Northwest berry Foundation

Management Detail

Orange Rust

in Blackberries

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Latin name: Arthuriomyces peckianus and Gymnoconia nitens

Description:

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.

Identification/Symptoms:
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.

Disease Cycle:
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.

Links:

Scouting:

  • Unlike most rusts, this is a systemic disease.
  • Infected plants can be easily identified shortly after new growth appears in the spring. Newly formed shoots are weak and spindly. The new leaves on such canes are stunted or misshapen and pale green to yellowish.
  • Infected leaves are covered with blister-like pustules that are waxy at first but soon turn powdery and bright orange.
  • Later in the season, the tips or infected young canes appear to have outgrown the fungus and may appear normal. Identification is then difficult even though plant infection remains.
  • Notify state small fruit extension pathologist if disease is found.

 
Cultural Controls:

  • Quickly destroy infected plants.
  • Establish new planting from a clean source.

 

For information about chemical controls, check with our Pesticide Guide.

images


photo from Oregon State University


photo from Ohio State University

 

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