Rhizoctonia disease in sugar beet: Disease screening and cyto-histopathology of sugar beet, Rhizoctonia solani interaction

Rhizoctonia solani AG2-2 attacks sugar beet at the seedling stage causing damping off and, at maturity causing crown and root rot. It is estimated that an average 2% yield is lost to these diseases annually, and it is not uncommon to observe more than 50% losses in individual fields under the disease conducive conditions. The main goals of this research were to develop a robust Rhizoctonia seedling damping off disease screening method and critically observe the sugar beet- R. solani interaction during compatible and incompatible disease outcomes. An efficient protocol to screen Rhizoctonia seedling damping-off (RSD) disease in sugar beet was developed and this method was used to study the disease progress pattern. The RSD disease progress curve consisted of three distinct stages---an initial rapid disease progress stage, an intermediate stationary phase and a final resolution phase resulting in death or recovery. R. solani AG2-2 R-1 and W22 fungal isolates penetrated the sugar beet seedlings in susceptible and resistant cultivars but death was the uniform outcome only when the R-1 isolate infected a susceptible cultivar, USH20 (compatible interaction). Both the susceptible host infected with the W22 isolate and the resistant cultivar, EL51 infected with either R-1 or W22 isolates survived. Cultivar USH20 was highly susceptible to Rhizoctonia seedling damping off and for the first time resistance to Rhizoctonia seedling damping off was detected in EL51. In the field when USH20 and EL51 were artificially inoculated with R. solani AG2-2 R-1 and W22 fungal isolates, EL51 showed resistance to both isolates but USH20 succumbed to R-1 isolate and recovered from W22 infection. Predicted sugar yield from EL51 was much higher than USH20 in this experiment, primarily due to preservation of plant stand.; The cyto- and histopathology in compatible and incompatible interactions were examined using light, fluorescence, confocal and scanning electron microscopy. R. solani AG2-2 R-1 and W22 fungal isolates produced typical infection structures that evidently penetrated the epidermis and were seen in the cortex tissue in both resistant (EL51) and susceptible (USH20) sugar beet seedlings. During the compatible interaction (USH20/R-1) the R-1 isolate ramified the host stele ground tissue. During incompatible interactions, resistant plant EL51 limited the growth and penetration of R-1 and W22 isolates to just beneath the endodermis via cork layer formation. When susceptible plant USH20 was inoculated with W22 isolate, the host produced a very thin cork layer that was breached by the fungus and the fungus established in the stele tissue without causing disease. The EL51/R-1 interaction produced higher autofluorescence in the cortex cells compared to interactions EL51/W22, USH20/R-1 and USH20/W22.