Approximately 260,000 ha of alfalfa is grown in Illinois. Two soil samples were collected randomly from slowly drained thin patches in each of four established alfalfa fields near Urbana in 2001. Plants in the thin patches were asymptomatic. Aphanomyces euteiches Drechs. was baited from the soil with cv. Saranac alfalfa seedlings and was isolated from 3- to 4-week-old infected seedlings using a medium containing metalaxyl and benomyl (1,2). It is difficult to isolate A. euteiches from field-grown roots. One to seven isolates were obtained per field, and all were identified as A. euteiches based on morphology (1,2). A. euteiches (races R1 and R2) causes root rot of alfalfa in slowly drained fields in Iowa, Kentucky, and Wisconsin (1,2). The race of 13 isolates was determined in tests repeated once with alfalfa populations Saranac (susceptible to R1 and R2), WAPH-1 (resistant only to R1), and WAPH-5 (resistant to R1 and R2) (1). Twelve 7-day-old seedlings in each of three pots per population were inoculated with 103 zoospores per seedling in a growth chamber (25°C). A disease index (DI) was determined 12 days later by scoring plants on a 1 to 5 scale, where 5 is a dead plant (1). Race was based on DI, R1: DI ≥3 for Saranac and <3 for WAPH-1, and R2: DI > 3 for Saranac and WAPH-1. The DI was 1.0 for noninoculated plants. All isolates were R2; the DI was >3.0 for inoculated Saranac and WAPH-1 and <3.0 for WAPH-5. To our knowledge, this is the first report of A. euteiches races in Illinois, and this pathogen was reported previously only from northwest Illinois. Control of Aphanomyces root rot is based on resistance; however, few alfalfa cultivars are resistant to R2.
References: (1) D. Malvick and C. Grau. Plant Dis. 85:740, 2001. (2) G. Munkvold and W. Carlton. Plant Dis. 79:1251,1995.