Identifying Pathogens of Switchgrass and Investigating Antimicrobial Activity of Switchgrass-Derived Extractives

Switchgrass is an increasingly important biofuel crop, but knowledge of switchgrass fungal pathogens is not extensive. The purpose of this research was to identify the fungal pathogens that decrease crop yield of switchgrass grown in Tennessee and to investigate a potential value-added by-product of the switchgrass biofuel conversion process. The specific objectives were 1) to identify and characterize prevalent fungal pathogens of switchgrass in Tennessee, 2) assess switchgrass seed produced in the United States for seedborne fungal pathogens, and 3) evaluate switchgrass extractives for antimicrobial activity against plant pathogens.

Diseased switchgrass samples were collected from several locations in East Tennessee. Tissue was surface-sterilized and plated on potato dextrose agar (PDA), or water agar. Emergent fungi were isolated and identified based on colony, physiological, and molecular characteristics. Pathogenicity was confirmed with Koch's postulates in growth chamber studies. Nine pathogenic species were identified, several of which had not been reported on switchgrass previously.

To assess presence of seedborne pathogens, seed of seven cultivars from 11 commercial seed production companies located in the United States were tested, including multiple entries of ‘Alamo’, ‘Blackwell’, ‘Cave-in-Rock’, and ‘Kanlow’. Three hundred surface-sterilized seed per lot were plated on PDA. Rates of fungal infection among the 30 sampled seed lots varied from less than 1 % to 87%. The most frequently observed pathogens were Bipolaris oryzae, Alternaria alternata, and Fusarium graminearum/ pseudograminearum. Additional species of Bipolaris and Fusarium were present with less frequency.

To test the switchgrass extractives for antimicrobial activity, six ethanol-soluble extractives treatments, extracted from switchgrass collected from three farms at two different plant ages, were tested in a 3 × 2 factorial design for activity against four bacterial plant pathogens. Significant differences were shown for the main effect of bacteria (P=0.09) and the interaction effect of farm × plant age (P=0.04). Xanthomonas perforans was more sensitive to the extractives treatments than Pseudomonas syringae pv. tomato and extractives from 112 day-old switchgrass from farm C04 inhibited bacterial growth more than the extractives from 112 day-old switchgrass from farms C19 and C33.