| Late blight, incited by the fungus Septoria apiicola , is the most important foliar disease of celery in Michigan and results in necrotic lesions on leaves and petioles that can reduce yields by up to 80%. Most celery growers begin applying the fungicide chlorothalonil one to three weeks after transplanting and reapply it at 7- to 10-day intervals to protect the crop, but some of these applications may not be needed early in the season or when environmental conditions do not favor disease development. The purpose of this research was to evaluate late blight management programs that reduce fungicide use and incorporate a new reduced-risk fungicide, azoxystrobin, which poses a lower risk to non-target organisms and the environment than chlorothalonil.; Disease predictors were evaluated, using a standard and a reduced-risk fungicide program, for managing late blight on 'Dutchess' celery in 2003 to 2005. The TOM-CAST 10-DSV predictor provided disease control that was comparable to the weekly application program during each year. It required up to five fewer sprays, while reducing fungicide costs up to {dollar}215/ha, compared to the weekly program. Other disease predictors, such as the Septoria predictor, Cercospora predictor, and TOM-CAST 15-DSV, also provided control similar to the weekly program, but their efficacy was less consistent than TOM-CAST 10-DSV and was often dependent on the fungicide program. The fungicide programs frequently provided similar control, but azoxystrobin alternated with chlorothalonil was more effective than chlorothalonil alone when disease pressure was high in 2003.; Weekly sprays of azoxystrobin alternated with chlorothalonil were initiated early (one week after transplanting), preventively (four weeks after transplanting), or when disease symptom were detected at a trace, 5%, or 10% level on 'Dutchess' celery in 2003 and 2004. Preventive applications required three fewer sprays, reduced fungicide costs by up to {dollar}134/ha, and provided similar control compared to the early fungicide program. Delaying the initial application until any level of disease developed subsequently resulted in unacceptable levels of disease at harvest and cannot be recommended in Michigan due to the risk of extensive yield loss.; A reduced-risk fungicide program was initiated preventively and reapplied weekly or according to the Septoria predictor or TOM-CAST 10-DSV and was compared with a weekly management program initiated early at a research farm and a commercial field in 2004 and 2005. Combining the use of preventive initial applications with disease predictors reduced the number of sprays by two to six while providing similar disease control compared with the weekly fungicide program initiated early. (Abstract shortened by UMI.)... |
