|Samac, Deborah - Debby|
|ALLEN, SHEVAWN - Dairyland Seed Company, Inc|
|WITTE, DAVE - Forage Genetics International|
|MILLER, DOUG - Cal/west Seeds|
|PETERSON, JENN - Pioneer Hi-Bred International|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/24/2013
Publication Date: 6/1/2014
Citation: Samac, D.A., Allen, S., Witte, D., Miller, D., Peterson, J. 2014. First report of race 2 of Colletotrichum trifolii causing anthracnose on alfalfa (Medicago sativa) in Wisconsin. Plant Disease. 98(6):843.
Interpretive Summary: Anthracnose is a serious disease of alfalfa in the United States. It occurs wherever alfalfa is grown, causing loss of forage as well as causing crown rot, which reduces stand life and predisposes plants to winter injury. Although three physiological races have been identified, only one race, called race 1, was reported to be widespread. Most alfalfa cultivars have resistance to this race of the pathogen but not to other races. Recently, anthracnose was observed in four locations in Wisconsin. Isolation and testing of the pathogen on race 1 and race 2 resistant alfalfa cultivars showed that the pathogen causing disease is a race 2 strain. The isolation of race 2 strains in major alfalfa growing regions in Wisconsin indicates that this physiological race is currently more widespread than previously observed and this needs to be considered in developing new disease resistant alfalfa cultivars with resistance to anthracnose.
Technical Abstract: Anthracnose of alfalfa (Medicago sativa), caused by Colletotrichum trifolii, is widespread in the United States. Three physiological races have been described. Race 1 is reported to be the dominant race that is present wherever alfalfa is grown, while race 2 was reported in a limited area in the Mid-Atlantic states, and race 4 was found in Ohio. Conspicuous, straw-colored dead stems with a “shepherd’s crook” wilt and large, sunken, diamond-shaped lesions with a dark border were observed in experimental plots in Clinton, WI in August 2011 and in West Salem and Arlington, WI in August 2012. Acervuli with black setae and orange spore masses were observed in lesions placed in moist chambers. Conidia were germinated on 1% water agar and then single hyphae were transferred to potato dextrose agar (PDA) plates. DNA was extracted from pure cultures of strains DA-1 (Clinton, WI) and FGI-3 (West Salem, WI), the rDNA internal transcribed spacer (ITS) was amplified with primers ITS1 (5’-TCCGTAGGTGAACCTGCGG-3’) and ITS4 (5’-TCCTCCGCTTATTGATATGC-3’), and the sequences compared to known race 1 and race 2 strains. The sequences were identical to the ITS sequence of C. trifolii 2sp2 (race 1) and C. trifolii SB-2 (race 2), but clearly distinct from the ITS sequence of C. destructivum and C. dematium, which can cause anthracnose on alfalfa. Conidia from DA-1 and FGI-3 were used to inoculate10-day-old plants of three differential cultivars: Saranac (susceptible to race 1 and race 2), Arc (resistant to race 1, susceptible to race 2), and Saranac AR (resistant to race 1 and race 2). In the three repetitions of the experiment using 75 plants of each cultivar in each experiment, less than 10% of the Saranac and Arc plants survived, while survival of Saranac AR was 31-44%. The susceptibility of Arc when inoculated with DA-1 and FGI-3 is consistent with the reaction to race 2 strains, indicating that both strains are race 2. The isolation of race 2 strains in major alfalfa growing regions in Wisconsin indicates that this physiological race is currently more widespread than previously observed and should be considered in alfalfa breeding programs when developing multi-pest resistant alfalfa cultivars.