Greenhouse experiments, where sunflower plants (cv. Peredovic) were inoculated with Puccinia helianthi at the vegetative stage of plant growth, showed that the effect of rust on growth parameters varied according to the moisture stress to which plants were subjected. Rust infection caused a marked reduction in the diffusive resistance of leaves in darkness. However, the diffusive resistance in light of leaves not stressed or subjected to mild moisture stress was unaffected by rust infection. In uninfected plants subjected to moderate moisture stress, the diffusive resistance in light increased markedly, whereas in rust-infected leaves it remained the same as in leaves not stressed or subjected to mild moisture stress. Carbon-14 uptake by the first pair of true leaves of non-stressed plants or those subjected to mild moisture stress was unaffected by a low level of rust infection (9% disease rating). Carbon-14 uptake by uninfected leaves was reduced when they were subjected to moderate moisture stress. Rust infection and moderate moisture stress caused a reduction in the rate of translocation of labelled assimilate from leaves fed with 14CO2 and also altered the distribution pattern of labelled translocate to various sinks.
Oil yield losses in sunflower (cv. Peredovic) caused by rust infection (Puccinia helianthi) were greatly influenced by the growth stage of the plant when infection occurred and by the intensity of infection. Simulated epidemics produced under greenhouse conditions, where plants were first inoculated at the vegetative, budding, anthesis or seed development stages of growth and thereafter at 10 day intervals, resulted in reductions in oil yield of 85, 73, 38 and 13% respectively relative to those in uninoculated controls. In contrast, plants inoculated once only at each of these growth stages showed reductions in 011 yield of 13, 42, 35 and 10% respectively. When plants were inoculated at the vegetative stage and thereafter at 10, 20 and 40 day intervals, the reductions in oil yield were 85, 79 and 64% respectively relative to uninoculated controls. The reduction in oil yield of infected plants was caused by a reduction in the number of seeds produced per head, the weight of individual seeds and the percentage oil content of seeds. The effects of rust infection on other parameters showed similar trends to that on oil yield.
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