Evaluation of host status of garlic varieties for a plant-parasitic nematode, Ditylenchus destructor, by using in vitro inoculation.

Ditylenchus destructor is a plant-parasitic nematode that severely damages garlic (Allium sativum L.) in Japan. D. destructor is detected in roots, bulbs, and outer bulb skins of garlic at harvest; however, the resistance of garlic to D. destructor infection is not well understood. Here, we investigated the propagation of D. destructor in storage organs and roots using in vitro plantlets of six Japanese garlic varieties to exclude the effects of microbes and to uniform growing conditions. In vitro inoculation can proceed simultaneously with vegetative growth, storage organ formation of garlic plantlets, and D. destructor infection. In 'Fukuchi-white', a variety susceptible to D. destructor, nematodes successfully propagated in storage organs and roots. Furthermore, the nematodes invaded and propagated in the newly formed storage organs. By contrast, 'Kirishima', 'Hirado', and 'Shishimaru' substantially suppressed more the propagation of the nematodes in storage organs and roots than 'Fukuchi-white'. Additionally, the propagation of nematodes in newly formed storage organs was inhibited in these three varieties. 'Shishimaru' showed unique responses to D. destructor infection: nematode propagation was the lowest among six varieties in inoculation tests and the nematode-inoculated cloves turned brown. Our results suggest that several garlic varieties have resistance mechanisms that suppress the propagation of D. destructor in storage organs and roots, and that in vitro inoculation methods are useful for selecting resistant garlic varieties. These findings will help developing novel D. destructor-resistant garlic varieties and our further understanding of garlic-nematode interactions.