Photo 1. Early symptom of orange rust. Photo: K. Demchak
Question: I'm attempting to manage orange rust in a bramble planting. I know what the early spindly growth looks like in the spring, and I am familiar with the orange cheese puff-looking stage. But what happens between these 2 stages and what symptoms should I be looking for? Chris Claridge, Albany, NY
Photo 1 shows the early symptoms where growth appears spindly, and has a golden-orange hue to it. Leaves on these relatively weak primocane shoots may also appear pale green to yellowish and narrow with a twisted or cupped edge. Bright orange rust pustules will eventually be apparent on the lower side of the leaves of these plants. This is a good stage at which to dig out the entire infected plant--including the root system--as you can remove the plant before the spores are produced, which have potential to spread the disease to healthy plants. Orange rust is systemic within the plant, and cannot be cured.
Photo 2 below shows some eruptions of blister-like rust pustules of various sizes on the leaf undersides, which are often described as "waxy" in appearance. Some are quite small, and much of the leaf underside still appears green. Spores (aesiospore: one of the many spore types of orange rust) start to be produced at this stage followed by their release and spread by wind.
Photo 2. K. Demchak, Penn State
Photo 3 shows the same plants about 2 weeks later at the most apparent and familiar stage of the disease cycle. At this stage much of the leaf underside is covered with orange pustules and abundant spores are being released.
Photo 3. K. Demchak, Penn State
Finally, in Photo 4, the badly-affected leaves dry, curl up, and die and will eventually fall off the plant. In this last photo, you can see the accumulated spores lying on the upper surface of the leaves in the background.
Photo 4. K. Demchak, Penn State
It should be pointed out that there is another rust--yellow rust--which may also be present in the spring. Yellow rust is present usually only on red raspberry. Orange rust, on the other hand, has not been reported on red raspberry, instead being present on black raspberry and blackberry. Plant removal is not necessary for yellow rust, as it is not a systemic disease. With yellow rust, yellow pustules in the early spring will be present only on the upper side of the leaves and also on the canes, compared with lower sides of leaves for orange rust. Yellow rust will also not deform the leaves as it happens in case of orange rust (spindly, pale green to yellowish narrow leaves with a twisted edge).
How can orange rust be managed?
First, since orange rust is a systemic disease, keep in mind that any plants propagated from infected plants will also be infected. That means that canes dug from infected plants, or plants grown from root pieces taken from infected mother plants, will also be infected. So it is important to obtain clean plants from a reputable nursery source. Removed entire infected plants early in the spring whenever possible. Cassandra Swett, Berry Pathology, University of Maryland wrote an article that provides additional information on the life cycle of orange rust, and the additional control measures that you can take, including which fungicides are effective and the timing for their use.