Latin name: Arthuriomyces peckianus and Gymnoconia nitens
Reason for Concern:
1) A rare disease, but serious economically.
2) Floricanes will never produce flowers.
3) Unlike all other fungi that infect brambles, orange rust grows perennially and systemically throughout the roots, crown and shoots of an infected plant.
4) Once infected, a plant is infected for life.
5) Does not normally kill plants, but causes them to stunt and weaken, producing little or no fruit.
New shoots are small and typically weak from plant base. Emerging leaves are usually stunted, deformed, and yellow. From afar, the plant has a lightly foliated, spindly appearance. Irregularly shaped, waxy orange blisters turn powdery bright orange on leaf undersides and release spores. Rusted leaves wither and drop in late spring or early summer. Late in the season brown-black pustules develop on lower leaf surfaces. Canes produced on diseased plants may appear healthy; however, these infected canes are usually spineless and do not produce blossoms. Following spring: orange pustules produce on cane leaves. Infected plants appear bushy as many short upright shoots arise from one bud. This disease can be confused with Cane and Leaf Rust.
Causal fungi are almost identical, except for a few differences in life cycle. G. nitens is predominate species in blackberries and dewberries where spores that develop in spring develop systemic infections. A. peckianus predominates black raspberries. Spores that develop in spring from this form do not develop systemic infections, but will if infected by fungi in fall. To tell the difference between the two, germinate the aeciospores on water agar. In late spring, wind and rain-splash spread bright orange spores from infected leaves to healthy leaves. In favorable conditions spores germinate 21-40 days after infection, creating small, brownish black fungi on the underside of infected leaflets. Spores also infect buds on cane tips as they root and also may infect new shoots being formed at the crowns. The fungus becomes systemic; as a result, a few canes from the crown will show rust the following year. Orange rust favors low temperatures and high humidity; higher temperatures decrease spore germination.