259 MELAMPSORA LINI (PERS.) LeV. UREDOSPORE LONGEVITY AND GERMINATION. By H. B. Keer. (Three Text-figures.) [Read 29th October, 1958.] Synopsis. During-genetical investigations of rust resistance at Sydney University between 1948 and 1953 it was found that a boiled aqueous extract of host tissue greatly stimulated the germination of stored uredospores of Melampsora Hni (Pers.) Lev. (Kerr, 1952). Exploratory studies were carried out to determine the relative efficiency of water, gelatin solution and aqueous host extract as germinating media for these spores. These showed that germination of stored spores on water was highly capricious, that better but still somewhat capricious germination obtained on gelatin, while maximum germination was obtained on the host extract. Preliminary experiments were carried out to determine the nature of the substance or substances responsible for the germination stimulating potential of the extract. These showed that the substance was part of or comprised the ether-soluble oily fraction adsorbed by activated charcoal. Other experiments were set up to determine the longevity of uredospores of different races of the pathogen under different storage conditions. These showed a significant difference between races when temperature and specially humidity were carefully controlled. Low temperature and intermediate relative humidity were most favourable to the maintenance of uredospore viability during-storage. Introduction. In any experiment depending on infection of host material or in which the results are assessed in terms of the germination of fungal spores it is essential to obtain maximum germination of the spores. Since water is the most common natural germinating medium it is one of the most widely used in the laboratory (Hwang, 1942; Prasada, 1948). Many workers have found that water was a poor germinating medium even for fresh spores with no dormant period (Noble, 1924; Wilcoxon and McCallan, 1943). Germination was often improved on a decoction of host tissue (Chiu and Walker, 1949). Increases in germination often as good as that with host tissue were sometimes induced by extracts of non-host plants. Germination was also improved, in some cases, by the addition of chemicals to the germinating medium (Thiele and Weiss, 1920; Noble, 1924). The favourable effect of the host tissue was not always confined to an improvement in germination. Chupp (1917) found that spores which failed to germinate at room temperature did so at this temperature when young seedlings were added. Chiu and Walker found that the optimal temperature range for germination of cucurbit black rot spores on water was 20°C. to 24°C., but was 24 °C. to 28 °C. on plant extract. Whitehead found that spores of Urocystis cepulae germinated slowly on water, but much faster on onion juice. Sharvelle (1936) found that the percentage germination, the length of the germ tubes, and type of germ tubes of Melampsora lini uredospores sown on cold extracts of different varieties of flax and linseed were correlated with the rust reaction of the variety to the race used. The effect was destroyed by brief heating of the extract. Cold extracts of immune and resistant varieties retarded the percentage germination and length of the tubes, but extracts of susceptible varieties neither retarded nor stimulated germination. It is generally agreed that low temperatures and low humidities are most favourable for the maintenance of uredospore viability. Bailey (1923) reported an optimal humidity range of 20% to 40% RH for uredospores of Puccinia helianthi. Raeder and Bever (1931) studied the effects of five humidities (100%, 76%, 49%, 25% and 1-5% RH) on the longevity of P. glumarum uredospores. They survived best at 49% RH at most temperatures but in the range -2°C. to 5°C. they survived longest at 75% RH. Chester (1946), summarizing results of many workers, showed that uredo-Proceedtngs of the Linnean Society of New South Wales, 1958, Vol. lxxxiii, Part 3.