Relative Host Resistance to Black Spot Disease in Field Pea (Pisum sativum) is Determined by Individual Pathogens

Black spot, also known as Ascochyta blight, is the most important disease on field pea (Pisum sativum). It is caused by a complex of pathogens, the most important of which in Australia include Didymella pinodes, Phoma pinodella, and P. koolunga. The relative proportions of these and other component pathogens of the complex fluctuate widely across time and geographic locations in Australia, limiting the ability of breeders to develop varieties with effective resistance to black spot. To address this, 40 field pea genotypes were tested under controlled environment conditions for their individual stem and leaf responses against these three pathogens. Disease severity was calculated as area under disease progress curve (AUDPC), and subsequently converted to mean rank (MR). The overall rank (OR) for each pathogen was used to compare response of genotypes under inoculation with each pathogen. The expressions of host resistance across the field pea genotypes were largely dependent upon the individual test pathogen and whether the test was on stem or leaf. Overall, P. koolunga caused most severe stem disease; significantly more severe than either D. pinodes or P. pinodella. This is the first report of the host resistance identified in field pea to P. koolunga; the five genotypes showing highest resistance on stem, viz. 05P778-BSR-701, ATC 5338, ATC 5345, Dundale, and ATC 866, had AUDPC MR values <250.4, while the AUDPC MR values of the 19 genotypes showing the best resistance on leaf was less than 296.8. Two genotypes, ATC 866 and Dundale, showed resistance against P. koolunga on both stem and leaf. Against D. pinodes, the four and 16 most resistant genotypes on stem and leaf had AUDPC MR values <111.2 and <136.6, respectively, with four genotypes showing resistance on both stem and leaf including 05P770-BSR-705, Austrian Winter Pea, 06P822-(F5)-BSR-6, and 98107-62E. Against P. pinodella, four and eight genotypes showing the best resistance on stem and leaf had AUDPC MR values <81.3 and <221.9, respectively; three genotypes, viz. 98107-62E, Dundale, and Austrian Winter Pea showed combined resistance on stem and leaf. A few genotypes identified with resistance against two major pathogens of the complex will be of particular significance to breeding programs. These findings explain why field pea varieties arising from breeding programs in Australia fail to display the level or consistency of resistance required against black spot and why there needs to be a wider focus than D. pinodes in breeding programs.