Poster Session: Host Resistance

248-P

Predisposition to Phytophthora root rot varies among rhododendron genotypes subjected to flooding stress.
S. KREBS (1), P. Bonello (2)
(1) The Holden Arboretum, Kirtland, OH, U.S.A.; (2) The Ohio State University, Columbus, OH, U.S.A.

Root rot caused by the invasive fungal pathogen Phytophthora cinnamomi is a major source of mortality for Rhododendron and other popular ornamental genera. A breeding program is underway to produce resistant rhododendrons that can be grown more successfully and sustainably under a broad range of production and landscape conditions. Current hybridization is based on R. hyperythrum, a species from Taiwan that exhibits more root rot resistance under flooding conditions than other resistant genotypes. In a replicated field trial where rhododendrons were subjected to repeated flooding throughout one growing season, three resistant cultivars included as benchmarks showed advanced disease symptoms, with necrosis occurring in the crown tissue (average disease rating = 4.1 on a scale of 1-5 where 5 is a dead plant). In contrast, symptoms in R. hyperythrum were limited to fine and coarse roots, resulting in a significantly lower disease rating (2.7). Eight genetically diverse F₁ hybrids – crosses between R. hyperythrum and cold hardy, susceptible cultivars – averaged a root rot rating of 3.3 and were significantly less diseased following flooding stress than the resistant benchmarks. Presence of P. cinnamomi in symptomatic tissue was confirmed by isolate morphology and sequence data. Compared to other resistant genotypes, R. hyperythrum and its F₁ hybrids are less predisposed to root rot in poorly drained soils, a common and often fatal feature of the home landscape.