|HU, XIAOPING - Northwest Agricultural & Forestry University|
|GURUNG, SURAJ - University Of California|
|SHORT, DYLAN - University Of California|
|SANDOYA, GERMAN - University Of California|
|SHANG, WENJING - Northwest Agricultural & Forestry University|
|SUBBARAO, KRISHNA - University Of California|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/5/2015
Publication Date: 5/11/2015
Citation: Hu, X., Gurung, S., Short, D.P., Sandoya, G., Shang, W., Hayes, R.J., Subbarao, K.V. 2015. Non-defoliating and defoliating strains from cotton correlate with races 1 and 2 of Verticillium dahliae. Plant Disease. doi: 10.1094/PDIS-03-15-0261-RE.
Interpretive Summary: Verticillium wilt is a serious wilting disease of plants caused by the soil dwelling fungus Verticillium dahliae. The fungus is known worldwide and attacks many crop plants. In cotton, strains of the fungus are classified as defoliating or non-defoliating based on the symptoms they cause on cotton. The defoliating strains cause the plant to rapidly lose leaves, while the non-defoliating strains cause disease but the plant’s leaves stay attached. In tomato and lettuce, fungal strains are classified differently. Strains are characterized as race 1 or race 2 based on their ability to cause disease on specific cultivars of the crops. Specifically, some tomato and lettuce cultivars are able to resist race 1 strains but not race 2. A well characterized gene in the fungus controls the difference between race 1 and 2. This study investigated the frequency and distribution of races and defoliation phenotypes in 331 fungal strains collected from cotton grown in Australia, China, Israel, Turkey, and the United States. The relationship between the race and defoliation classification systems was also determined. The classification of strains was conducted using DNA tests that predict defoliating vs. non-defoliating status or determine the version of the gene controlling race 1 vs. 2. DNA test results were confirmed by inoculating several cotton cultivars with a sample of fungal strains and observing the symptoms that resulted. The experiments determined that there was a close correlation between the classification systems. 96.4% of strains classified as defoliating were determined to be race 2, while 90.8% of strains classified as non-defoliating were determined to be race 1. The greenhouse assays confirmed the molecular identification of the race and defoliation classifications. While the existence of races has not been previously established among fungal strains of cotton, the long established non-defoliating and defoliating classification corresponded closely with race designations.
Technical Abstract: Verticillium wilt caused by Verticillium dahliae is an important disease of cotton worldwide. Isolates of V. dahliae can be characterized as race 1 or race 2 based on the responses of differential cultivars of tomato and lettuce or as defoliating or non-defoliating based on symptom expression in cotton. To investigate the frequency and distribution of races and defoliation phenotypes of cotton-associated V. dahliae, 331 isolates from Australia, China, Israel, Turkey, and the United States were tested by polymerase chain reaction (PCR) using defoliating, non-defoliating, race 1- and race 2-specific primers DF/DR, NDF/NDR, VdAve1F/VdAve1R, and VdR2F/VdR2R, respectively. Of the total, 96.4% of isolates genotyped as defoliating were also characterized as race 2, while 90.8% of isolates that were genotyped as non-defoliating were genotyped as race 1. To verify these results, three cotton cultivars, ‘FM 2484B2F’ (highly resistant), ‘98M-2983’ (highly susceptible) and ‘CA4002’ (partial resistant) used as differentials were each inoculated with ten isolates characterized by PCR: six defoliating/race 2 strains (GH1005, GH1021, HN, XJ2008, XJ592, and reference strain Ls17) and four non-defoliating/race 1 strains (GH1015, GH1016, GH1020, and reference strain Ls16). All defoliating/race 2 isolates except for Ls17 caused defoliation on cultivars 98M-2983 and CA4002. Isolate Ls17 caused defoliation in cultivar 98M-2983 only. The non-defoliating/race 1 isolates caused Verticillium wilt symptoms devoid of defoliation on cultivar 98M-2983. The greenhouse assays confirmed the molecular identification of race and defoliation phenotype. While the existence of races has not been previously established among V. dahliae isolates from cotton, the long established non-defoliating and defoliating population structure corresponded with V. dahliae races 1 and 2, respectively. Similar race structure may also exist among the defoliating and non-defoliating isolates infecting olives but this remains to be established.