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  • Author or Editor: E.K. Blythe x
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In the southern and eastern United States, azalea stems cut during the spring for propagation may be infested with Rhizoctonia spp. Multiple methods were evaluated in a series of laboratory experiments for the purpose of eliminating Rhizoctonia from stem cuttings of Rhododendron L. ‘Gumpo White’ [‘Gumpo White’ (Satsuki) azalea] to prevent spread of azalea web blight during the propagation phase of nursery production. Leafless stem sections were inoculated with an isolate of binucleate Rhizoctonia anastomosis group P (AG P). Disinfestants (sodium hypochlorite, hydrogen dioxide, and quaternary ammonium chloride) or fungicides (chlorothalonil + thiophanate-methyl and flutolanil) applied at several rates (below, at, and above label rates) did not eliminate Rhizoctonia AG P from stem sections. Recovery of Rhizoctonia AG P was not reduced by submersing stem pieces in 45 °C water, but was eliminated at water temperatures of 50 °C or greater. Mortality of Rhizoctonia infesting azalea stem pieces was best explained by a cubic regression model. Mortality increased with increasing time (0, 1.5, 3, 4.5, 6, 7.5, 9, 10.5, 12, 15, 18, and 21 min) in water at 50 and 55 °C and with increasing temperatures (52, 55, 58, 61, 64, 67, and 70 °C) when stem pieces were submerged for 30 and 60 s. The duration of hot water treatment at which 99% of stem pieces were predicted to be free of Rhizoctonia was 20 min 16 s at 50 °C and 5 min 19 s at 55 °C. The average water temperature at which 99% of the stem pieces were predicted to be free of Rhizoctonia was 60.2 and 56.9 °C when stem pieces were submerged for 30 and 60 s, respectively. Only minor leaf damage occurred on terminal, leafy stem cuttings when submerged in 50 °C water after 40 min. Severe leaf damage did occur if cuttings were submerged long enough in water of 55 °C or greater. Leaf damage was predicted to exceed a proportional leaf damage value of 0.25 (indicating severe damage) when leafy stem cuttings were submerged in 55 °C water for longer than 13 min 54 s or for 30 and 60 s with water temperature greater than 57.4 and greater than 56.8 °C, respectively. Of the methods tested, submersion in hot water has the greatest potential for eliminating Rhizoctonia AG P from azalea stem cuttings. Submerging stem pieces in 50 °C water for 21 min eliminated Rhizoctonia and provided the least risk for development of severe leaf damage.

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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. Before 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 before propagation in one experiment. All 12 cultivars tolerated 50 °C water for 20 min. Cuttings collected from the 12 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% or greater leaf area was removed. Despite the risk imposed by submersing azalea cuttings in 50 °C water, all 12 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.

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