A study of Aphanomyces euteiches Drechs. root rot of field pea (Pisum sativum L.) in Manitoba
Slusarenko, Kirsten Lea
Aphanomyces euteiches Drechs. is an important yield reducing pathogen of field pea (Pisum sativum L.) in Manitoba. Currently, disease control is achieved through crop rotation or avoidance of fields with high inoculum pressure. The purpose of this study was to determine A. euteiches incidence, pathogenicity and host-range, as well as the effect that fungicidal and biological seed treatments have on pathogen control. A survey of commercial field peas was conducted in 1999 and 2000 to determine the incidence of A. euteiches in Manitoba. Aphanomyces euteiches was isolated from 9 of 44 fields surveyed. The Aphanomyces isolates originated from fields within a 400 km range extending from Morden to Russell, Manitoba. Growth cabinet experiments were carried out in 2000 and 2001, to determine if certain environmental conditions and hosts were favourable to the pathogen. Using the pea variety Carneval, seven A. euteiches isolates (15,22,24,25,26,27 and 41) were studied at four growing temperatures (16,20,24, and 28o C) and three seedling ages (1,2, and 3 weeks). Measurement to determine the extent of disease development included disease severity and root and shoot dry weights. Disease development was favourable at all four temperature regimes tested. Seedling age had no consistent effect on disease development. Disease severity between the seven A. euteiches isolates were significantly different (p=0.05), suggesting that pathogenic variability exists between the isolates of A. euteiches. To determine each isolates host-range, all seven isolates were tested against four pea varieties (AC Tamor, Carneval, Marjoret, and Trapper), eight lentil varieties (CDC Glamis, CDC Robin, Crimson, Eston, French, Indianhead, Laird, and Richlea), two chickpea varieties (CDC Desiray and CDC Yuma), six bean varieties (AC Clack Diamond, CDC Pintium, AC Scarlet, Envoy, Navigator, and Pintoba), one Alfalfa variety (OAC Minto) and one soybean variety (Alta). All seven isolates were virulent on the four pea varieties tested and avirulent on the soybean, chickpea and bean varieties tested. Lentil and alfalfa were the only hosts to with both susceptible and resistant lines to the seven isolates. Field experiments to determine the effect of seed applied fungicides and biological agents on pathogen control were conducted in 1999 and 2000 at sites in and near Morden and in Winnipeg, Manitoba. A natural source of inoculum was relied upon at each location. The thirteen fungicide and biological seed treatments tested included seven fungicides (Aliette, Apron, Crown, Ridomil, Ronilan, Thiram, and Vitaflo-280) and two biological agents (ACM941 and AR101). Three combination treatments were also tested, consisting of both a fungicide and a biological agent (ACM941+Aliette, ACM941+Apron and ACM941+Vitaflo-280). Emergence, disease severity, root and shoot dry weights and yield were measured to determine the effect each treatment had on Aphanomyces root rot. Emergence was significant (p=0.05) at each field site, although the overall effect that individual treatments had on emergence varied between sites. Average disease severity and yield ranged from 0.3 to 3.9 and 467g to 1227g between sites, respectively. These differences may have resulted from variable environmental conditions (temperature, moisture and soil type) between sites and years, and/or the relative aggressiveness of the A. euteiches isolates present. An increase in inoculum pressure, resulting from growing the same crop in consecutive years could also explain the increase in disease severity. These results suggest that A. euteiches is more extensively spread in Manitoba than previously believed and that the isolates present are pathogenically variable. The importance of crop rotation as a method of controlling this disease is also evident since pea, lentil and alfalfa were obvious hosts and none of the thirteen seed treatments tested significantly controlled A. euteiches root rot of field pea.