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Mites KO Knockout Roses

How to fight back

Knockout roses suffering from rose rosette are covered with purple-red scales and twisted stems with ­distorted flower buds.

     Popular as Knockout roses are, they are not immune to viruses. They are susceptible to witches broom and to rose rosette, which is becoming a frequent problem. Rose rosette is spread by infested pruners and by a microscopic eriophyid mite. 
    Rose rosette appears as vigorous, new multi-stem growth with purplish-red stems, leaves and flower buds. The flower buds are not as prominent as those on normal plants. The infected buds are covered with purple-red scales. Some of the foliage may appear as if it had been sprayed with weed killer. Stems with flower buds may become twisted and distorted.
    Recent research indicates that removing the infected stems by pruning them near the ground with clean sterile pruners can prevent the spread of this virus. The blades of the pruners need to be cleaned from all debris and dipped in rubbing alcohol between each cut.
    The virus migrates slowly down the stem, so the sooner you identify the problem the more likely you can prevent it from spreading. Infected branches should be incinerated or discarded but not dumped in the compost pile.
    If you are in the habit of deadheading the plants to encourage flower succession, use a sterilized sharp knife or learn to snap the wilted flowers from the stems. Avoid using pruners.
    If the disease has progressed, there is some evidence that pruning the infected plants to the ground and allowing the roots to generate new stems might be possible. Since Knockout roses are grown from rooted cuttings, the new growth will be of the original variety. However, the eriophyid mite can still infest the new growth, so you may wish to replant the area with another species that will not be affected by the virus. Consider abelia, southern hibiscus or dwarf crape myrtle.
    The eriophyid mite can be controlled by spraying with two percent horticultural oil when temperatures are below 80 degrees. It also appears to be susceptible to the liquid form of Sevin.


The Dying Tomatoes Mystery Solved

    To recap the problem with Pat and Paul Fessler’s Crownsville garden: In one six-by-30-foot area all the plants, tomatoes and cucumbers, withered and died over a 12-hour period.
    I requested a soil test, as the plants died quickly, an indication of high ­levels of sodium, salts, manganese or boron in soil. But that wasn’t it.
    Then the Fesslers provided more information: We gather several hundred bags of oak leaves every few years, spread to a depth of three feet, and let the garden rest the following year.
    That gave me the solution.
    The leaves were not fully composted, with too high a carbon-nitrogen ratio, thus starving the plants of nutrients and oxygen.
    Mix grass clippings and soil with the leaves and keep the area wet until it all composts in place. It will take two to three years for three feet of leaves to rot and break down sufficiently.