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Evaluation of the relative importance of Ascochyta pisi in the Ascochyta blight complex of pea in Saskatchewan



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Ascochyta blight may be caused by the fungal pathogens Peyronellaea pinodes, Ascochyta pisi, Phoma medicaginis var. pinodella, and Phoma koolunga. While P. pinodes has been the most important pathogen in Saskatchewan, A. pisi has become more prevalent in southern and southern-western parts of the province. Conidial germination of A. pisi and P. pinodes on glass slides and infection of plants of pea cultivar CDC Cooper by both pathogens were evaluated under controlled conditions at temperatures ranging from 10 to 30°C and wetness periods of 0 to 12 h to determine whether they had different optimal environmental requirements. For both pathogens, conidial germination and disease severity increased with increasing temperature and leaf wetness period up to the optimum of 20 to 25°C. Overall, P. pinodes had consistently higher germination and disease severity compared to A. pisi. For conidial germination, these differences became obvious starting at 20°C after more than 4 h incubation, and at 25 and 30°C P. pinodes had consistently higher germination after 2 h of incubation. Similarly, disease severity caused by P. pinodes was consistently higher at 20 and 25°C compared to that caused by A. pisi. The role of infected seed in the epidemiology of A. pisi in pea was studied with naturally infected seeds under field conditions in 2012, 2013 and 2014 at two locations where low natural inoculum were expected. Results revealed a significant effect of A. pisi seed infection on emergence of seedlings (P < 0.05). Seed infection levels of 10 and 14.5% resulted in reduced emergence compared to 0.5% seed infection, but the level of seed infection at planting had no impact on A. pisi disease severity, seed infection levels of harvested seed or seed yield. No visible symptoms caused by A. pisi appeared on the aerial parts of the seedlings. Results suggest that disease did not progress from seeds, or contributed to infection of aerial parts of the plants, hence infected seeds cannot be regarded as a source of inoculum in the epidemiology of this pathogen. Assessing seed components of seeds with varying levels of A. pisi infection and seed staining revealed that the pathogen was present in all components of the seed, regardless of the severity of seed staining. Field studies were conducted to assess yield loss caused by A. pisi between 2012 and 2014 at Swift Current and Stewart Valley where a high incidence of A. pisi had been reported in the past, and under irrigation with inoculation at Saskatoon (2014). Two (2012, 2013) or three (2014) fungicides (pyraclostrobin, chlorothalonil, or pyraclostrobin and boscalid) were applied to create plots with low levels of A. pisi infection that were compared with a non-sprayed treatment. The more susceptible pea cultivars Cooper and SW Midas were compared with the more resistant cultivars CDC Bronco and CDC Golden. Disease pressure was low in all three years and no differences in yield between fungicide treated and non-sprayed treatments were observed. Nevertheless, the incidence of A. pisi isolated from harvested seeds of the fungicide treatments was lower than that of the non-sprayed treatment, despite similar A. pisi severity on plants before harvest. The response of the recombinant inbred line (RIL) pea population PR-10 developed from a cross between the susceptible variety Cooper and the partially resistant variety CDC Bronco to A. pisi infection was evaluated in 2012, 2013 and 2014 under field conditions. RILs were inoculated with A. pisi in 2014 at Saskatoon under irrigation. Under low disease pressure in all three years, no difference in A. pisi severity was observed between the parents and among RILs. Yields differed among RILs and were attributed to genetic differences. Considering that the parents did not differ in resistance to A. pisi it was concluded that this population might not be suitable for the study of genetic control of resistance to A. pisi.



Ascochyta pisi, Peyronellaea pinodes



Master of Science (M.Sc.)


Plant Sciences


Plant Science


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