Azalea web blight, caused by certain binucleate species of Rhizoctonia, occurs yearly on some azalea cultivars during nursery production in the southern and eastern United States. Azalea shoots collected for stem cutting propagation can harbor the pathogen, thus allowing the disease to be perpetuated through vegetative propagation. A previous study demonstrated that submerging Rhizoctonia-infested stem pieces of ‘Gumpo White’ azalea in 50 °C water for 21 min could eliminate the pathogen without causing significant damage to leaf tissue (Copes and Blythe, 2009). Because the objective of the study was pathogen control efficacy, a level of acceptable leaf damage was determined based on reasonable expectations without documenting the effect on root development. In the same study, hot water treatment (HWT) was the only sanitation method found to be effective in eliminating Rhizoctonia anastomosis group-P from azalea stem pieces. Soaking infested stem pieces in selected chemical disinfestants or fungicides was ineffective.
Sanitation is a proven and cost-effective approach for limiting the entry of pathogens into a propagation facility (Daughtrey and Benson, 2005; Williams-Woodward and Jones, 2001). Ideally, vegetative material used for plant propagation should be collected from healthy plants that do not harbor pathogens. If pathogen-free plant material cannot be guaranteed, then procedures may be needed to eliminate or restrict pathogen carryover. Detection methods to screen for pathogens, isolation of newly received plant material, and control treatments have been used for this purpose. HWT is a sanitation method used to reduce pathogen carryover in seeds, tubers, fruits, and vegetables (du Toit and Hernandez-Perez, 2005; Gramaje et al., 2009; Grondeau and Sampson, 1994; Jarvis, 1992; Keck et al., 1995; Lurie, 2006; Sharma and Tripathi, 2008; Turechek and Peres, 2009). However, thermotherapy techniques have been infrequently reported for use with stem cuttings, probably as a result of the risk of damaging tender shoots and leaves (Jarvis, 1992).
The main purpose of heat treatment is to kill pathogens or insects that infest or infect plant tissue. However, both pathogen and plant genera vary in sensitivity to heat. Therefore, commercial practicality of thermotherapy depends on a large enough difference between the temperature at which a pathogen is killed and the temperature at which plant tissue is damaged. As an example of a moderately acceptable difference, bare-rooted strawberry runners submerged in a 48 °C bath for 2 h resulted in up to a 106 colony-forming units/mL reduction in Xanthomonas fragariae and minimal effect on vegetative growth, whereas submersion in a 52 °C bath for 60 min completely killed the bacterium and also killed the plant (Turechek and Peres, 2009). Plant sensitivity to heat may be affected by plant species suitability to climate zones, cultivar response, age of tissue, and moisture content of plant tissue (Baker, 1962; Jarvis, 1992; Lurie, 2006). Additional plant factors influence heat sensitivity of types of plant propagules other than terminal stem cuttings such as external cell tissue type and age of seeds and post-treatment handling of tubers (Baker, 1962; Coyne et al., 2010).
The first objective of the present study was to determine if HWT would damage leaf tissue and/or reduce root development by rooting 12 commonly grown cultivars of evergreen azaleas from different hybrid groups using conventional nursery propagation practices with and without submergence of newly prepared terminal cuttings in 50 °C water for 20 min. Because ‘Gumpo White’ azalea cuttings developed only minor leaf tissue damage when submerged in 50 °C water for 20 to 40 min and severe leaf damage when submerged in 55 °C water for 13 min in our prior study (Copes and Blythe, 2009), the lower 50 °C water temperature was selected for the current study. The second objective was to evaluate effects of 50 °C water treatment on terminal cuttings of the same 12 azalea cultivars when submerged for 20 to 80 min to determine the risk of damage if cuttings are accidentally submerged for longer than the recommended 20 min. Finally, because little is known about the rooting response of azalea after different types of tissue damage, the third objective was to evaluate root development and subsequent leaf production on cuttings of two cultivars in response to incremental reductions in photosynthetic leaf area resulting from either heat-induced leaf tissue damage or leaf removal.
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