Heat-induced predisposition to Phytophthora root rot disease in Rhododendron

S.L. Krebs
Resistance to pathogens may be an important attribute of plants that are adapted to warm climates, where disease pressure is often greater. However, resistance genes alone may not be sufficient to prevent disease because abiotic stress, including high temperatures, can reduce host resistance, a phenomenon known as 'predisposition'. This study compares the disease responses of diverse Rhododendron taxa to heat stress following inoculation with Phytophthora cinnamomi Rands, the primary pathogen causing root rot disease, in order to determine the variation in predisposition. The key experimental comparison was between non-stressed plants (26°C) and plants whose roots had received a high temperature treatment (40°C) prior to inoculation. Roots were sampled at time intervals (DAI, days after inoculation) and visually rated using a disease scale ranging from 1 (healthy) to 5 (dead). In highly susceptible taxa such as the cultivar R. 'Haaga' and the Rhododendron species R. ponticum, there was no significant effect of heat stress on disease development compared to non-stressed plants - severe root rot symptoms appeared early in both cases. Heat treatment significantly increased disease severity in the cultivars R. 'Ingrid Mehlquist' and R. 'Holden' based on comparisons between 14 and 48 DAI, a clear indication of heat induced predisposition. For two resistant Asian species, high temperature stress consistently increased root disease scores in R. keiskei while the opposite response pattern was observed in R. hyperythrum, where symptoms following heat stress were equal to or lower than non-stressed plants at most sampling times. These data suggest that the unusual ability of R. hyperythrum and its hybrids to adapt to warm climates (USDA hardiness zone 9) is due in part to a mechanism which prevents predisposition to disease at high soil temperatures.
Krebs, S.L. (2018). Heat-induced predisposition to Phytophthora root rot disease in Rhododendron. Acta Hortic. 1191, 59-68
DOI: 10.17660/ActaHortic.2018.1191.9
Phytophthora cinnamomi, high temperature stress, Rhododendron hyperythrum

Acta Horticulturae