Ash rust, caused by the fungus Puccinia sparganioides, is a disease which affects white and green ash in Delaware. Symptoms on ash appear in mid-May as small yellow to yellow-orange spots on upper leaf surfaces. About ten days later bright orange-yellow clusters of aecia (fungal spore producing structures) became apparent on leaf under surfaces. Leaves often become distorted and petioles may develop wart-like swellings which also bear clusters of aecia. Leaves with infected petioles wilt and die and severely affected trees appear scorched in June. Defoliation of heavily infected trees occurs, but in most cases, infected trees may push a flush of new growth. Repeatedly infected trees may be predisposed to winter damage and to secondary infection by wood decay organisms. 
The alternate host for ash rust is marsh and cord grass which is found in coastal areas. If the disease only occurs sporadically, chemical control is not needed, especially if the infected tree is otherwise healthy. It is very difficult to predict if the disease is going to occur and preventative fungicide applications would need to be applied at bud break. Spraying after infection has occurred will not result in control.
NFG 5/17/2017
