Sumac leaf spot

Pseudocercospora rhoina is a pathogenic fungus known to cause leaf spot disease in various species of sumac (Rhus). This fungal infection is a common concern for arborists and gardeners managing ornamental landscapes, as it directly impacts the photosynthetic efficiency and aesthetic value of the shrubs.

The host range includes several sumac varieties, with the symptoms manifesting primarily on the foliage. Under conducive environmental conditions, the infection can become widespread across a colony of shrubs, potentially leading to defoliation during the peak of the growing season if the disease is left unchecked.

Early symptoms include the appearance of small, circular to irregular spots on the leaf surface, which typically turn dark brown or reddish-brown as the infection progresses. A defining characteristic is the presence of the fungus's reproductive structures, which appear as a greyish, velvety mat of spores on the undersides of the affected leaf spots during humid weather.

The life cycle of the pathogen is heavily dependent on moisture and moderate temperatures. Rain-splashed spores spread the fungus from infected leaf litter on the ground to healthy foliage. The fungus survives the winter primarily within fallen leaves, serving as the primary inoculum source for new infections in the following spring.

Effective management strategies emphasize sanitation and preventative care. Key measures include:

• promptly removing and disposing of all fallen leaves during autumn to eliminate overwintering sites;
• improving air circulation within the shrub canopy through proper pruning;
• applying protective fungicides during periods of high humidity and rainfall;
• ensuring that plants are properly watered and fertilized to maintain overall vigor.