Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/4/2009
Publication Date: 8/1/2009
Citation: Copes, W.E., Blythe, E. 2009. Chemical and hot water treatments to control Rhizoctonia AG P infesting stem cuttings of azalea. HortScience. 44(5):1370-1376. Interpretive Summary: Rhizoctonia AG U, the fungus that causes azalea web blight, has been discovered to infest healthy appearing azalea stems used for propagation. Multiple control methods, including disinfestants, fungicides and hot water, were tested for their ability to rid azalea stems of the pathogen without damaging plant tissue. Disinfestants and fungicides applied after cuttings were taken were ineffective. However, results imply that fungicides applied 1 week before cuttings are collected will provide nearly 100% control. Additionally, stems submerged in 55 °C water for 5 minutes eliminated the pathogen from stems. This information will benefit ornamental plant producers, Extension agents, and research scientists.
Technical Abstract: In the southern and eastern U.S., azalea 'Gumpo' stems cut during the spring for propagation may be infested with Rhizoctonia spp. Multiple methods were evaluated for the purpose of eliminating Rhizoctonia spp. from stem cuttings to prevent spread into the propagation house. Stems were inoculated with isolate of binucleate Rhizoctonia anastomosis group U (=AG U). Application of disinfestants (sodium hypochlorite, hydrogen dioxide, quaternary ammonium chloride) and fungicides (chlorothalonil plus thiophanate methyl, and flutolanil) at several rates did not eliminate Rhizoctonia AG-U from stem cuttings. Application of flutolanil (0.315 g a.i. l-1) to stems prior to inoculation and prior to and after inoculation reduced recovery of Rhizoctonia AG-U by 94% and 100%. A factorial treatment design of inoculated stems (treated or not treated with flutolanil) that were glued in the center of cork discs (treated or not treated with flutolanil) were plated for recovery of Rhizoctonia AG U. Application of flutolanil to cork discs reduced recovery of Rhizoctonia AG U to nearly 0%. This implies that if the fungus can grow immediately out of a fungicide treated area, such as a treated stem cutting, into a fungicide-free area, such as an agar-based or peat media, is too short of exposure to cause mortality. Recovery of Rhizoctonia AG U was not reduced by submersing stems in 45 °C water, but was reduced by 100% by submersing stems in 55 °C water. Minor leaf damage resulted from submersion of stems in 55 °C water for 5 minutes. Leaf damage was severe when stems were submerged in 55 °C water for 25 and 45 minutes. Of the methods tested in bench top studies, the application of fungicides to plants prior to collecting stem cuttings or the submersion of stems in hot water have the most potential of eliminating Rhizoctonia AG U from azalea stems.