Histological observation of potato in response to Rhizoctonia solani infection

Rhizoctonia solani is the causal agent of stem canker and black scurf in potato, resulting in significant yield and economic losses. To understand the infection process of R. solani and potato responses to the pathogen, histological observation of inoculated potato was performed under light and electron microscopes, under laboratory conditions. R. solani was observed at the invasion site, for the initial infection time, and for the presence of infection structures (appressoria, infection cushions). These attributes were compared when R. solani affected different parts of the host: aboveground stems, underground stems, tubers; as well as in different potato cultivars: 'Desiree' (resistant to R. solani) and 'Atlantic' (susceptible). In aboveground stems of potato, the invasion of R. solani was mainly limited to intercellular spaces. In underground stems and tubers, the primary invasion sites were epidermal cracks and lenticels. Initial infection time was 8 to 12 h post inoculation (HPI) in aboveground stems, 8 HPI in underground stems, and 4 HPI in tubers. The hyphae of R. solani produced various infection structures, with a few infection cushions, and a large number of appressoria with different morphologies. In aboveground stems, more infection structures were found than in underground stems and tubers. 'Desiree' had fewer numbers and smaller sizes of lenticels, thicker cuticle and periderm, and fewer epidermal cracks compared to 'Atlantic'. Fewer numbers of infection structure of R. solani were observed in 'Desiree' than in 'Atlantic'. It was suggested that the epidermal and perithelial structures of potato were related to disease resistance, such as number and size of lenticels, thickness of cuticle and periderm, and epidermal cracks.