• Horticultural Science & Technology
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• v.32 no.6
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• pp.803-811
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• 2014

This study was carried out to investigate the effect of relative humidity (70, 80, 90, and 95%) and to examine the interrelationship of temperature (15, 20, and $25^{\circ}C$) and relative humidity (60, 70, 80, and 90%) in the greenhouse on growth and occurrence of white rust in chrysanthemum 'Baekma'. The developmental stages of white rust were monitored from the initial occurrence to sporidium formation in teliospores. When the relative humidity of greenhouse was adjusted to 90% or more for 40 days, the occurrence rate of white rust, the number of infected leaves, and the number of symptoms per leaf were more than 97%, 10, and 55, respectively. However, in the treatment with 70% relative humidity, the occurrence rate of white rust dropped to 30% and only two spots of symptoms were observed with only 1 or 2 infected leaves per plant. The growth of cut flowers, such as fresh weight and leaf number, was the highest in 70% relative humidity. In the 90% relative humidity treatment, the first symptom of white spot appeared at 6 days after planting in chrysanthemum infected with white rust (stage 1), and then after one day, teliospores came out through the epidermal tissue of leaf (stage 2). The telium was formed with increasing teliospores after 1 to 2 days (stage 3), and then the promycelia developed from teliospores (stage 4). After a lapse of 4 to 5 days, sporidia formed on promycelia (stage 5). Regardless of relative humidity, white rust did not occur at all in treatments at $25^{\circ}C$. In treatments at $20^{\circ}C$, the number of sporidia was high regardless of relative humidity, but white rust did not occur in 60% and 70% relative humidity. As the relative humidity was higher, the number of sporidia and occurrence rate of white rust greatly increased, but the occurrence rate of white rust decreased to less than 14% in 60% and 70% relative humidity in treatment at $15^{\circ}C$.