Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/12/2010
Publication Date: 7/1/2010
Citation: Goates, B. J. 2010. Survival of secondary sporidia of floret-infecting Tilletia species: Implications for epidemiology. Phytopathology 100:655- 662. Interpretive Summary: Survival of secondary sporidia of floret infecting Tilletia species under laboratory and field conditions: Implication for epidemiology Blair J. Goates, USDA-ARS, Aberdeen, ID The smut fungi Tilletia horrida, T. indica, and T. walkeri infect rice, wheat, and ryegrass respectively causing disease in seed. The diseases cause conversion of the seed into a mass of spores that are deposited in the soil during harvest. Spores are viable for years in soil. Spores germinate under proper conditions and form cells called sporidia that are forcibly discharged into the air and infect the developing seed. Sporidia are considered to be very fragile and short lived, giving rise to thoughts that spores must germinate during a short period during plant heading stages when plants are susceptible to infection. However, the research conducted in this study demonstrated that sporidia are quite durable during 4-5 weeks of 10-20% relative humidity in the laboratory, and also after several weeks in grain fields that included up to 15 days with very dry periods (<20% humidity) and eleven days with hot (>38C) periods. Dried sporidia produced new sporidia that had germinated to produce long hyphae 18 hours after the dried sporidia were rehydrated. It appears that sporidia produced by germinating spores well prior to susceptible heading stages can lay dormant in the field, even in adverse environments for extensive periods, and then rapidly regenerate to infect plants during humid rainy conditions normally associated with the diseases. This information provides a better understanding of the epidemiology of these diseases which will aid efforts to control them.
Technical Abstract: Abstract Secondary sporidia resulting from germinating teliospores of Tilletia horrida, T. indica, and T. walkeri initiate local infection of rice, wheat, and rye grass florets respectively, leading to disease in seed. Secondary sporidia are considered to be fragile and short lived. To examine this, secondary sporidia from potato dextrose agar and water agar cultures of these species were deposited via natural liberation onto lids of petri dishes and were air dried and maintained in the laboratory at 10-20% RH at 20-22°C, and at 40-50% RH at 18°C. Lids were periodically inverted over fresh agar to determine viability of dried sporidia. Sporidia held up to 31 to 49 days at 10-20% RH and 56 to 88 days at 40-50% RH regenerated rapidly. Commonly, 18 hours after lids containing dried sporidia were inverted over fresh PDA, newly produced secondary sporidia were deposited on the agar that had produced extensive hyphal growth. In separate tests, there was no difference in the viability of sporidia that were initially dried rapidly or dried slowly over 10 hours followed by incubation at 10% RH for four days prior to assessment. Experiments were also conducted under field conditions. Dried sporidia of T. indica or T. horrida on petri dish lids placed in the lower canopy of wheat or barley fields in ID and AZ during early flag leaf to soft dough stages, and held until crops were near or beyond maturity regenerated rapidly despite wide diurnal fluctuations in temperature up to 46°C and several days of RH < 20%. These results suggest that sporidia produced well prior to susceptible periods of the host can lay dormant even in very dry field environments for extensive periods and then rapidly regenerate under humid rainy conditions normally associated with the diseases.