Carbon-utilization patterns in the photoresponsive fungus Cercospora beticola.

The leaf-spot disease of sugar beet caused by Cercospora beticola is of considerable economic importance in these countries where the conditions of high light-intensity (Calpouzos & Stallknecht, 1967) and relative humidity (Pool Rr McKay, 1916) favour spore germination and the establishment of infection. High light-intensity greatly increases carbohydrate exudation from leaves (Tukey et al., 1959, and any treatment, such as increasing light-intensity, which raises the nutrient status of the leaves, increases exudation, and hence spore germination ( P r m & Dickinson, 1971). The nature of the carbon source present in the leaf leachates will affect the host-parasite relationship by regulating germination and growth. Further, production of the phytotoxin, cercosporin, by C. beticola (Balis & Payne, 1971) will be regulated by the nature of the leaf nutrients present. In culture, C. beticola is also highly photosensitive, cercosporin production being photoregulated (Balis & Payne, 1971 ; Lynch & Balis, 1973; Lynch & Geoghegan, 1975). The organism used was strain C-1 of C. beticola (Balis & Payne, 1971). The basal medium contained (g/litre): KHzP04, 0.5; KZHPO4, 0.5; KCl, 0.5; MgSO,, 0.5; FeSO,, 0.01; agar, 15.0; NaN03, 2.0. ZnSO, (1Op.p.m.) and CuS04 (5p.p.m.) were also included, unless otherwise stated. Pigments were extracted with ethyl acetate and chromatographed on acidified silica gel, with ethyl acetatelbenzene (2: 3, v/v) as eluent, as described by Balk & Payne (1971). The red phytotoxin, in ethanolic solution, was determined spectrophotometrically at 480nm, by using a standard curve. The cultures were grown in Erlenmeyer flasks, incubated at 25°C and 90% relative humidity in specially constructed light cabinets containing Philips fluorescent lamps to give a light intensity of 161 .51~~ (15ft-candles). Since it was found (Lynch & Balis, 1973) that the carbonxitrogen ratio greatly influenced growth patterns in this organism, the carbon substrates were initially incorporated, after sterilization, in the basal medium at constant pH (5.8) to give a carbon:nitrogen ratio of 86:l. Growth was estimated, as colony diameter, after 9 days incubation, and the phytotoxin was then extracted and assayed. The results of one experiment are presented in Fig. 1. Growth in the light (unshaded areas) was optimized on maltose, fructose, sucrose and arabinose, whereas significantly less growth was produced on glucose and xylose in particular. Growth in the parallel dark-grown cultures was best on arabinose and poorest on xylose. The normal photoinduction of cercosporin (Balis & Payne, 1971 ; Lynch & Geoghegan, 1975) was optimal on arabinose under these conditions. In spore-germination tests on 1 % sugar solutions, at 85-90% minimum relative humidity, assessed after 24 h, glucose and fructose supported good germination (72-740/,), but sucrose was more inhibitory and starch proved quite inimical to germination. With xylose, arabinose, glucose, fructose, galactose, mannose, lactose, sucrose, maltose and cellobiose as carbon sources, at a concentration of 20g per litre, without addition of ZnSO, or CuSO,, which are known to facilitate amino acid uptake, slightly different patterns of carbon utilization were noted, with galactose being optimal for growth in light and arabinose and lactose supporting best growth in dark-grown cultures. Phytotoxin yields were highest on arabinose and lactose, yields in light on lactose being three times greater than those on sucrose. The differences in carbon utilization patterns between lightand dark-grown cultures, in the presence and absence of copper and zinc ions, suggest differential enzyme-induction effects and indicate one mechanism of action of light on C. beticola. Metal ions have been shown to