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Isolates of Venturia inaequalis were sampled from monoculture and mixed orchards of three apple cultivars: Bramley, Cox and Worcester. In addition, single-ascospore progeny isolates were obtained from three crosses between pairs of isolates originating from the three cultivars in monoculture orchards. These field isolates and single-ascospore progenies were inoculated onto each of the three cultivars in a glasshouse. The patterns of infection showed that all three cultivars, commonly regarded as susceptible to scab, contained some specific resistance factors and that scab isolates from both mixed and monoculture orchards appeared to have co-evolved with cultivars. A much higher proportion of isolates from cv. Worcester in the mixed orchard were unable to infect any of the three cultivars than isolates from any other combination of cultivar and orchard type, but there was no other difference between isolates from mixed vs. monoculture orchards. Many isolates could infect both Bramley and Cox, or Cox and Worcester; but only a single isolate could infect both Bramley and Worcester. Results from the testing of 61 single-ascospore progeny isolates suggested that virulence towards Bramley, Cox and Worcester was controlled by at least one, two and three factors, respectively. Moreover, the exact nature of the interactions between these factors in determining virulence depended on the particular pair of isolates concerned.

Apple scab, caused by Venturia inaequalis, is one of the most of damaging diseases worldwide on apple and currently is managed mainly by scheduled applications of fungicides. Understanding pathogen population structure is important for breeding and deployment of resistant cultivars. Isolates of V. inaequalis were sampled from a number of cultivars in China, India, and the United Kingdom to estimate differences in pathogen populations. Amplified fragment length polymorphism (AFLP) markers were used to genotype isolates, mostly from China and the United Kingdom. The AFLP data indicated that, overall, there were significant differences in V. inaequalis populations from China and the United Kingdom. Within China, there was no significant differentiation associated with their geographical or cultivar origins. In contrast, populations from four cultivars in two U.K. orchards (monoculture of Gala and a mixture orchard of Bramley, Cox, and Worcester) differed significantly. Furthermore, populations from Gala and Worcester were more homogenous than expected but those from Cox were more diverse than expected. In total, 80 isolates were selected randomly from three countries for virulence testing: 20 from the United Kingdom (10 from Gala and 10 from Cox), 30 from China (10 from Gala, 10 from Fuji, and 10 from Qingquan), and 30 from India (10 from Gala, 10 from Golden Delicious, and 10 from Black Ben Davis); of these 80 isolates, 41, 47, and 59 were inoculated against each of these cultivars in the United Kingdom, India, and China, respectively. The two local cultivars from India (Black Ben Davis) and the United Kingdom (Cox) were more resistant against non-indigenous isolates, particularly those from China, than they were against indigenous isolates; the Chinese local cultivar (Qingguan) showed a higher general level of resistance against isolates regardless of their origin.

Ri bonuclease, deoxyribonuclease, acid phosphatase and phenoloxidase were detected in preparations of extracellular melanoprotein isolated from cultures of Venturia inaequalis. The high initial levels of activity produced per g fungus declined within a few days of inoculation, to an approximately constant level. About six times as much activity was released by conidia germinating in medium enriched with wood extract than was released in basal medium. After 5 d incubation, the rate of production of enzymically active melanoprotein reflected the rate of growth of the fungus. Isolated melanoprotein stored at 4 "C for 3 months showed up to 190% more hydrolase activity than was measured originally, but longer storage caused a subsequent decrease in activity. Only phenoloxidase decreased continuously during storage. The apparent stability of the bond between melanin and protein under dissociating conditions contrasted with the reversible formation of complexes between the fungal product and polyelectrolytes.A set of re-isolates of different ages produced randomly variable levels of hydrolase activity when cultured repeatedly over a period of 16 months. On each occasion, however, the specific and total activity of each hydrolase was (with one marginal exception) lower in cultures of the oldest re-isolate than in those of the later re-isolates. In contrast, the specific activity of phenoloxidase was highest in cultures of the oldest re-isolate. These changes were associated with the decline in virulence observed in stored re-isolates of clone E, of V. inaequalis. I N T R O D U C T I O NExtracellular enzymes associated with melanoprotein produced by cultures of Venturia inaequalis were studied with a view to elucidating the biological stability of the pathogen. An attempt was also made to correlate the various levels of enzyme activity produced in culture by different clones with the corresponding levels of virulence observed on test plants. METHODS Growth of V. inaequalis andpreparation of extracellulav melanoproteins.Stock cultures of each isolate were maintained under paraffin oil at 4 "C (Kirkham, 1957). Using the methods of Hignett et al. (1977), extracellular melanoprotein was isolated from 40 ml shake cultures of V. inaequalis and fractionally precipitated with ethyl alcohol giving two fractions (N50 and N75). The cultures were usually grown on basal medium augmented with diffusible neutral wood extract (NWED) CHignett et a [., 1977). For cultures grown on basal medium without NWED, 0.05% (w/v) asparagine was added as an alternative nitrogen source. Growth curves were obtained by taking duplicate 40 ml shake cultures at intervals up to 18 d after inoculation. The mycelium was filtered off, washed exhaustively and dried to constant weight over phosphorus pentoxide. Non-diffusible material in the culture fluid was isolated by dialysis at 4 "C against five changes of 1 1 water

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