Development of Fungal Brown Spot and Spot Blotch on Cultivated Wild Rice in Minnesota

Symptoms of fungal brown spot of cultivated wild rice (Zizania palustris), caused by Bipolaris oryzae, initially occurred on the flag and lower aerial leaves of wild rice at the boot stage of development; whereas spot blotch symptoms, caused by B. sorokiniana, first occurred at the floating leaf stage of plant development on both the floating leaf and the first aerial leaves. The percentage of B. oryzae isolated from all lesions did not increase significantly until early grain formation. Lesion numbers then increased rapidly until plant maturity, when B. oryzae was isolated from 36.9 and 49.3% of all flag and bottom aerial leaf lesions, respectively. On both the flag and lower aerial leaves, the percentage of lesions yielding B. sorokiniana increased slowly until early grain formation, then increased rapidly until plant maturity, 17.6 and 14.1% respectively. Numerous spots on the floating and first aerial leaves, previously thought to be caused by Bipolaris spp. infections, were caused primarily by Nakataea sigmoidea, Colletotrichum spp., and Phoma spp. The number of conidia of B. oryzae produced per lesion under laboratory conditions was greatest from lesions on lower leaves from early-mid flowering until plant maturity. The number of conidia of B. sorokiniana from lesions on both upper and bottom leaves increased until early-mid flowering, then remained relatively constant until plant maturity except that the number of conidia from lesions on bottom leaves declined.