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.