Effects of fungicide spray timing on winter wheat disease control

A field experiment of novel systematic design was established on 17 sites i n the three years 1988-90 to compare the effect of spray timing on the dise ase control provided by fungicides. Ar each site one set of plots received applications of a mixture of fenpropimorph and prochloraz or triadimenol, t ridemorph and chlorothalonil fungicides, applied at successively later star t dates to produce a series of temporally and spatially related, but discre te, epidemics curtailed at consecutively later stages of development betwee n GS32 and GS75. A second set of plots was treated with a single spray of o ne of a set of the nine candidate fungicides between GS32 and GS71. Disease developed at each site, with moderate septoria leaf spot (Septoria tritici ) at five of the eight sites in 1988 and four of the seven sites in 1989. E rysiphe graminis f.sp. tritici was present at two sites in 1988, four sites in 1989 and one site only in 1990. Puccinia striiformis (four sites only i n 1989 and 1990) and brown rust (one site in each year) were less common. T he systematic treatments showed consistent relationships between green leaf retention during grain filling and yield. They also provided clear indicat ions that epidemics of foliar disease initiated before flag leaf emergence had the greatest impact on yield. After this stage, yield loss averaged 27. 2 kg ha(-1) for each day that elapsed before fungicide was applied. The seq uential single spray treatments showed marked differences between the prote ctant and eradicant activity of the different fungicides used. Each disease was most effectively controlled on the final leaf by treatment at or immed iately after its emergence. Chlorothalonil was as effective as any of the f ungicides tested against septoria, when applied before flag leaf emergence, but was inferior when applied after this stage. The fungicides were equall y effective against yellow rust when applied just before symptoms were dete cted, when spray timing appeared to be more important than choice of fungic ide. The response of brown rust to fungicides was similar, except that chlo rothalonil was the least effective. For mildew, timing seemed to be less im portant, although sprays applied immediately after leaf emergence provided the best control on each individual leaf layer. Yield was well related to d isease and green leaf area late in the season, these factors being more imp ortant than which disease was present, which fungicide, and which cultivar were used.