Orange rust of caneberries—Arthuriomyces peckianus and Gymnoconia nitens
Blackberry, black and purple raspberry, and dewberry are susceptible to infection by one of two orange rust pathogens. Blackberry and dewberry can be infected by G. nitens. Black and purple raspberries can be infected by A. peckianus. Red raspberry is not affected by these fungi.
Identification
Caneberries infected with orange rust fungus produce spindly, weak shoots from the base of the plant. These weak shoots usually never produce blossoms. Viewed from afar, the infected caneberries are easily distinguished from healthy plants. The infected plants have sparse foliage and an overall spindly appearance.
In spring newly emerging, infected leaves on primocanes (first-year, green shoots) are deformed, narrow, undersized, and pale green to yellowish. Later the underside of leaves develops irregularly shaped, waxy, orange pustules of fungal fruiting bodies (aecia, photograph by Oregon State University). Aecia soon turn a powdery, bright orange as they rupture and release aeciospores. Leaves that produced aecia generally dry up and drop prematurely by midsummer. Late in the growing season brown to black fruiting body pustules (telia) develop on the lower surface of leaves of infected plants and release teliospores.
Note that cane and leaf rust, late leaf rust, and yellow rust also infect caneberries. These diseases can produce symptoms resembling those of orange rust.
Life cycle
In spring, when the bright orange spores are being produced, they are released from the pustules and scattered by wind. During cool, moist weather the spores germinate. If the spores land on a host, they penetrate the leaf surface and the fungus colonizes the leaf. In late summer and fall teliospores are produced. These are also spread by air currents and during cool, moist weather can cause additional infections of shoot tips and buds on rooting cane tips. Fall infections of buds, leaves, and shoot tips can result in a permanent, systemic infection of the crown, roots, and shoots.
The two orange rust fungi cause the same damage. They are distinguished by differences in their hosts, spore appearance when examined under magnification, and whether they have a life cycle that is relatively long (A. peckianus) or short (G. nitens).
Damage
Infected leaves may wilt and drop prematurely. Although these pathogens are not known to kill plants, they severely weaken caneberries. Systemically infected plants produce few or no fruit.
Solutions
Obtain disease-free planting stock from a reputable, commercial nursery. Thin healthy canes and control weeds; these practices increase air circulation, which helps to reduce orange rust spore germination and infection.
To prevent the spread of orange rust to other caneberries, regularly inspect caneberries and promptly remove infected plants including the roots. Dispose of the plant debris away from where caneberries are grown. Remove infected plants in late winter or early spring before they produce fruiting bodies and release spores.
Also remove nearby wild blackberries because they are a host of orange rust from which fungal spores may move to caneberries grown for their fruit. Note that where host plants occur in riparian areas there are regulations that limit vegetation removal. Before managing vegetation along waterways, contact the California Department of Fish and Wildlife to learn of any rules that may apply to your situation.
Fungicides are not effective for managing orange rust. As discussed in Pest Notes: Wild Blackberries, an herbicide that kills broadleaf plants, such as glyphosate or triclopyr can be used as a spot treatment to kill infected caneberries and wild blackberries before removing them.
Adapted from Orange Rust: Gymnoconia sp. and Arthuriomyces sp.(PDF), Cornell University and Pest Management Guidelines: Caneberries, University of California Statewide Integrated Pest Management Program (UC IPM). |
Small, deformed leaves of black raspberry infected with orange rust.
A spindly, rust covered shoot of blackberry with deformed, undersized leaves (right) in comparison with healthy foliage.
A black raspberry with the orange fruiting bodies (aecia) of orange rust on the underside of leaves.
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