Mountain ash (Sorbus spp.) grows in more than 30 states across the U.S. and in nearly all Canadian provinces. The American mountain ash (S. Americana) and northern mountain ash (S. decora) are native to North America, while other species, including the European mountain ash (S. aucuparia), were introduced from other continents. Mountain ash is vulnerable to several types of fungal pathogens, including those responsible for leaf spot diseases and cankers.
Leaf spot diseases of mountain ash and other ornamental tree species result from an infection by one of several genera of parasitic fungi. Various fungi, including those from the genus Alternaria and Phyllosticta, are responsible for this disease in mountain ash. Initial symptoms appear as discolored spots on the upper and lower surface of leaves. The spots eventually merge and cover the entire surface of infected foliage. Infested leaves rot and fall from the tree as the tissue decay worsens. Leaf spot diseases are rarely serious on their own, but they can compound with other damaging agents, like environmental stress or insect pests, to cause permanent or fatal damage.
The combination of sun damage and infection of canker-causing fungi is a leading cause of fatal injury to young mountain ash trees in Wisconsin, according to a University of Wisconsin publication. This ailment is called sunscald-fungal canker complex. The relatively thin bark of the ash tree is susceptible to damage from excessive heat or sunlight. Tissue damaged by sun exposure is an easy target for parasitic fungi, which infiltrate the cracks in the bark of the tree's trunk and branches. Several fungal pathogens can cause cankers, which are unsightly, distorted growths along the branches and trunk of a tree. These cankers house the reproductive bodies of the fungal invader, allowing it to survive through the winter inside the malformed growth. Leaves and stems along infected branches wilt and become discolored. Affected branches may completely die if left untreated.
Apple scab is a well-known ailment of apple and crabapple trees (Malus spp.), but it can also infect mountain ash and other species. This disease produces brown, green or yellow spots on its host's leaves. The fungus responsible, Venturia inaequalis, can also destroy fruit by forming dark, rotting legions on its surface. Like most other fungal diseases, apple scab thrives in humid and warm environments. The pathogen will readily spread to neighboring plants by releasing wind-borne spores.
Management and Prevention
Planting mountain ash in a conducive environment and keeping it healthy is key to preventing the onset of fungal diseases. During the summer, mountain ash should be watered regularly to prevent scalding. Foliage and branches of mountain ash and surrounding trees should also be inspected routinely for signs of fungal damage. Diseased tissues can be pruned and removed early to prevent a serious outbreak. It is critical to remove cankers, which house fungal growths, and plant debris of infected plants each year. Clean garden tools in a sterilizing solution to prevent the transmission of fungal pathogens from infected plants to healthy ones.