|Blythe, Eugene - Mississippi State University|
Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/5/2010
Publication Date: 1/1/2011
Citation: Copes, W.E., Blythe, E. 2011. Rooting response of azalea cultivars to hot water treatment used for pathogen conrol. HortScience. 46:52-56.
Interpretive Summary: Binucleate Rhizoctonia species that cause azalea web blight are spread on new stem growth collected for vegetative propagation. Submerging terminal leafy cuttings of 'Gumpo White' azalea in 122 °F water for 21 min was previously shown to eliminate the fungal pathogen from plant tissues. The objective of this research was to evaluate sensitivity of a broad diversity of azalea cultivars to hot water submergence. Twelve azalea cultivars, representing eleven hybrid groups, rooted well after exposure to 20 to 40 minute hot water treatments. Most, but not all of the cultivars, suffered injury from being submerged for 60 to 80 minutes. Submersing azalea cuttings in 122 °F water for 21 minutes appears to present a low risk of negatively affecting rooting. The information will be beneficial to research and extension scientists and commercial ornamental plant producers.
Technical Abstract: Submerging terminal leafy cuttings of Rhododendron L. 'Gumpo White' ('Gumpo White' azalea) in 50 °C water for 21 min was previously shown to eliminate binucleate Rhizoctonia species, the cause of azalea web blight, from plant tissues. Prior to considering commercial use of this practice, a better understanding of the rooting response and tissue sensitivity of evergreen azalea cultivars to 50 °C water was needed; therefore, the current study was conducted. Terminal cuttings of the azalea cultivars 'Conleb' (Autumn Embers), 'Fashion', 'Formosa', 'Gumpo White', 'Hardy Gardenia', 'Hershey Red', 'Macrantha Pink', 'Midnight Flare', 'Red Ruffles', 'Renee Michelle', 'Roblel' (Autumn Debutante) and 'Watchet' were collected and submerged or not submerged in 50 °C water for 20 min prior to propagation in one experiment. All twelve cultivars tolerated 50 °C water for 20 min. Cuttings collected from the twelve cultivars were submerged in 50 °C water for 20, 40, 60, and 80 min in a second experiment. The cultivars varied in sensitivity when exposed to 50 °C water for 60 to 80 min resulting in differing responses in root development and final leaf count. In a third experiment, degrees of leaf damage caused by hot water submersion or by leaf removal were evaluated for the effect on root development and subsequent leaf count on rooted cuttings of ‘Gumpo White’ and ‘Roblel’. Induced incremental increases in leaf damage from hot water resulted in incremental reductions in the final leaf count and extent of root development for ‘Gumpo White’ and ‘Roblel’, while increasing percentage of leaf removal caused no reduction until =75% leaf area was removed. Despite the risk imposed by submersing azalea cuttings in 50 °C water, all twelve azalea cultivars were tolerant of submersion durations long enough to eliminate binucleate Rhizoctonia species from stem and leaf tissue with only a low likelihood of sustaining detrimental damage.