Location: Southern Horticultural ResearchTitle: Seasonal prevalence of species of binculeate Rhizoctonia fungi in growing medium, leaf litter, and stems of container-grown azalea) Author
Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/13/2011
Publication Date: 6/1/2011
Citation: Copes, W.E., Rodriquez-Carres, M., Toda, T., Rinehart, T.A., Cubeta, M.A. 2011. Seasonal prevalence of species of binculeate Rhizoctonia fungi in growing medium, leaf litter, and stems of container-grown azalea. Plant Disease. 95:705-711. Interpretive Summary: Rhizoctonia web blight is an annual problem in the southern to eastern U.S., but little is documented about the population levels of the pathogen in container-grown azalea. Sixty azaleas 'Gumpo White' with web blight were collected in Aug 2005 and 2006 and maintained under irrigation. Ten plants were destructively sampled into four bark medium zones, one leaf litter zone, and three stem zones. All zones were sampled in December, February, and May and only stem zones were sampled in June and July. 3550 isolates of Rhizoctonia were recovered, and came from all zones on all sample dates. Percent recovery of total Rhizoctonia species and specific binucleate Rhizoctonia anastomosis groups differed between substrate zones but not between sample dates. Because binucleate Rhizoctonia species extensively colonize the entire container-grown azalea, the possibility of propagating Rhizoctonia-free stock and minimizing contamination of plants on the nursery will be investigated. This information is valuable to research scientists for understanding the habitat and colonization habits of Rhizoctonia, which is valuable for deriving new control strategies.
Technical Abstract: Rhizoctonia web blight is an annual problem on container-grown azalea in the southern and eastern U.S., but little is documented about the distribution or persistence of Rhizoctonia in container-grown azalea. Sixty azalea plants (cv. >Gumpo White=) with greater than 35% web blight severity were collected in Aug 2005 and 2006. Ten plants were labeled per each of six sample periods and all plants were arranged in a completely randomized design on an outdoor irrigation pad. A nylon mesh bag containing 30 necrotic leaves, collected from ‘Gumpo White’ azaleas exhibiting web blight symptoms, were placed on top of the bark medium under the plant canopy in each container to simulate leaf litter. Ten plants were destructively sampled into eight zones by dividing stems into three zones (0-2 cm, 4-6 cm, and 9-15 cm lengths from the bark media), the bagged leaves as one leaf litter zone, and the bark media into four zones (an upper horizontal 3 cm layer, middle horizontal 7 cm layer which was further divided by removing the central 7.5 cm diameter core, and a lower horizontal 3 cm layer) in December, February, and May. Also, the three stem zones were sampled from ten plants in early- and late-June and late-July. Of 8940 total isolations, 3550 fungi were recovered with characteristics of Rhizoctonia species. Percent recovery differed from the eight zones (P<0.0001), but did not differ between experiment years (P=0.3950) and the three sample times (P=0.1896). The highest frequency of recovery of binucleate Rhizoctonia species was from the lower stem length (0 to 2 cm from the bark media) and the leaf litter, and frequency decreased with distance from the leaf litter. In a separate analysis, percent recovery differed between the three stem zones (P<0.0001), six sample times (P=0.0478), and between the two experiment years (P<0.0001), and no interaction occurred between stem zones and sample time (P=0.4334). The trend from the outer stem length zone was a decrease in recovery of Rhizoctonia from May to June and an increase in July. From a subsample of 167 isolates, 94.6% of the isolates were identified as binucleate Rhizoctonia (=BNR) anastomosis group (=AG) -A, -G, -K, -R, -S, and -P based on sequence analysis of internal transcribed spacer (ITS) of ribosomal DNA (rDNA) and comprised 9.6, 1.1, 9.6, 7.8, 9.0, 57.5% of the isolates, respectively. R. solani and an unidentified multi-nucleate basidiomycete comprised 3.6 and 1.8% of the 167 isolates, respectively. Based on frequency analysis, recovery of BNR AGs differed per plant zone (P<0.0001) but not over sample times (P=0.2473). Container-grown azaleas with web blight symptoms are colonized by several BNR AG and a few AG of R. solani. Results from this study indicate several BNR AG can colonize several different habitat niches of a container-grown azalea plant and occupy those niches over one year.