Gymnosporangium globosum

Kingdom: Fungi
Phylum: Basidiomycota
Class: Urediniomycetes
Subclass: Incertae sedis
Order: Uredinales
Family: Pucciniaceae
Genus: Gymnosporangium
Species: G. globosum

 

Cedar-Hawthorn rust is a disease caused by a fungal pathogen called Gymnosporangium globosum, which has as hosts members of the genus juniperus, crataegus, malus, pyrus and amelanchier. This fungus requires two living hosts plants in order to complete its life cycle. It must move from one host to another, in this case (Cedar-Hawthorn Rust) from juniper spp. to crataegus spp.

Galls develop on infected junipers, and when mature produce reddish-brown, pointless, gelatinous horns during rainy spring weather. They release spores that are infectious to the other hosts. These spores are dispersed through wind at night and early morning to fruit or leaves of the broadleaf host (hawthorn). Infections will occur if moisture is available for 4-6 hours, and temperatures are adequate. When the spots appear on the surface of leaves, light colored bristles appear on the underside of these leaf spots. In late summer, these structures will release spores that will infect the juniper host, completing the life cycle.

Even though the bright red and orange leaf spots and the gelatinous galls calls attention to the leaves, the disease rarely causes serious damage to its hosts.

You can manage this with fungicide applications beginning when flowers are opening, use of resistant cultivars and avoiding planting the two hosts close together.

Infected fruits and leaves of Hawthorn tree

 

Leaf spot on Hawthorn tree

Sources: http://extension.psu.edu/pests/plant-diseases/all-fact-sheets/cedar-apple-and-related-rusts

http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/cedar-apple-rust-and-gymnosporangium-rusts/

https://en.wikipedia.org/wiki/Gymnosporangium_globosum

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