Background

The results of six studies using precision methods to obtain quantitative yield-loss relationships are reported. Losses ranged from 100 per cent for Pyrenophora graminea on barley, to 51 per cent in a simulated grazing trial on wheat, 20 and 8 per cent respectively for Fusarium nivale and Rhizoctonia solani causing culm lesions on wheat, and 12 per cent for yellow rust on barley. Evidence of interactions between seed treatment with mercury, E. nivale infection and mildew on barley is also presented, in which seed treatment was beneficial regardless of seed infection, and E. nivale intensified the loss due to mildew. 180 m^t 45 m^t 6 X 45 mplots 2 ha field 2 ha field 4 X 45 m'' plots YieM-loss Relationships in Cereals (see p. 21) Upper. Apparently healthy controls. Middle. Fusarium nivale lesions. Lower. Sharp eyespot {Rhizoctonia solani) lesions.

SUMMARY Very severe infection of several crops of spring barley by Cochliobolus sativus occurred in Scotland during 1971–73. The most severely infected cultivar was Clermont, the high, susceptibility of which, combined with extensive seed infection, was responsible for the outbreak. Seed‐borne infection by C. sativus was not wholly controlled by mercurial seed treatments, although no mercury tolerance was detected. The fungus is able to overwinter in the soil in Scotland and can also survive by infecting other graminaceous hosts. Several of these are believed to be new host records for this fungus in Britain.

Susceptibility to smut disease in sugarcane is normally estimated in small plots raised from inoculated buds. Standardisation of inoculation techniques (particularly concentration of spores) and of measuring infection are shown to be important. Due to the variable pattern of symptom development and the statistical limitations of small sample numbers, assessment of infection must be made on a stalk basis to be meaningful. Parallel quantitative field survey data of major varieties during the Guyana epidemic has enabled inoculation tests to be related to natural infection to give thresholds of susceptibility below which a variety is unlikely to generate an epidemic, and which can therefore be considered safe for more extensive planting.

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