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ISHS Acta Horticulturae 990: II International Symposium on Woody Ornamentals of the Temperate Zone

RESISTANCE TO PHYTOPHTHORA ROOT ROT VARIES AMONG RHODODENDRONS SUBJECTED TO REPEATED FLOODING IN THE FIELD

Author:   S. Krebs
Keywords:   Rhododendron hyperythrum, Phytophthora cinnamomi, root rot resistance, predisposition
DOI:   10.17660/ActaHortic.2013.990.28
Abstract:
A replicated field trial containing Rhododendron cultivars, species, and experimental hybrids was repeatedly flooded during one growing season to test for resistance to Phytophthora cinnamomi under stress conditions. At the end of the season root rot disease scores were assigned based on visual assessment of root, crown, and shoot necrosis using a numerical rating scale of 1 (healthy fine roots) to 5 (dead plant). Under flooding conditions, the average disease score of three resistant cultivars (controls used as benchmarks) was 4.1, which was a 90 percent increase above their previously determined average of 2.2 under non-flooded conditions. In contrast, disease scores of the resistant species R. hyperythrum were 35 percent higher under flooded (2.7) than non-flooded (2.0) treatments. Eight F1 hybrids derived from R. hyperythrum had an average disease score of 3.3 and were significantly less diseased than the resistant benchmark cultivars under flooded field conditions. Loss of root rot resistance in flooded soils could result from conditions that favor pathogen development and infection and from physiological changes in host plants that predispose them to disease. Under flooding conditions, R. hyperythrum appears to be less predisposed to root rot than resistant genotypes with different genetic backgrounds. While the basis for this difference in stress response is not currently known, it appears to be heritable in the F1 generation and represents a valuable trait for root rot resistance breeding.

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