Hope on the horizon

Rose rosette is a viral disease that is specific to roses and often leads to the eventual death of infected plants. The disease is transmitted by a microscopic mite. The eriophyid mite (Phyllocoptes fructiphylus) tends to hide in the buds of the plant. The mite picks up the virus and transfer it from one section of the rose to another.

Multiflora rose (Rosa multiflora) is particularly susceptible to rose rosette disease. The invasive species’ decades-long march across much of the Eastern U.S. has likely been responsible for spreading the disease steadily eastward from its origin in the Rocky Mountains. Rose rosette disease has worked its way into New England and is also found in the Midwest, Texas and the Southeast U.S. In January, Florida became the latest state to report the disease. However, because it is not thought to be a new disease, and the mite is thought to be endemic, many plant diagnostic labs are not required to report it to APHIS.
 

Control

If you catch it early, you can remove the disease by pruning the symptomatic portion. But if you have the mites and they have already spread to the healthy parts of the plant, pruning may not curb the disease’s progress. In that case, destroying the infected plant material is the only option.

“If you think you’ve caught it early enough, you can prune it way back,” says Joe Bischoff, regulatory and legislative affairs director for AmericanHort. “It may be that it hasn’t made the transfer into the actual root or base of the plant itself. Then of course you want to get rid of the mite with side treatments.”

In a production setting, growers should destroy infected plants as soon as possible and observe other adjacent plants for symptoms. Infected plants should be burned and/or bagged.

AmericanHort has developed a website at www.RoseRosetteDisease.com with plenty of useful content for growers.
 

Symptoms

Early on, the leaves show smaller than normal, and are often distorted. Typically one or two shoots leaves may stay red or pink instead of turning green like a normal leaf. On certain types of roses, the disease affects thorn production. Excessive thorniness and unopened flower buds are other symptoms of rose rosette. Plants that have been infected for several months will show multiple deformed shoots bearing deformed flowers, and a dense “witch’s broom” cluster of leaves and stems.

Worried you’ve got rose rosette disease in your nursery? The first thing you need to do is figure out if what you have is really rose rosette disease. The best course of action is to submit it to a plant diagnostic lab for confirmation.

“It’s very easy to confuse because herbicide overuse can cause symptoms similar to rose rosette disease,” Bischoff says. “That is why a thorough diagnostic from someone who is really skilled and has the right background and tools is so important.”

Finding a laboratory willing to take the necessary steps to reliably diagnose the issue can be difficult, but Oklahoma State University’s Plant Disease and Insect Diagnostic Lab is accepting in-state and out-of-state samples. The lab’s plant pathology diagnostician, Jen Olson, has significant experience working with RRD and is a reliable resource on the issue.

To send a sample, snip off two or three symptomatic canes/leaves and place them inside two plastic bags (double bagged). Symptomatic leaves are the preferred tissue for testing. Mail everything to the address on the form, found at this link: http://entoplp.okstate.edu/pddl/pdidl-form.pdf

The cost is $35 per sample. Checks should be written to Oklahoma State University and be included along with the sample. Reports are sent directly back to the submitter, preferably by emailed PDF.
 

Prevention

Because R. multiflora is the most prevalent host for the disease, all R. multiflora plants should be eliminated from fields within a 100-meter radius of rose nurseries and gardens. This is important because contaminated mites are easily blown from wooded or forest edges where the diseased plants may reside to nearby nursery plants.

Conard-Pyle/Star Roses’ Growers Manual suggests using a combination of mechanical and chemical methods to eliminate R. multiflora. Frequent, repeated cutting or mowing at a rate of three to six times per growing season, for two to four years, and repeated applications of systemic herbicides to freshly-cut stumps or regrowth.

Rose rosette is particularly challenging because of the multiple components involved with the disease.

“You have an invasive plant species, a mite that vectors the pathogen, and then the virus itself,” Bischoff says. “It is a really dynamic situation and one you don’t encounter very often when trying to manage pests and pathogens. It’s a unique challenge, which makes it difficult in some respects but it opens the opportunity for different management strategies to be effective.”

Plant diseases are often perceived as difficult because of one particular tricky issue. A disease spreads quickly, or perhaps it is maintained in the soil for a long period. But rose rosette’s multiple components allow for multiple tactics to combat the disease.

“If you control the mite, you control the spread,” Bischoff says. “If you figure out a way to disarm the disease, then you’ve covered it. If you somehow eradicate multiflora rose from your environment, chances are you won’t encounter the disease. There are multiple avenues we can take to tackle this. In that respect it’s promising for the long term sustainability of roses. We’re going to figure this out. There will be a path to figure it out. We just need to find the most effective path.”

Bischoff says there are researchers working to find that path. One research group has submitted a pre-proposal for a Specialty Crops Research Initiative grant, hoping to receive funding to tackle some of the work. Joseph Albano, research programs director for the Horticultural Research Institute is on the advisory committee for the group, which is led by David Byrne, professor and Basye Endowed Chair in Rose Genetics at Texas A&M University’s Department of Horticultural Sciences.

“What those researchers could end up finding is that the best way is a multi-pronged attack, or a way to slowly chip away at three components, or maybe they find a silver bullet to deal with one issue and take care of (the disease) that way. That is the most promising aspect of this pest-pathogen complex,” Bischoff says.

For more: www.roserosettedisease.com

Photos courtesy of Missouri Botanical Garden