First Report of Lentil Root Rot Caused by Aphanomyces euteiches in Idaho

In June 2008, lentil plants (Lens culinaris Medik. cv. Crimson) in a field in Kendrick, ID exhibited symptoms including stunting, leaf chlorosis, reddening of abaxial leaf surfaces, root browning, and necrosis. Roots were surface sterilized, plated on water agar, and pure cultures were obtained through hyphal tips. DNA was extracted from mycelia and amplified with PCR primers that produced a 1,332-bp fragment specific for Aphanomyces euteiches Drechs (4). DNA of A. euteiches isolated from pea (Pisum sativum L.) was used as a positive control for PCR. A PCR product of the expected size was amplified from the positive control and also from a single isolate obtained from symptomatic lentils. Mycelial mats of the isolate (No. 823) were incubated overnight in a dilute mineral salt solution (1), a process that routinely produced 350,000 zoospores/ml, which were used to screen 33 advanced lentil breeding lines and three lentil cultivars (Eston, Crimson, and Pardina). Plants were grown in greenhouse flats containing perlite. At the time of planting, all flats were inoculated with a liquid suspension of Rhizobium leguminosarum bv. viceae. One week after emergence, all seedlings were inoculated with 20,000 or 5,000 zoospores of A. euteiches isolate 823. Plants were scored 14 days after inoculation using a 1 (no discoloration of roots) to 5 (dead seedling) disease severity index (DSI) commonly used to evaluate disease reaction to A. euteiches in pea (3). For each inoculum level, six plants of each lentil genotype were scored for disease reaction and the experiments were repeated once. A pooled analysis of both replications indicated that all entries were at least moderately susceptible to A. euteiches (mean DSI >or=3) at both inoculum levels, and the majority was highly susceptible (DSI >or=4). Oospores approximately 20 μm in diameter were observed in lentil roots 14 days after inoculation. In addition, five plants each of 73 NPGS PI accessions and four lentil cultivars (Eston, Crimson, Pardina, and Riveland) were inoculated with 250 zoospores per plant and scored for disease reaction. These experiments were repeated twice. Significant differences in DSI were observed among entries. The grand mean DSI of all entries was 3.56. The mean DSI of entries based on a pooled analysis of three replications ranged from 2.67 (Riveland) to 4.0 (PI 472561). All plants for each replication were bulked and dried in an oven, as were five noninoculated plants for each entry, and weighed. The average percent weight of inoculated plants relative to that of noninoculated plants for all 77 entries was 32.7% and the correlation between this percentage and DSI was –0.25 (P = 0.03). The results suggest that resistance to A. euteiches is lacking in the lentil genotypes examined and that plant biomass is reduced by the pathogen. Moussart et al. (2) recently demonstrated that a French pea isolate of A. euteiches caused severe disease among a set of 28 lentil genotypes. We detected A. euteiches in a field in which peas were recently grown and this isolate likely originated from peas. Additional lentil fields in Washington and Idaho will be surveyed for the presence of A. euteiches. Preliminary lentil breeding lines and additional accessions will be screened for tolerance to A. euteiches in the greenhouse and field.