Lentil anthracnose: epidemiology, fungicide decision support system, resistance and pathogen races

Abstract

Colletotrichum lentis causes anthracnose of lentil (Lens culinaris) in Canada that results in defoliation, stem girdling and severe yield losses, but the disease is rarely reported elsewhere. The pathogen survives as microsclerotia on lentil debris for up to 3 years when buried in the soil, but loses viability on the soil surface. Windborne debris spreads the pathogen to neighbouring fields, while seedborne infection is less important. Foliar fungicides were registered, and a fungicide decision support system was developed which assessed disease risk with 85% accuracy. Around 2300 L. culinaris accessions from 50 countries were screened for resistance. Congruently, two races – Ct1 and Ct0 – were identified on differential lentil lines. Resistant lines were generated by cycles of inoculation and selfing of single resistant plants which resulted in the following three accessions resistant to both races: VIR2633 (Georgia), VIR2058 and VIR2076 (Czech Republic), while six and 49 lines had resistance to Ct0 and Ct1, respectively. Ct1 resistance is controlled by recessive and dominant genes crt1 and CtR3 in variety ‘Indianhead’, ctr2 and CtR5 in accession PI345629 and CtR4 in PI320937. Molecular markers linked to Ct1 resistance were identified on linkage group six, close to Ascochyta lentis resistance, and were used to combine resistance to both pathogens in breeding lines. Two repeat rich regions in the intergenic spacer (IGS) of ribosomal DNA can be used to differentiate the two C. lentis races. Utilizing length polymorphisms in a 39 nucleotide repeat region showed the races were equally frequent among isolates collected between 1991 and 1999, while 95% belonged to race Ct0 in 2010, likely because lentil varieties are susceptible to race Ct0, but around one-third of the varieties had Ct1 resistance.