Seasonal Prevalence of Species of Binucleate Rhizoctonia Fungi in Growing Medium, Leaf Litter, and Stems of Container-Grown Azalea

Abstract

Rhizoctonia web blight is an annual problem on container-grown azalea (Rhododendron spp.) in the southern and eastern United States but little is documented about the distribution or persistence of Rhizoctonia spp. in container-grown azalea. Sixty web-blight-damaged azalea plants (‘Gumpo White’) were collected in August 2005 and 2006 and arranged in a completely randomized design on an outdoor irrigation pad. A nylon mesh bag containing 30 necrotic leaves collected from web-blight-damaged ‘Gumpo White’ azalea plants were placed on the surface of the 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 (lengths of 0 to 2, 4 to 6, and 9 to 15 cm above the medium surface), bagged leaves into one leaf litter zone, and the medium into four zones (three horizontal layers: 1 to 3, 3 to 7, and 7 to 10 cm below the medium surface, with the middle layer further divided by removing the central 7.5-cm-diameter core) in December, February, and May. Only the three stem zones were sampled from 10 plants in early and late June and late July. Of 8,940 total isolations, 3,655 fungi with morphological characteristics of a Rhizoctonia sp. were recovered. Percent recovery differed from the eight zones (P < 0.0001) but did not differ between years (P = 0.3950) and sampling times (P = 0.1896). Frequency of recovery of Rhizoctonia spp. was highest from the lower stem and the leaf litter, and decreased with distance from the leaf litter. Recovery from stems over the six sample times was analyzed separately. Percent recovery differed between stem zones (P < 0.0001), sample times (P = 0.0478), and experiment years (P < 0.0001). In both years, mean recovery of Rhizoctonia spp. was higher from the lower stem and decreased with distance to the upper stem layer. From a subsample of 145 isolates, 95.1% were identified as binucleate Rhizoctonia (BNR) anastomosis groups (AGs)-A, -G, -K, -R, -S, and -U (-P), and 2.8 and 2.1% were Rhizoctonia solani AG-2 and an uncultured Laetisaria sp., respectively. Based on frequency analysis, recovery of BNR AGs differed by plant zone (P < 0.0001) but not over sample times (P = 0.4831). The six AGs of BNR are the predominant Rhizoctonia fungi occupying the habitat niches in container-grown azalea, with little change in population frequency and composition from fall to summer; thus, BNR pathogenic and nonpathogenic to azalea have established a mixed Rhizoctonia community on container-grown azalea.

Rhizoctonia Web Blight Development on Azalea in Relation to Leaf Wetness Duration in the Glasshouse

“…; Copes et al. ). The diagnostic symptom pattern is a leaf blight that develops without intermediate stages.…”

Weather-Based Forecasting of Rhizoctonia Web Blight Development on Container-Grown Azalea

“…Temporal studies on Rhizoctonia web blight on azalea in south ern M ississippi have reported several different weather conditions that are conducive for disease development (4)(5)(6)(7)(8). Rhizoctonia web blight development on azalea was initially correlated with air tem peratures between 25 and 30°C for 6 to 8 h per day, relative humid ity >95% for >8 h per day, and/or leaf wetness for >6 h per day (7).…”

“…The rate and degree o f web blight development differed between wet, moderate, and dry years, and weather variables o f tem pera ture, rainfall, and duration o f leaf wetness do influence disease development and control (4)(5)(6)(7)(8). Rhizoctonia web blight on azalea behaves like a polycyclic disease (7,8,14).…”

“…M ycelium serves as primary and secondary inoculum. In the southeastern United States, mycelium persists in the azalea canopy 12 months o f the year (6). The mycelium density declines by early spring and builds up again by summer, probably in a cyclic growth pattern that corre lates with episodic leaf blight development.…”

“…The mycelium density declines by early spring and builds up again by summer, probably in a cyclic growth pattern that corre lates with episodic leaf blight development. Binucleate Rhizoctonia fungi that cause web blight produce psuedosclerotia, but it is un clear if it plays a distinct role as primary inoculum (6). The pres ence of basidiospores has not yet been determined.…”