Controlled environment assessment of partial resistance to Mycosphaerella pinodes in pea (Pisum sativum L.) seedlings

We evaluated the effect of various genotypes of pea (Pisum sativum L.) to delay the development of ascochyta blight, caused by the fungus Mycosphaerella pinodes, at the seedling stage under controlled conditions. We assessed leaflets, stipules and stems, separately. We compared genotypes on the basis of four variables assessing resistance: incubation period (IP), appearance of disease scoring 3 (DS3), disease progress rate (r(d)) and area under the disease progress curve (AUDPC). For leaves, a continuous variation was observed between pea lines in disease severity at all times and disease progress curves were similar for the various genotypes. Partial resistance was defined as a delay in first symptom appearance and disease development. Resistance factors are thought to be involved in the very early stages of the interaction. Similar results were obtained for stipules and leaflets, suggesting similar mechanisms of resistance in these two organs. Stem and leaf resistances were also strongly correlated. We found that IP could discriminate genotypes but that factors associated with disease progress (DS3, AUDPC and r(d)) were more informative. We suggest that further genetic analyses should be based on growth-chamber methods, with AUDPC as the variable measured.