Basic Info: Rust infections occur more frequently on the fruit, but can also be found on twigs, buds or sometimes leaves. The rust fungus requires two hosts to complete its disease cycle; a Rose family host and a Juniper host. Spores are produced on infected junipers in spring, spreading by wind to susceptible hawthorn and Saskatoon plants nearby. Beginning a few weeks after initial infection, the fungus produces a spore stage resulting in conspicuous long white tubes covering the fruit, that eventually rupture releasing reddish spores which spread by wind to infect nearby junipers. Cankered areas on juniper hosts can continue to produce spores for several years as the fungus expands. Infected leaves often have small flecks or lesions of necrotic tissue. Infected fruit may either fall prematurely or remain on the tree, but be misshapen with sunken lesions and necrotic tissue deep in the fruit. A spore stage on fruit produces distinguishable long white tubes that eventually release reddish coloured spores. Infections on the alternate host (junipers) produce swollen, spindle-shaped cankers on stems, which exude a red/orange gelatinous (spore producing) mass during rain periods in spring.
What We Recommend:
- Avoid planting hawthorns and Saskatoons near Junipers, which are alternate hosts for these rusts.
- Fungicides containing cholorothalonil applied before bloom or myclobutanil applied every 14 days after disease appearance provide some control.
- Remove all infected material completely from the area.
Trees Effected by Juniper Hawthorn Rust: Junipers, Hawthorns and other Rose family hosts.
Government Informational Source Link: Gymnosporangium Rust - agriculture.canada.ca
