Survival of secondary sporidia of floret-infecting Tilletia species: implications for epidemiology.

Secondary sporidia of Tilletia horrida, T. indica, and T. walkeri initiate local infection of rice, wheat, and ryegrass florets, respectively, leading to disease in seed. Secondary sporidia are considered to be fragile and short lived. To examine this, secondary sporidia from agar cultures of these species were naturally discharged onto petri dish lids and were air-dried and maintained in the laboratory at 10 to 20% relative humidity (RH) at 20 to 22 degrees C, and at 40 to 50% RH at 18 degrees C. Lids were periodically inverted over fresh agar to determine viability of dried sporidia. Sporidia held 31 to 49 days at 10 to 20% RH and 56 to 88 days at 40 to 50% RH regenerated rapidly. Commonly, 18 h after lids with dried sporidia were inverted over agar, newly produced secondary sporidia had discharged onto the agar and produced extensive hyphal growth. There was no difference in the viability of sporidia that were initially dried rapidly or dried slowly over 10 h. Sporidia of T. horrida or T. indica dried on petri dish lids placed in the lower canopy of barley or wheat fields in Idaho and Arizona during early flag leaf to soft dough stages and held until crops were near or beyond maturity regenerated rapidly despite temperatures up to 46 degrees C and several days of RH < 20%. These results suggest that sporidia produced well prior to susceptible growth stages of the host can lay dormant in very dry field environments and then rapidly regenerate under humid rainy conditions associated with the diseases.