​​​PPDL Picture of the Week for

June 6, 2016

Orange Rust in Blackberries and   

Black Raspberries

Bruce Bordelon, Professor, Department of Horticulture and Landscape Architecture, Purdue University

Orange rust is one of the most common diseases of blackberries and black raspberries in the Midwest. It is a systemic disease that is confined to Rubus spp. (autoecious).  There are two forms, a long-cycle form that occurs on black raspberry, and a short-cycle form that occurs on blackberries. There are two causal organisms:Arthuriomyces peckiamus (the long-cycle form) andGymnoconia nitens (the short-cycle form). All varieties of black raspberry and many varieties of erect blackberries are susceptible. Red raspberries are immune to orange rust.
Symptoms: Symptoms of the disease can be seen in early spring when new primocanes emerge. Infected plants produce an abundance of spindly canes that have misshapen leaves that are often pale green or yellow. Within a few weeks of emergence, the lower leaf surfaces become covered with blister-like masses of orange aeciospores. This is the most characteristic stage of the disease. Infected plants are quite obvious with brilliant orange leaves. 
Disease cycle: Aeciospores from infected canes spread the disease to mature leaves on canes of other plants. Infected leaves develop teliospores later in the summer, which germinate to produce a basidium, which in turn produces basidiospores. The basidiospores infect buds on primocanes at the base of the plants. These infections will spread down through the canes and into the crown, eventually infecting the entire plant.  In subsequent years, new canes that emerge from infected crowns will be infected. The fungus persists in the crowns as perennial mycelium. 
Damage: Infected plants seldom die, but are very weak and do not produce quality fruit. Thus they represent a complete loss. But heavy infestation of plantings is not common. 
Management and Control: Orange rust is a systemic fungal disease. Once infected, a plant cannot be cured. Always start by planting disease free plants from a reputable nursery. Removing infected plants entirely, including the roots, is the best way to prevent spread. Remove infected plants as soon as they appear in the spring before they release spores. Eradicate infected wildRubus plants in the near vicinity of your planting. Any management practice that encourages air circulation within the canopy such as thinning canes within the row, removing floricanes immediately after harvest, weed management and proper nutrition will aid in disease control by reducing the duration of leaf wetness. The multiple cycles of infection by the various spore stages make fungicide management very difficult. While a few fungicides are registered for use, chemical control alone is impractical.  Proper management is key to controlling orange rust.

References: Compendium of Raspberry and Blackberry Diseases and Insects. 1991. Ellis, M.A., R.H. Converse, R.N. Williams, and B. Williamson, editors. APS Press

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Fig.1-Wild blackberry showing normal and infected leaves, upper and lower surface

Fig.2-Close-up of blister-like pustules on the underside of a blackberry leaf

 

Fig.3-Infected and normal wild blackberries

 

Fig.4-Black raspberry infected by orange rust, upper leaf surface

 

Fig.5-Black raspberry infected by orange rust, lower leaf surface

 
Fig.6-Thornless blackberry plant with about half the canes infected and half not infected

 

Fig.7-Orange aeciospores deposited on a mature leaf​