The use of rainfall and accumulated mean temperature to indicate fungicide activity in the control of leaf diseases of winter wheat in the UK1

During 1988/1990, a series of 21 experiments was established in commercial crops of winter wheat. Chlorothalonil, fenpropimorph and propiconazole were chosen as protectant, eradicant or curative fungicides active against leaf diseases of winter wheat in the UK. To test their properties each one was applied once only to separate plots during a period of 7–8 consecutive weeks in May and June (GS 32–39). Disease progress was assessed weekly on adjacent unsprayed control and sprayed plots up to GS 85. Septoria tritici leaf blotch (Mycosphaerella graminicola) was the disease that occurred most frequently and severely across the 21 sites. Powdery mildew (Erysiphe graminis), brown rust (Puccinia recondita) and yellow rust (P. striiformis) occurred at fewer sites and were sufficiently severe to distinguish differences between the active ingredients at only two or three sites. Analysis of the disease progress curves for 75% control of each disease at one site only indicated that chlorothalonil possessed very good protectant but shorter‐term eradicant activity against M. graminicola and P. striiformis. Fenpropimorph exhibited only short‐term eradicant control of M. graminicola, but gave excellent protectant and eradicant control of E. graminis and P. striiformis; against P. recondita only eradicant activity was apparent. Propiconazole showed activity similar to that of fenpropimorph against P. recondita and excellent protectant and eradicant activity against M. graminicola and P. striiformis; against E. graminis, it gave good protectant and eradicant control. From disease progress curves, it was possible to calculate the period of protectant and eradicant activity in thermal time (accumulated degree days above zero) for each of the three active ingredients and to identify the most effective timing(s) for fungicide application in relation to rainfall or imputed infection.