Fungicides for controlling leaf diseases of winter wheat: Evaluation, timing and importance of varietal resistance
About this project
During the three years 1991-1993, a series of three related collaborative experiments investigated the interaction between fungicides and cultivar resistance to leaf disease to provide guidance on improvements to wheat disease control. The three multi-site experiments evaluated the biological properties of new fungicide molecules, assessed the value of cultivar partial resistance to disease and compared single sprays of new fungicide products with a set of standard programmes.
In the first experiment, the mildew susceptible cultivar Apollo and the Septoria susceptible Riband were each grown at two sites in each year. Single sprays of fungicide were applied at successive seven day intervals from early May until mid June. Additional plots were treated with a second application of the same fungicide three weeks later from mid May until late June.
Differences between fungicides for control of Septoria tritici were smaller than the differences between spray intervals. For the single treatments disease control and yield increases were greatest in 1992 and 1993 for treatments applied during the second two weeks of May and in 1991 in early June. For the two-spray treatments benefits were greatest when the second spray was applied to plots first treated during May and each of these two-spray treatments gave comparable disease control and yield benefits. They were generally superior to those of the single sprays on Riband but not on the Apollo.
On Riband severe S. tritici developed at each of the sites in each year. The time of infection of each of the final three leaves (the flag leaf and the two leaves below) was related to specific rain splash events at most sites. In each case a single well timed spray gave disease control comparable to the best of the two-spray treatments on each of the final leaves.
The mildew (Erysiphe graminis) epidemics on Apollo were generally less severe than the Septoria epidemics on Riband at Morley and ADAS Rosemaund. The best single spray treatment generally controlled disease on each of the final three leaves as well as the best two-spray treatment. There was a very small effect of the early single sprays (those applied before the start of flag leaf emergence) on final disease levels of both Septoria and mildew on both cultivars. Where mildew was severe later in the season, there was a yield benefit when the first spray of a two-spray treatment was applied before stem extension, in early May.
In the second experiment a set of six standard one, two and three-spray programmes was compared with a targeted disease management treatment using the ADAS Managed Disease Control system on the four cultivars Riband, Beaver, Apollo and Hereward. Each cultivar was selected for their specific differences in host partial resistance. The three-spray programme, with treatment at second node detectable, flag leaf emerged and ear emerged gave the best overall disease control and greatest yield benefit on all cultivars but was the most cost effective treatment only on the disease susceptible cultivar Riband. By contrast, the single flag leaf emerged treatment generally proved to be most cost effective on Beaver and Hereward. On the mildew susceptible Apollo, the reduced dose treatment, incorporating an early (second node detectable) application of a mildew fungicide was the most profitable treatment. The targeted treatment was the most cost effective programme on 40% of the sites on Riband but on only 20% of the Hereward sites. The results indicate that targeted disease control is difficult. Operators are risk averse and tend to err on the side of overuse of fungicide.
The third experiment used the mildew susceptible Apollo at five sites, Mercia was grown at one site in two years with the Septoria susceptible cultivar Riband at seven sites. It provided independent information on new fungicides before they reached the market place and compared their performance as single sprays applied at flag leaf emerged with a set of standard one, two and three-spray programmes.
In each year the new conazole fungicides, tebuconazole and cyproconazole, gave superior disease control and yield response on both cultivars compared with the standard single spray treatment when applied at flag leaf emerged. These products were particularly effective on the Riband sites. Where severe epidemics of Septoria developed. The yield and disease control following treatment with these products was superior to that provided by the standard treatment and equal to that of either of the two-spray programmes used.
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