Development of Bioassay for Screening of Resistant Roses against Root Rot Disease Caused by Pythium helicoides Drechsler

Abstract

Rose root rot disease caused by Pythium helicoides has caused serious economic losses in rose production in Japan. The use of resistant rootstock against root rot disease is a valuable strategy toward legal restrictions and consumer concerns against fungicide application. So, the establishment of a bioassay for the selection of resistant roses is very important. In this study, the utility of the original inoculation method by a self-made Ebb & Flow watering system was examined using a bioassay for the selection of resistant roses. Rosa multiflora ‘Matsushima No. 3’ and R. ‘Nakashima 91’ were used as resistant and susceptible plant materials, respectively. Rooted cuttings were inoculated by soaking them with a zoospore suspension for 1 h in the situation of them being planted in a plug tray without transplanting, and then zoospore-containing nutrient solution was given from the tank of the Ebb & Flow system 4 times every day, for 2 min each time. Although the conventional phytopathological bioassay could not distinguish between ‘Matsushima No. 3’ and ‘Nakashima 91’ in terms of disease severity, the resistibility of ‘Matsushima No. 3’ and susceptibility of ‘Nakashima 91’ to root rot disease were confirmed through the original bioassay developed in this study, and the reproducibility was high. Therefore, it could be concluded that this bioassay was suitable for the screening of resistant varieties. Histological observation indicated that the density of hyphae in cortical cells of ‘Matsushima No. 3’ was less than that of ‘Nakashima 91’. Hyphae penetrated into the endodermal tissue in ‘Nakashima 91’, whereas they were inhibited from expanding to cortical cells of ‘Matsushima No. 3’.