Genetic Diversity of Verticillium dahliae Isolates From Mint Detected with Genotyping by Sequencing

Verticillium wilt is the most important disease threatening the commercial production of mint grown for essential oil. An important long-term goal for mint breeders is the production of cultivars with resistance to Verticillium wilt. Before that can be accomplished, a better understanding of the genetic variation within and among populations of Verticillium dahliae is needed. We characterized the extent of phenotypic and genetic diversity present in contemporary and archival populations of V. dahliae from mint fields in Oregon and other production regions of the United States using genotyping by sequencing, PCR assays for mating type and pathogenic race, vegetative compatibility group (VCG) tests, and aggressiveness assays. We report that the population in the Pacific Northwest can be described as one common genetic group and four relatively rare genetic groups. Eighty-three percent of the isolates belonged to VCG2B, and all isolates possessed the MAT1-2 idiomorph and were characterized as pathogenic race 2. These results indicate low levels of genetic diversity and a negligible risk of sexual recombination in populations of this host-adapted pathogen population. Knowledge of the genetic structure of V. dahliae in the Pacific Northwest will inform breeders about the diversity of pathogenicity factors that may need to be considered in their breeding programs.

Characterization by RAPD-PCR of races of Fusarium oxysporum f.sp. ciceri infecting chickpea

Isolates of Fusarium oxysporum f.sp. ciceri from chickpea (Cicer arietnum L.) can be characterized as races and wilt and yellowing pathotypes by pathogenicity tests on varieties of differentials chickpea. This study shows that, DNA bands generated by RAPD-PCR can be used to assign Fusarium oxysporum f.sp. ciceri isolates to pathotype and pathogenic race.

A New Pathogenic Race of Tilletia caries Possessing the Broadest Virulence Spectrum of Known Races

Common bunt, caused by the fungi Tilletia caries and T. laevis, is one of the most destructive seedborne diseases of wheat. In conventional agriculture, common bunt is managed almost exclusively with chemical seed treatments. However, in organic farming, synthetic chemicals are prohibited. Because of this, there has been a resurgence of this disease in organic wheat. In order to maintain high yields and excellent seed quality, organic growers must rely heavily on resistant wheat cultivars. To breed cultivars with resistance against common bunt, and to effectively deploy resistance genes, it is necessary to identify and monitor the pathogenic races of the local pathogen population. Towards this goal, races of T. caries present in Washington, Oregon, California, and Idaho were identified by inoculating field collections of the pathogen on 13 differential wheat cultivars. Results of three years' testing show that there is a new pathogenic race in Washington State, which possesses the broadest virulence spectrum to date compared with known bunt races. Furthermore, two-year data indicates the presence of other new races in Washington, California, and Oregon. Accepted for publication 5 April 2011. Published 20 May 2011.

Two New Races of Phytophthora phaseoli from Lima Bean in Delaware

Downy mildew, incited by Phytophthora phaseoli Thaxt., is the most important disease of lima bean (Phaseolus lunatus L.) on the east coast of the United States. It has been a serious threat to commercial lima bean production in Delaware, Maryland, and New Jersey for the past 5 years. Growers have attempted to manage this disease using resistant cultivars and copper hydroxide fungicides. In August and September 1995, a new pathogenic race of P. phaseoli was isolated from infected pods of the lima bean cv. Packer in a production field near Milton, DE. Races of P. phaseoli are determined using a modification of a cultivar differential developed by Wester (3). The cv. 184–85, which is resistant to races A, B, C, and D (1), is susceptible to the new race, designated as E. In August 2000, another new pathogenic race of P. phaseoli was isolated from infected pods of cv. 184–85 near Middletown, DE. The lima bean line BG2-408, which is resistant to races A, B, C, D, and E, is susceptible to the new race, designated as F. Symptoms produced on lima bean plants infected by races E and F are similar to each other, and to those produced by all other races. All races of P. phaseoli have the same cultural characteristics on lima bean pod agar. Evaluations of in field weather station data and disease occurrence indicate that races E and F may have temperature maxima greater than 32°C, whereas race D has a maximum of less than 32°C (2). During the 2000 growing season, 118 isolates of P. phaseoli were collected from 44 production fields in Delaware and the eastern shore of Maryland, with 86% characterized as race E and 5% as race F. References: (1) C. R. Davidson et al. Biol. Cult. Tests 2001:V80. (2) R. A. Hyre and R. S. Cox. Phytopathology 43:419, 1953. (3) R. E. Wester. Phytopathology 60:1856, 1970.

Characterization of an Isolate of Phytophthora Nicotianae Var. Nicotianae From North Queensland

An isolate of P. nicotianae var. nicotianae from North Queensland was shown to be of pathogenic race 0, since it was non-pathogenic to Nicotiana longifiora and N. plumbaginifolia, but was strongly pathogenic to N. rustica and N. tabacum. The pH optimum, the pH-growth response curve, the temperature optima for growth on various media, and growth in the presence of sterol were similar to those reported previously for isolates from America.