The typical symptoms of this fungal disease are a rapid blighting or dieback of the foliage during hot humid conditions. The infected plants may turn dark brown or black and will often quickly collapse. The pathogen can be seen on blighted tissue as a fine tan webbing that usually adheres tightly to the leaves and stems. The fungus Rhizoctonia is very common in most soils and also causes root rots and stem cankers. Rhizoctonia can attack nearly all herbaceous perennials and is especially common on dianthus, coreopsis, ferns, goldenrod, hibiscus, statice, sweet woodruff, and yarrows.
Management
Avoid overhead irrigation and thin plantings to increase air circulation that will promote faster leaf drying. Thinning can be accomplished by selective pruning of infected leaves or by mechanical removal such as with a lawn mower set at the highest setting. Removal of fallen debris and clean up of old leaves etc. will also help reduce the amount of moisture trapped and will promote faster drying of the planting bed. Under severe conditions, a registered fungicide may need to be applied.