Temporal and Spatial Changes in the Pea Black Spot Disease Complex in Western Australia

Black spot (also referred to as Ascochyta blight, Ascochyta foot rot and black stem, and Ascochyta leaf and pod spot) is a devastating disease of pea (Pisum sativum) caused by one or more pathogenic fungi, including Didymella pinodes, Ascochyta pisi, and Phoma pinodella. Surveys were conducted across pea-growing regions of Western Australia in 1984, 1987, 1989, 1996, 2010, and 2012. In total, 1,872 fungal isolates were collected in association with pea black spot disease symptoms. Internal transcribed spacer regions from representative isolates, chosen based on morphology, were sequenced to aid in identification. In most years and locations, D. pinodes was the predominant pathogen in the black spot complex. From 1984 to 2012, four new pathogens associated with black spot symptoms on leaves or stems (P. koolunga, P. herbarum, Boeremia exigua var. exigua, and P. glomerata) were confirmed. This study is the first to confirm P. koolunga in association with pea black spot symptoms in field pea in Western Australia and show that, by 2012, it was widely present in new regions. In 2012, P. koolunga was more prevalent than D. pinodes in Northam and P. pinodella in Esperance. P. herbarum and B. exigua var. exigua were only recorded in 2010. Although A. pisi was reported in Western Australia in 1912 and again in 1968 and is commonly associated with pea black spot in other states of Australia and elsewhere, it was not recorded in Western Australia from 1984 to 2012. It is clear that the pathogen population associated with the pea black spot complex in Western Australia has been dynamic across time and geographic location. This poses a particular challenge to development of effective resistance against the black spot complex, because breeding programs are focused almost exclusively on resistance to D. pinodes, largely ignoring other major pathogens in the disease complex. Furthermore, development and deployment of effective host resistance or fungicides against just one or two of the pathogens in the disease complex could radically shift the make-up of the population toward pathogen species that are least challenged by the host resistance or fungicides, creating an evolving black spot complex that remains ahead of breeding and other management efforts.