No abstract

In tunnel experiments, the efficacy of dicarboximide sprays in controlling grey mould of strawberries was greatly decreased by the presence of dicarboximideresistant forms of Botrytis cinerea. The use of dichlofluanid, as a tank-mix or in an alternating programme, with a dicarboximide fungicide, procymidone, helped to maintain the efficacy of disease control but failed to prevent an increase in the proportion of dicarboximide-resistant forms of the pathogen.Alternative 'partner' fungicides (thiram, chlorothalonil) delayed build-up of resistance to dicarboximides. Build-up of resistance was absent or relatively small in unsprayed plots. Application of dichlofluanid alone was always associated with a substantial increase in dicarboximide resistance, although less than in procymidone-treated plots. Monitoring dicarboximide resistance in the tunnels during the winter, when no further sprays were applied, revealed a gradual decline in the proportion of dicarboximide-resistant forms in all previously treated plots.In laboratory studies on inoculated leaf debris, dichlofluanid treatment induced the build-up of dicarboximide-resistant forms of B . cinerea. Leaf-disc tests revealed cross-resistance of dicarboximide-resistant isolates towards dichlofluanid but not towards thiram or chlorothalonil. Dichlofluanid is widely used for control of B . cinerea and the implications of these results for the practical management of dicarboximide resistance in this pathogen are discussed.

No abstract

In crops of winter barley, cvs Igri and Halcyon, crop structure and intensity of foliar diseases were altered by delaying the main top dressing of nitrogen until mid‐April. A single application of fungicide then was sufficient to decrease substantially the severity of fungal disease and improve grain yield at harvest. Disease control and yield responses were greatest when fungicides were applied during a 3‐week period after the main nitrogen application in April. No additional yield benefits were obtained by making two fungicide applications at growth stage (GS) 31 and GS 39.

At Long Ashton Research Station serious outbreaks of fireblight (caused by Erwinia amylovora) occurred on apples in 1978, 1980 and 1982. and on pears in 1979, Mid‐season and late‐flowering cider apple cultivars were particularly affected. On cider apple, outbreaks were associated with years of profuse blossom coinciding with unusually warm temperatures during bloom. Spread of infection between hosts, including hawthorn and cotoneaster, was undoubtedly important. These and other factors underlying the outbreaks are discussed.