The fungus <i>Colletotrichum truncatum</i> (Schw.) Andrus and Moore causes lentil anthracnose, which is a major challenge to lentil production in Western Canada. The pathogen infects leaves and stems, resulting in defoliation, stem girdling, plant wilting, and possibly plant death. Two races, Ct0 and Ct1, have been identified in the pathogen population in Canada. However, the differences in the infection process between the two races have not been described in detail. Currently, several lentil cultivars, such as CDC Redberry, CDC Robin, CDC Rosetown, CDC Rouleau, and CDC Viceroy, have resistance against race Ct1, whereas there are no cultivars showing resistance to race Ct0. The objective of this study was to investigate differences in the infection process between race Ct0 and race Ct1 using the fully susceptible cultivar Eston and the race Ct1-resistant cultivar CDC Robin. Experiments on glass well slides showed that race Ct0 had no inherently different conidium germination rate compared to race Ct1, and that differences in conidium germination between the two races on lentil plants were the result of specific interactions between the two races and lentil resistance. Investigations of the infection process of the two races on detached and attached leaves of both lentil cultivars were conducted starting 12 h postinoculation (hpi) until 72 hpi, including conidium germination, appressorium formation, and leaf penetration. Results indicated that differences in virulence of the two races may be related to the ability of conidia to germinate and form appressoria, as well as the ability of primary infection hyphae to grow in response to cues from the lentil cultivars. Furthermore, resistance of lentil to isolates of race Ct1 appeared to involve an inhibition in and/or delay of the spread of primary infection hyphae inside the plant tissue. Results of infection studies of one isolate from each race on attached leaves did not completely agree with results of the same isolates on detached leaves. Based on this study, race Ct0 and race Ct1 do not appear to be classical physiological races, but may represent aggressive races or some intermediate forms.

The fungus <i>Colletotrichum truncatum</i> (Schw.) Andrus and Moore causes lentil anthracnose, which is a major challenge to lentil production in Western Canada. The pathogen infects leaves and stems, resulting in defoliation, stem girdling, plant wilting, and possibly plant death. Two races, Ct0 and Ct1, have been identified in the pathogen population in Canada. However, the differences in the infection process between the two races have not been described in detail. Currently, several lentil cultivars, such as CDC Redberry, CDC Robin, CDC Rosetown, CDC Rouleau, and CDC Viceroy, have resistance against race Ct1, whereas there are no cultivars showing resistance to race Ct0. The objective of this study was to investigate differences in the infection process between race Ct0 and race Ct1 using the fully susceptible cultivar Eston and the race Ct1-resistant cultivar CDC Robin. Experiments on glass well slides showed that race Ct0 had no inherently different conidium germination rate compared to race Ct1, and that differences in conidium germination between the two races on lentil plants were the result of specific interactions between the two races and lentil resistance. Investigations of the infection process of the two races on detached and attached leaves of both lentil cultivars were conducted starting 12 h postinoculation (hpi) until 72 hpi, including conidium germination, appressorium formation, and leaf penetration. Results indicated that differences in virulence of the two races may be related to the ability of conidia to germinate and form appressoria, as well as the ability of primary infection hyphae to grow in response to cues from the lentil cultivars. Furthermore, resistance of lentil to isolates of race Ct1 appeared to involve an inhibition in and/or delay of the spread of primary infection hyphae inside the plant tissue. Results of infection studies of one isolate from each race on attached leaves did not completely agree with results of the same isolates on detached leaves. Based on this study, race Ct0 and race Ct1 do not appear to be classical physiological races, but may represent aggressive races or some intermediate forms.

Characteristics of quantitative resistance in pea (Pisum sativum L.) to Erysiphe pisi DC, the pathogen causing powdery mildew, were investigated. Cultivars and seedlines of pea expressing quantitative resistance to E. pisi were identified and evaluated, by measuring the amounts of pathogen present on plant surfaces in field and glasshouse experiments. Disease severity on cv. Quantum was intermediate when compared with that on cv. Bolero (susceptible) and cv. Resal (resistant) in a field experiment. In glasshouse experiments, two groups of cultivars, one with a high degree of resistance and the other with nil to low degrees of resistance to E. pisi, were identified. This indicated either that a different mechanism of resistance applied in the two groups, or that there has been no previous selection for intermediate resistance. Several other cultivars expressing quantitative resistance were identified in a field experiment. Quantitative resistance in Quantum did not affect germination of E. pisi conidia, but reduced infection efficiency of conidia on this cultivar compared with cv. Pania (susceptible). Other epidemiological characteristics of quantitative resistance expression in Quantum relative to Pania were a 33% reduction in total conidium production and a 16% increase in time to maximum daily conidium production, both expressed on a colony area basis. In Bolero, the total conidium production was reduced relative to Pania, but the time to maximum spore production on a colony area basis was shorter. There were no differences between the cultivars in pathogen colony size or numbers of haustoria produced by the pathogen. Electron microscope studies suggested that haustoria in Quantum plants were smaller and less lobed than those in Pania plants and the surface area to volume ratios of the lobes and haustorial bodies were larger in Pania than in Quantum. The progress in time and spread in space of E. pisi was measured in field plots of cultivars Quantum, Pania and Bolero as disease severity (proportion of leaf area infected). Division of leaves (nodes) into three different age groups (young, medium, old) was necessary because of large variability in disease severity within plants. Disease severity on leaves at young nodes was less than 4% until the final assessment at 35 days after inoculation (dai). Exponential disease progress curves were fitted for leaves at medium nodes. Mean disease severity on medium nodes 12 dai was greatest (P<0.001) on Bolero and Pania (9.3 and 6.8% of leaf area infected respectively), and least on Quantum (1.6%). The mean disease relative growth rate was greatest (P<0.001) for Quantum, but was delayed compared to Pania and Bolero. Gompertz growth curves were fitted to disease progress data for leaves at old nodes. The asymptote was 78.2% of leaf area infected on Quantum, significantly lower (P<0.001) than on Bolero or Pania, which reached 100%. The point of inflection on Quantum occurred 22.8 dai, later (P<0.001) than on Pania (18.8 dai) and Bolero (18.3 dai), and the mean disease severity at the point of inflection was 28.8% for Quantum, less (P<0.00l) than on Pania (38.9%) or Bolero (38.5%). The average daily rates of increase in disease severity did not differ between the cultivars. Disease progress on Quantum was delayed compared with Pania and Bolero. Disease gradients from inoculum foci to 12 m were detected at early stages of the epidemic but the effects of background inoculum and the rate of disease progress were greater than the focus effect. Gradients flattened with time as the disease epidemic intensified, which was evident from the large isopathic rates (between 2.2 and 4.0 m d⁻¹) Some epidemiological variables expressed in controlled environments (low infection efficiency, low maximum daily spore production and long time to maximum spore production) that characterised quantitative resistance in Quantum were correlated with disease progress and spread in the field. These findings could be utilised in pea breeding programmes to identify parent lines from which quantitatively resistant progeny could be selected.