Rosa multiflora with Root Rot Tolerance Has No Tolerance Mechanism on the Root Surface during the Early Infection Process

Abstract

Rosa multiflora ‘Matsushima No. 3’ shows tolerance to root rot disease caused by Pythium helicoides Drechsler. However, there has been no report on the tolerance mechanism of ‘Matsushima No. 3’. The infection process of Pythium in diseased plants was divided broadly into two stages, as early and later infection processes. This study focused on the early infection process: chemotaxis, attachment, and encystment of zoospores and germ tube germination on the root surface, in order to clarify the tolerance of ‘Matsushima No. 3’ in comparison with a susceptible rose. ‘Matsushima No. 3’ and R. ‘Fashion Parade’, which were estimated as showing tolerance and susceptibility to root rot disease in the field, respectively, were used as plant materials. Rooted cuttings of ‘Matsushima No. 3’ and ‘Fashion Parade’ were inoculated by zoospore suspension of P. helicoides, and root rot symptoms of these roses were evaluated at 7 days after inoculation. From the results of inoculation, we concluded that ‘Matsushima No. 3’ was tolerant, and ‘Fashion Parade’ was susceptible to root rot disease. Visual observation showed that zoospores of P. helicoides aggregated at the end of capillaries filled with root extract agar of ‘Matsushima No. 3’ as well as ‘Fashion Parade’, and zoospores encysted on the root surface of ‘Matsushima No. 3’ too. Therefore, ‘Matsushima No. 3’ didn’t inhibit the chemotaxis and encystment of zoospores. From SEM observations, most cystospores germinated on the root surface in ‘Matsushima No. 3’ and ‘Fashion Parade’, and the percentages of germ tube penetration to the epidermis in ‘Matsushima No. 3’ and ‘Fashion Parade’ were 55.0 and 71.2, respectively. The result that no differences were recognized between the two roses during the early infection process indicated that the tolerance mechanism in ‘Matsushima No. 3’ was not active in the early but in the latter infection process, that is, some tolerance mechanism might inhibit hyphae from expanding to the cortical tissue.