Observations of growth stages of Diaporthe citri on epidermal strips of onion bulb.

Abstract

The whole cycle of fungal growth of Diaporthe citri, i.e., pycnospore germination, appressorial formation, hyphal penetration, penetrated mycelia, pycnidium formation and pycnospore formation was observed on alcohol-treated epidermal strips of onion bulb which might be free from some inhibitors against hyphal penetration. The results obtained on epidermal strips of onion bulb were nearly same as those obtained on citrus leaves or twigs. Therefore, the method using epidermal strips of onion bulb would be very useful for assaying the effects of fungicides on the different growth stages of D. citri. The maximum rate of pycnospore germination was approximately 80 to 100% and that of hyphal penetration was approximately 50 to 70%. The shapes of appressoria were mostly fist-like or globular form and they did not necessarily have relation to hyphal penetration. Therefore, the function of appressorium of D. citri seems to adhere on host plants, but the role does not seem to be so important, judging from the fact that the appressorial formation rate is only 20 to 30%. When suspension of pycnospores in pycnidia formed on epidermal strips of onion bulb was inoculated on young citrus leaves, typical symptoms of melanose were observed, indicating that the pycnospores were virulent.