Reports on Plant Diseases

Orange rust, the most common and serious of the several rust diseases attacking brambles, is caused by the fungus Gymnoconia peckiana (G. interstitialis). Orange rust infects most wild blackberries and their domesticated cultivars, in cluding the thornless types, all cultivars of black raspberries, and most purple raspberries and dewberries. Orange rust rarely kills plants but causes them to be stunted and weakened so that they produce little or no fruit. Symptoms Orange rust is easily identified shortly after new growth appears in the spring. Newly forming shoots are weak, spindly, lack spines, and are more susceptible to powdery mildew. Leaves are stunted and misshapen and pale green to yellowish. Several weeks later, lower surfaces are covered with blister-like pustules (sori) that are initially waxy but turn powdery and bright orange. These "rusted" leaves wither and drop by early summer.

Click on image for larger version Orange Rust on Blackberry

Young, apparently healthy canes, with normal leaves can be found toward the end of June. Unfortunately, diseased plants are systemically infected, and the fungus is present in the roots, canes, and leaves. "Healthy-looking" canes will not blossom the following spring. Each succeeding spring, the undersides of the leaves will develop the characteristic orange pustules.

Diseased shoots of rust-infected plants are normally too weak to form rooted tips, which limits cane growth and spread. Instead of one shoot arising from the bud, several stunted canes give infected plants a bunchy, "witches-broom" appearance. A rust-infected plant remains diseased throughout its life.

Back to Top

Disease Cycle

In midspring, masses of bright orange spores (aeciospores) are dispersed by wind. These spores contact mature susceptible leaves and produce a germ tube with a swelling (appressorium) that attaches to the host. An infection peg develops from the appressorium, penetrates the cuticle or a stomate, and invades epidermal cells. The fungus sends out branching filaments (hyphae) which produce food- absorbing organs (haustoria) within soft-walled parenchyma cells. The rust fungus gradually spreads throughout the canes and runners until the entire plant is infected. In late summer and early fall, pustules turn black or dark brown because of the formation of another type of spore (teliospores). These teliospores either infect directly or produce sporidia (or basidiospores) capable of infecting (a) the buds on the cane tips, and (b) the buds or new shoots on crowns of healthy plants. The rust fungus overwinters in the infected host tissues. The orange aeciospores form from new pustules the following spring as the canes start to grow. With the formation of the aeciospores, the life cycle is complete. The possibility of overwintering teliospores producing basidiospores in the spring has not been clarified.

Back to Top

Control

Several strawberry cultivars, which include Canoga, Guardian, and Honeoye, appear to be partially resistant to gray mold. Under certain conditions, however, these cultivars may also become infected. Cultivars that produce the most exposed fruit suffer the least damage.

Cultural Practices

1. Plant only certified, disease-free planting stock from a reputable nursery.

2. Before setting out new plants, remove and burn all wild brambles and any cultivated plants that are rust infected, including the roots. If rusted plants cannot be destroyed, do not plant susceptible brambles.

3. When the disease first appears in early spring, dig up and burn infected plants before the pustules break open and discharge spores.

4. Prune out and burn fruiting canes immediately after harvest. Improve air circulation by thinning out healthy canes in the rows and keeping the planting free of weeds.

5. Timely fungicide sprays for control of anthracnose and other foliar diseases does not eliminate rust but will reduce the number of new infections.

6. Some blackberry cultivars have been reported as resistant to orange rust, but their availability and trueness-to-name are questionable. In a properly managed planting, including the control measures outlined above, the disease is usually not serious.

Back to Top

Authors:

Darin M. Eastburn (eastburn@uiuc.edu)
Stephen M. Ries (s-ries@uiuc.edu]