A few years ago, a Master Gardener friend tore out all her KnockOut roses because of rose rosette disease. Since then, I’d heard about other gardeners having this problem; luckily, it had skipped our place.
But last May it showed up on the 100-foot row of KnockOuts along our driveway; their loss would be an unmitigated disaster. If your roses begin to look like this, here’s what to do.
Rose rosette is caused by a virus spread by a very small mite. The mites don’t fly, but are blown around by the wind. Symptoms include red leaf veins and red-colored new growth, followed by deep red side shoots. Rose rosette is often called “witches’ broom disease” because of how the infected foliage looks.
The mites overwinter on living roses, and then in spring move to developing shoots to lay eggs that hatch in a few days. The young mature in about a week, and multiple generations occur each year.
Mites transmit the virus when plants are in active growth. Inside the plant, the disease slowly spreads downward from new top growth to the roots.
What can you do if your plants get rose rosette disease? Unfortunately no cure exists. If the plant is totally infected, cover it with a garbage bag so that you don’t spread the mites to adjacent plants, and dig it out, roots and all (you don’t need to remove the soil).
If the damage is only on new top shoots, here is what you can do to maybe save your plant:
— First, spray to kill the mites. Horticultural oil and insecticidal soap are effective organic choices.
— Cut all stems that are affected down to the base of the plant.
— Remember the rule that nobody follows about dipping cutting tools in rubbing alcohol or a bleach solution after each cut? Be sure to do this or you might spread the virus to other plants or other parts of the same plant.
Then follow these Integrated Pest Management (IPM) tactics to control the mite that transmits the virus:
— The mite’s main hosts are wild multiflora roses; keep cultivated roses at least 300 feet away from multifloras.
— Prune bushes in February before the mites become active, then spray with horticultural oil to kill any that have overwintered. (As an aside, we’ve noticed less freeze damage on February-pruned vs. fall-pruned roses, especially when winter temperatures fall below zero.)
— Follow a regular spray program from bud break through July, concentrating on the new growing tips.
Put all diseased plant material in the regular garbage; do not compost.
Of our 30 KnockOuts, only one plant was infected, and symptoms were limited to new top growth on several of that plant’s stems. I sprayed it and the other plants in the row with insecticidal soap to kill the mites, and then cut all stems of the infected plant down to the base.
I repeated the spray program each week for a couple of weeks. No symptoms have appeared on any other roses. Initially, all the new sprouts on the cut-down rose were healthy, leading me to believe that I had caught the problem early enough.
Alas, later in the season more “witches brooms” appeared, so the entire plant was removed.
Have a home gardening question? Call the Master Gardener Plant Clinic at 456-3575.
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