Yield responses to the application of foliar fungicides to wheat are highly variable. As the cost of treatment is known, some method of estimating future yield response is required if treatments are to be applied only when the value of the yield beneÐt will exceed the cost. Treatment decisions are often supported by disease thresholds, either formalised or developed by crop managers through practical experience. In farm practice, the proportion of yield that is due to fungicide treatment is not usually known, so the success of a spray decision is often judged by the level of disease later in the season. This paper presents data from Ðeld experiments in 1994 and 1995, showing that variation in the current level of visible disease (yellow rust and Septoria tritici) explains little of the variation in future e †ects of the disease on the host, and that the yield e †ect of a unit disease is not constant across sites and seasons. These Ðndings suggest that traditional disease thresholds may be unreliable predictors of the need to spray and that estimating the success of a treatment decision by observing disease levels later in the season is prone to error. A "negative thresholdÏ, deÐned as that level of disease below which an economically damaging epidemic cannot develop within a known time-period, may be more reliable. Below the negative threshold, treatment is not required. Above the negative threshold, other factors a †ecting the rate of epidemic development and sensitivity of the host to green leaf area loss need to be considered, in order to quantify the need for treatment. Measurements which reÑect the cropÏs ability to intercept solar radiation may prove more reliable tools than percentage disease for judging the success of treatment decisions and, experimentally, for quantifying the e †ect of variation in risk determinants.

Rendezvous is a new cultivar of winter wheat with potent eyespot resistance derived from Aegilops ventricosa. In 11 field experiments conducted over 6 years with both natural and artificial inoculum, Rendezvous was significantly more resistant than other commercial wheat cultivars, many of which have resistance derived from Cappelle‐Desprez. It was also more resistant than its parent VPM 1, the donor of the potent eyespot resistance, suggesting that it possesses resistance genes from both Ae. ventricosa and Cappelle‐Desprez. In severe eyespot epidemics, Rendezvous lodged less and had a greater mean yield than the moderately resistant cultivar Norman. Treatment with fungicide reduced disease and increased yield in both cultivars, so that they were equal in yield. Rendezvous should not need routine fungicidal treatment for eyespot control, but under exceptional disease pressure may benefit from treatment.

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Data from field experiments were used to test whether disease observations on lower leaves of wheat were good predictors of future epidemic development in the upper canopy. Fungicide treatments to replicated plots in 1994 and 1995 caused variation in levels of initial inoculum of urediniospores of yellow rust ( Puccinia striiformis ). Observations of symptom severity were made on individual leaf layers throughout the season. Sporulating lesions on lower leaves were considered to be a measure of inoculum source strength for transfer to upper culm leaves. Low source strengths were associated with delays in epidemic development on the upper leaves, as quantified by a location parameter, t m , the time at which severity reached half of the asymptote value in logistic fits to disease progress data on the upper leaves. However, there was much unexplained variation, probably due to extraneous variation in upward transfer efficiency of inoculum and rates of epidemic development. Vanderplank's sanitation ratio theory was used to account for variation in inoculum transfer and epidemic rate. The analysis revealed that if the aim is to predict disease on a newly emerged culm leaf, during the period when it can be treated effectively with fungicide, then observations of disease two leaves further down the culm were of the greatest predictive value ( R 2 = 86%). These observations, however, need to be integrated with information about factors affecting upward inoculum transfer and rates of epidemic development if acceptable predictive precision is to be achieved.

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