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Orange
Rust of Brambles
October
1996
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
Figure 1. Orange Rust on Blackberry
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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.
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
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Plant only certified, disease-free planting stock from a reputable
nursery.
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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.
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When the disease first appears in early spring, dig up and burn infected
plants before the pustules break open and discharge spores.
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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.
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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:
Darin M.
Eastburn (eastburn@uiuc.edu)
Stephen M. Ries (s-ries@uiuc.edu]
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