Applications of urea after harvest but before leaf-fall restricted perithecial production by Venturia inaequalis. Immersion of detached leaves in urea appeared to be the most effective method of preventing perithecial formation, although spraying attached leaves was equally effective when leaf abscission occurred within a week of treatment.A high nitrogen content within the leaf was one of the major factors contributing to suppression. Urea-treated leaves decomposed rapidly, thus destroying the overwintering substrate for the fungus.urea, followed by a second (pre-bud-burst) application at 2 yo, ascospore production in the spring was suppressed. The second treatment appeared to prevent the release of ascospores from mature perithecia.When apple plants (clone M. I I I ) were sprayed in autumn with 5

SUMMARYUsing phenylmercuric chloride (PMC) as the experimental fungicide it has been shown that spraying trees once in autumn at 0.05% and again just before bud‐burst at 0.01% reduced scab infection on blossom truss leaves by over 99% in the following spring. In another orchard 0.01% PMC sprays in autumn and just before bud‐burst, followed by fungicide sprays in spring and early summer, gave complete control of scab. In a third orchard, which was subject to contamination by ascospores from non‐treated apple trees, infection was reduced by 56.9% by one autumn and one pre‐bud‐burst spray of 0.01% PMC without subsequent anti‐scab sprays.

SUMMARY Treatment of detached apple leaves in October with urea induced a rapid and prolonged increase in the microbial populations present in the leaves during the winter. These effects were accompanied by a major shift in the population balance from a predominantly Gram‐positive, chromogenic type of microflora to one dominated by Gram‐negative and non‐chromogenic organisms. Fluorescent pseudomonads became particularly numerous in the urea‐treated leaves and many were found to suppress the development in vitro of Venturia inaequalis. An organism forming yellow colonies, possessing exceptional activity against the scab fungus, which was also found in urea‐treated leaves, was shown to be a Gram‐negative, peritrichous bacterium. No antagonistic organisms were isolated from untreated control leaves, but many were found–usually chromogenic forms–which appeared to stimulate the scab fungus in vitro. Treatment with urea accelerated the decomposition of the leaves and suppressed the development of V. inaequalis in the portions of the laminae which remained structurally intact until the following spring. Many of the effects of urea were enhanced by the addition of glucose, streptomycin, or a suspension of a Pseudomonas sp. isolated from leaves during the winter. Streptomycin alone reduced the total numbers of microorganisms in the leaves but increased the proportion of Gram‐negative organisms: this was associated with diminished ascospore production in the spring. There was no direct evidence that the antagonistic flora stimulated by urea was responsible for the inhibition of V. inaequalis, and alternative explanations are suggested.

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A selection of fungicides, herbicides and surfactants and urea were tested for their effect on the production of pseudothecia and ascospore release of Leptosphaeria maculans present on oil-seed rape straw and turnip seed crop straw. The fungicides ethyl mercury phosphate, triarimol, fenarimol, carbendazim, tridemorph and benomyl, each at 1 g/litre, the herbicides dinoseb and diquat, each at 10 g/litre the surfactants Bradasol, Cetrimide, Deciquam 222, Hyamine 1622 and Maxonol N, each at 50 g product/litre, and urea at 150 g/litre, applied to straw before pseudothecia had formed were more than 90% effective in preventing further development. These chemicals were also effective in preventing further ascospore production when applied to straw bearing mature pseudothecia but only dinoseb and urea prevented the release of mature ascospores. The results suggest that it may be possible to break the life cycle of L. maculans by chemical treatment and thereby obviate the need for subsequent control measures.