Fungal rusts belonging to the genus Melampsora are making Poplars (Populus spp.) in southwest Ohio look pretty rough. Although Eastern Cottonwoods (P. deltoides) are particularly hard hit, all poplar species are being affected to some degree.
Affected leaves are at first chlorotic but rapidly turn grayish brown to dark brown and drop. Symptoms mimic drought stress; however, poplars are shedding leaves where soil moisture has been relatively consistent this season.
Poplar Leaf Chlorosis from Rust Upper Leaf Surface
Telltale burnt orange to orangish-yellow uredinial pustules are evident on the affected leaves' underside. The uredial spores infect new leaves, supporting the escalation of symptoms throughout the season. Eventually, crusty telia form on the dying leaves, which is the overwintering stage.
Two hosts are required for Melampsora leaf rusts to complete the disease cycles. Teliospores arise from the telia in the spring to infect conifers. However, the infections cause little noticeable damage. Eventually, spores that develop on the conifer hosts drift to poplars to initiate infections and complete the fungal disease cycle.
Heavy infections can cause poplars to suffer significant defoliation; however, the impact depends on when the defoliation occurs. Leaf loss at this time of the year has minimal impact on overall tree health because trees have had plenty of time to produce and store enough carbohydrate to support new growth next spring.
Poplar Defoliation from Rust Infections
Fungicide applications may protect poplars; however, their effective use is problematic. Timing is critical because the applications should commence with the first appearance of leaf symptoms. Close monitoring is required. Fungicide applications must also be thorough, and multiple applications may be needed.
The complexity of protecting poplars with fungicide applications means this approach is most effective in nurseries. Raking and destroying fallen poplar leaves can help break the disease cycle, making this the most effective management approach in landscapes.