Host, pathogen and environmental effects on incomplete resistance to anthracnose, Colletotrichum truncatum, in lentil, Lens culinaris
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Incomplete resistance has recently been identified in a few lentil (Lens culinaris) lines/cultivars to anthracnose (Colletotrichum truncatum). The effectiveness of this incomplete resistance in controlling anthracnose, the responses of incompletely resistant lines/cultivars to the pathogen under various environmental factors favourable to anthracnose development and the components of incomplete resistance were investigated in one or more of five incompletely resistant lines/cultivars Indianhead, 458-57, PI 299331, PI 320937, PI 345629 and the susceptible cultivar Eston. Line 458-57 was evaluated for control of anthracnose with and without applications of the fungicide chlorothalonil in fields with low, moderate and high anthracnose levels. Yield losses in untreated plots across disease pressures ranged from 26-57% in Eston and 20-28% in 458-57. Line 458-57 with and without chlorothalonil was most effective in reducing disease severity (DS) and yield losses over those of Eston at high disease pressure where the effectiveness of incomplete resistance was better or equal to a single application of 1 or 2 kg a.i./ha chlorothalonil to Eston In a study with Indianhead, 458-57 and Eston inoculated with a single isolate, DS was found to increase with temperature from 16 to 28$\sp\circ$C, inoculum density (up to 1 $\times$ 10$\sp5$ conidia/ml) and longer leaf wetness period (up to 48 h), and plants were more susceptible at early flowering. In all the tests, DS was reduced on the incompletely resistant lines/cultivars compared to Eston. Analysis of the components of resistance in the six lines/cultivars in the field and growth chamber using one isolate of C. truncatum indicated that incomplete resistance was characterized by longer incubation and latent periods, fewer and smaller lesions and reduced sporulation. Disease severity and the area under the disease progress curve values were also lower on incompletely resistant lines/cultivars than on Eston.