to exist within the take-all pathogen, even though sexual recombination is not a common part of its ecology, apparently are reflected in the differences among G. graminis var. tritici isolates in sensitivity to 2,4-DAPG. TAD occurs worldwide (22,35,36) and follows a similar pattern everywhere, yet the speed of its development and the robustness and longevity of the suppressiveness vary among fields (22). The long-term goal of our research is to identify biotic and abiotic factors that impact the robustness of take-all suppressiveness in Corresponding author: D. M. Weller; E-mail address:[email protected] doi:10.1094 / PHYTO-99-5-0472 This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological