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Friday - April 17, 2015

From: austin, TX
Region: Select Region
Topic: Diseases and Disorders, Edible Plants
Title: Rust on Blackberry
Answered by: Anne Van Nest

QUESTION:

I have two new blackberry bushes that I planted fall 2014. Only weeks after I planted the plant began to have rust color balls under the leaves then leaves began to die and fall off. When I returned to the help desk at the Austin nursery I was told this was normal for fall and all would be fine in spring. I had googled prior to going back, and reading about rust had me nervous for the soil in my vegtable garden. Spring is here and small orange colored balls are back and the leaves are turning yellow and dying. My number one concern is my soil and many other healthy plants. I have never had a problem like this and do not want to pass it to my other vegetables. The bare root Dewberry I planted several feet away looks fabulous. Should I treat it or cut my losses and plant two healthy ones next fall.

ANSWER:

Take a look at the AgriLife Extension Texas Fruit & Nut Production factsheet on Blackberries that was written by Monte Nesbitt, Jim Kamas and Larry Stein (Extension Fruit Specialists). There is a section on orange rust fungus and a picture of the disease that you can use for comparison. Orange rust fungus produces masses of orange colored spots on leaves in the spring. It moves through the plant, and all canes produced after that will be non-productive. It is a serious problem for susceptible varieties, and the thornless varieties as a class have a lot of problems with this disease. Start new plantings with disease-free nursery stock and quickly remove infected plants exhibiting symptoms.
The University of Illinois Extension also has a good website with information on Orange Rust of Brambles.

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, including 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.

Figure 1. 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.

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.

Cultural Practices

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

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.

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

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.

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

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.

Authors are Darin M.Eastburn([email protected]) and Stephen M. Ries ([email protected]]

Cornell University also has a factsheet on Orange Rust.

From the Image Gallery

Dwarf red blackberry
Rubus pubescens