Cause Gymnoconia nitens (formerly Arthuriomyces peckianus), a fungus that was rarely found in the Pacific Northwest but common in eastern North America. Records at OSU's Mycological Herbarium and Plant Clinic indicate that "orange rust" was found from 1913 to 1923 in Douglas, Hood River, Umatilla, Union, and Wasco counties of Oregon. In 1997, G. nitens was found on 'Kotata' blackberries in an isolated field in the Willamette Valley. The field was removed a few years later. The disease was found again in 2018 and in 2021 in several 'Black Diamond' fields in Marion County. The fungus was also reported from California on loganberry and thimbleberry. The disease has been seen on wild Himalayan blackberry (R. bifrons) in Benton and Marion counties.
The systemic nature of this disease is what distinguishes it from the other rust fungi on blackberry. Primocanes are already infected and form spores soon after sprouting from the crown. Infected plants cut in May to limit spread will resprout in June with spores.
Red raspberry is resistant while black raspberry are susceptible. Susceptible cultivars of blackberry include Black Diamond, Chester, Chickasaw, Navaho, Ouachita, and Triple Crown. Resistant cultivars have been listed but some have been found susceptible in other areas.
Two forms of the fungus have been described. One form is predominant on blackberries and dewberries, does not produce telia, and is referred to as a short-cycle form (and associated with the name Gymnoconia nitens). Aeciospores that develop in the spring from this form develop systemic infections. The other form is predominant on black raspberries, forms telia, and is referred to as a long-cycle form (and associated with the name Arthuriomyces peckianus). Aeciospores that develop in the spring from this form do not develop systemic infections but produce telia. Infections from teliospores that are produced in the fall will eventually develop into systemic infections. The only way to tell the difference between the two forms of this fungus is to germinate the aeciospores on water agar.
Symptoms Many weak, spindly shoots arise from infected roots and crowns in spring. Sometimes only one primocane may be infected making the disease difficult to find. Infected shoots may lack thorns and resemble weeds or plants with herbicide injury. Some primocanes and floricanes may look healthy later in the season, but floricanes never produce flowers due to systemic infection. In spring, unfolding leaves can be stunted, misshapen, and pale green to yellow. Lower leaf surfaces become covered with blister-like pustules (aecia). Aecia are waxy at first, then turn a powdery bright, almost fluorescent orange. Infected black raspberry leaves may wilt and drop off or may develop brownish black pustules (telia) on the lower leaf surface. Might be confused with cane and leaf rust, which has dull, more yellowish pustules, or with blackberry rust.
- Scout for the disease during spring and summer.
- Quickly remove and destroy infected plants including roots.
- Establish new plantings from a clean source or from tissue-cultured plants.
Chemical control None will cure existing infection as they only prevent new infections. Herbicides can be used as a spot treatment to kill infected plants.
- Abound at 6 to 15.5 fl oz/A is registered for Phragmidium spp. and may be useful. Do not apply with silicone-based surfactants. May be applied on the day of harvest. Group 11 fungicide. 4-hr reentry.
- QuiltXcel at 14 to 21 fl oz/A is registered for Phragmidium violaceum and may be useful. Do not use within 30 days of harvest. Sprayers should not be used on apples. Group 3 + 11 fungicide. 12-hr reentry.
- Rally 40 WSP at 1.25 to 3 oz/A. Applications may be made up to the day of harvest. Group 3 fungicide. 24-hr reentry.
- Spectracide Immunox Multi-Purpose Fungicide Spray Concentrate for Gardens at 0.67 fl oz/gal water. May be applied up to the day of harvest. Group 3 fungicide. H
Note: Some registered products offer only suppression of this disease and thus are not recommended for use. These products include Cabrio and Pristine.
References Dodge, B.O. 1924. Uninucleated aecidiospores in Caeoma nitens, and associated phenomena. Journal of Agricultural Research. 10:1045-1057.
Laundon, G.F. and Rainbow, A.F. 1969. Gymnoconia nitens. CMI Descriptions of Pathogenic Fungi and Bacteria, No. 201. Surrey, England: Commonwealth Mycological Institute.