| Summary: | Recent trials of Antirrhinum majus cultivars have revealed a wide range of susceptibility to rust (Puccinia antirrhini). The best plants were used as parents in a breeding programme to produce a useful level of durable field resistance. The F1 generation was self-pollinated but the F2 to F4 were selectively cross-pollinated within lines. The segregating generations were grown outdoors at two sites in Surrey, where they were subject to natural rust epidemics encouraged by the use of spreader rows. Individual plants were selected for rust-resistance and horticultural quality. By the F4 generation, lines showed greater resistance to rust than existing varieties and were becoming uniform in flower colour, growth habit and horticultural quality. Many floral abnormalities arose, especially in the F3. Their expression was usually highly variable and was enhanced by environmental stress. Tests of rust isolates against individual antirrhinum plants showed that there is considerable genetic variation within the rust population. Some monitoring of resistant varieties is desirable to identify future changes in the rust population. The spread of spores between widely spaced patches of susceptible host was simulated using a computer, which indicated that small, isolated patches may escape infection. The effect of rate-reducing resistance is increased when plants are widely separated. This is in agreement with practical experience. The urediniospores of Puccinia antirrhini cannot survive the winter in Britain and, though teliospores are regularly produced, no alternate host is known. However, overwintered antirrhinum plants produce viable urediniospores which start epidemics in early summer: there is no need to postulate an alternate host in the British Isles. Treating antirrhinum as a summer annual might be an important hygienic measure, reducing the local build up of inoculum early in the season. Acceptable control of rust should be possible, provided only the more resistant varieties are grown.
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