Characterization of Fungal Species Associated with Ascochyta Blight of Dry Pea in Montana and North America and Development of a Differential Medium for Their Detection

Abstract

Montana leads the production of dry pea in the United States. About 530,000 acres were planted to pea in 2019, accounting for 48% of the total national production ( USDA-NASS 2019 ). A predominant foliar disease of dry pea in Montana is Ascochyta blight, which is caused by multiple fungal pathogens including Didymella pisi, Peyronellaea pinodes, and Peyronellaea pinodella. D. pisi is the predominant pathogen causing Ascochyta blight of dry pea in Montana. Recently, an anticipated shift in pathogen composition has been observed in northeastern Montana from D. pisi to P. pinodes. Also, a Phoma sp. was found associated with infected dry pea seeds and included in this study. To characterize these fungi, we evaluated the effects of temperature (15, 20, 25, and 30°C) on mycelial growth rate and sporulation. The optimum temperature for mycelial growth and sporulation was either 20 or 25°C depending on the species. Analysis of variance supported that at all evaluated temperatures, Phoma sp. had the highest growth rate and produced more spores than the other species (P value < 0.001). In pathogenicity assays, P. pinodes caused more severe disease than the other species when inoculated on pea plants (cv. Carousel, P value ≤0.001). The Phoma sp. was not pathogenic. Peameal agar (PMA) was developed as a diagnostic tool for these pathogens. On PMA, the fungal species showed different mycelial morphology, which was used to visually discriminate them. Results from this study will be used as a base to understand the adaptability, pathogenicity and aggressiveness, and current status and changes in the population composition of fungal species causing Ascochyta blight of dry pea in Montana and North America.

Genome and Transcriptome Analysis of Ascochyta pisi Provides Insights into the Pathogenesis of Ascochyta Blight of Pea

“…Until now, at least seven fungal species have been identified as the causative agents of ascochyta blight of pea ( 2 , 3 , 5 – 7 ). They are soilborne, pathogenic fungi and survive as chlamydospores, mycelia, or sclerotia in soil, and they can coexist on peas or exist independently from each other ( 9 ). However, the component pathogens of this disease complex are varied in different pea-growing regions.…”

“…For instance, A. pinodes and Phoma koolunga are the dominant pathogens of the ascochyta blight complex in the Canadian prairies and southern Australia, while A. pisi is the predominant causal pathogen in North America, western Canada, and southern France ( 4 , 30 , 31 ). These geographical population structures of the ascochyta blight complex might be determined by the precipitation temperature, the ability for chlamydospore formation, and the exchange of seed germplasms ( 9 , 13 , 31 ). In our previous study, A. pinodes was isolated from ascochyta blight lesions on field peas in Zhejiang Province, China ( 3 ).…”

“…These pathogens cause similar symptoms on pea plants, with foot rot, black stem and leaf, and pod spot ( 3 ). The identification of pathogens was based largely on morphological characteristics such as growth, colony formation, and pigment on media; the shape and size of conidia; conidial characteristics; survival structures; and toxin production during the early days ( 8 , 9 ). Sequence-based molecular analyses are now routinely used to identify the Ascochyta complex and analyze the population structures using marker genes such as the internal transcribed spacer (ITS) region, the RNA polymerase II second-largest subunit (RPB2), the partial β-tubulin (TUB2) gene region, partial 28S large-subunit (LSU) nuclear ribosomal DNA (nrDNA) sequences, and microsatellite markers ( 5 , 7 , 9 – 13 ).…”

Genome-wide association study for morphological traits and resistance to Peryonella pinodes in the USDA pea single plant plus collection

“…Ascochyta blight is a disease complex caused by several related fungal pathogens individually or in combination ( Bernard Tivoli et al 2007 ). In North America, the species associated with the disease are Didymella pisi , Peyronellaea pinodella , and Peyronellaea pinodes ( Skoglund et al 2011 ; Owati et al 2020 ); the last of these being the most aggressive and prevalent worldwide ( Rubiales et al 2019 ; Xue et al 1997 ; Bretag et al 1995 ; Tivoli et al 1996 ). Pathogens are very hard to discriminate based on the symptoms they cause, as all of them cause black to brown spots or lesions on leaves, stems, and pods ( Skoglund et al 2011 ).…”

“…Pathogens are very hard to discriminate based on the symptoms they cause, as all of them cause black to brown spots or lesions on leaves, stems, and pods ( Skoglund et al 2011 ). However they can be distinguished based on other characteristics such as spore characteristics, appearance on culture plates, and toxin production ( Owati et al 2020 ).…”