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Title: A degree-day model for the latent period of stagonospora nodorum blotch in winter wheat

Author
item ZEARFOSS, ASHLEY - North Carolina State University
item Cowger, Christina
item OJIAMBO, PETER - North Carolina State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/4/2011
Publication Date: 5/1/2011
Citation: Zearfoss, A., Cowger, C., Ojiambo, P. 2011. A degree-day model for the latent period of stagonospora nodorum blotch in winter wheat. Plant Disease. 95:561-567.

Interpretive Summary: Stagonospora nodorum blotch (SNB), which is caused by the fungus Stagonospora nodorum, occurs frequently in the southeastern United States, and severe epidemics can lead to substantial economic losses. The latent period of the fungus is the time interval between infection of the wheat plant and the production of spores that can create new infections. We studied the relationship between temperature and latent period. We found that latent period is best described in terms of degree-days, rather than time per se, indicating that the latent period becomes shorter at higher temperatures. Our findings will be applied to development of models to forecast when SNB is likely to reach economically damaging levels. This will be useful in helping wheat growers make fungicide applications profitable.

Technical Abstract: Stagonospora nodorum blotch (SNB), which is caused by Stagonospora nodorum, occurs frequently in the southeastern United States and severe epidemics can lead to substantial economic losses. To establish a model for the development of SNB based on the effects of temperature on the latent period of the pathogen relative to host growth, batches of two winter wheat cultivars (AGS 2000 and USG 3209) were inoculated with pycnidiospores of S. nodorum at weekly intervals for 16 weeks in 2009. After 72 h of incubation, plants were exposed to outdoor conditions where temperatures ranged from -6.6°C to 35.8°C, with a mean batch temperature of 9.7°C to 24.7°C. Latent period, expressed as time from inoculation until the first visible lesions with pycnidia, ranged from 13 to 34 days. The relationship between the inverse of the latent period and mean temperature was best described by a linear model, and the estimated thermal time required for the completion of the latent period was 138.9 degree-days. A shifted cumulative gamma distribution model with a base temperature of 0.5°C was used to describe the relationship between the number of lesions with pycnidia and accumulated thermal time. When defined as time to 50% of the maximum number of lesions with pycnidia ( L50 ), the latent period was estimated as 336 and 323 degree-days above 0.5°C for AGS 2000 and USG 3209, respectively. The relationship between 1/ L50 and mean temperature was also best described using a linear model (r2 = 0.93, P < 0.001). The shifted gamma distribution model developed in this study will be useful in predicting available thresholds of SNB which are based on lesions with pycnidia. This study also provides data that link wheat growth with disease progress, which will facilitate accurate identification of thresholds for timing of fungicide applications.