Chemical and Hot Water Treatments to Control Rhizoctonia AG P Infesting Stem Cuttings of Azalea

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.