Rose rosette and microscopic mites

A writer from the News Journal heard about invasive microscopic rose bud mites in Delaware that are killing plants.  She became concerned and wanted to learn more about this situation.  These are very small eriophyid mites that spread rose rosette disease, also known as witches’-broom of rose, which is caused by a virus (Emaravirus sp.). The disease is limited to plants in the genus Rosa but some roses are believed to be resistant. Its main host is the multiflora rose, which is considered an invasive plant in Delaware. Multiflora rose has a small white flower in early spring and has taken over many natural areas in Delaware.  It can grow as a shrub or a vine and become intertwined with other shrubs and trees in the woods.  Interest in rose rosette has been generated by the threat to garden roses and also its possible use as a biocontrol for multiflora rose.

Rose rosette disease starts as a red color on the underside of leaf veins and quickly progresses to sharply increased growth at branch tips.  The growth is more succulent than normal and colored in various shades of red. Leaves often become deformed, crinkled, and brittle with yellow mosaics and red pigmentation. As the disease progresses, leaves become very small, petioles are shortened, and most lateral buds grow, producing short, intensely red shoots (witch’s broom). The disease causes the plant to be exceptionally susceptible to freeze damage. Symptoms on cultivated roses are typically less severe than on multiflora rose. Symptoms can be confused with some forms of herbicide damage.

The disease is transmitted by an eriophyid mite, a wingless mite that can travel passively in the wind as well as on contaminated clothing and equipment. The mites feed and reproduce in the tips of rose shoots. Females overwinter under bark or on bud scales of living roses. The females move to newly developing shoots where they lay one egg a day for about 30 days. The young hatch in 3-4 days. They can reach adulthood in about a week depending upon temperatures. Multiple generations occur each year until fall when females seek overwintering sites.  The mites are hampered by low humidity and can only survive about 8 hours without being on a host plant.

The virus is transmitted most readily between May and mid-July when plants are actively growing. Symptoms from new infections usually start appearing in mid-July. In general, smaller plants go through the disease stages more quickly than larger plants. Small plants are usually killed in about 2 years, while a large plant may survive for five years in a deteriorated condition.

To control this disease, you can:

  1. Remove ornamental roses with symptoms. Remove and destroy the entire plant including the roots by burning or placing in a plastic bag. Take care when working with diseased plants as you can spread the mites that spread the disease. Bag the plant before removal, cut it at ground level and then dig out the plant’s roots. Soil need not be removed. Clean tools and put on fresh clothing before moving to a disease-free plant or area.
  2. Plant ornamental roses as far away as possible from known stands of multiflora rose. Maintain at least 300 feet between your roses and any stands of multiflora rose. Even greater distance is preferred especially if they are upwind of your desirable rose plants.
  3. Control the disease by controlling the mite. Start mite control early by pruning your roses hard in late winter (back by 2/3) to remove as many overwintering mites as possible and then spray with horticultural oil to kill any remaining mites. Use horticultural oil or soap, which are less harmful to natural predators that feed on the problem mites. Apply weekly during the months of June and July paying particular attention to the new growing tips where the mites will congregate. Refrain from using leaf blowers around roses as they can spread mites.
  4. Help to isolate your roses. Do not plant roses too close together. With extra space between the plants mite movement can be reduced. Also, consider interplanting roses with other ornamental plants.
  5. Using rose rosette disease as an IPM strategy. The multiflora rose is an exotic invasive species that is responsible for the degradation of millions of acres of farmland and recreational areas. Using the disease to control this invasive weed can cut costs and be considered environmentally friendly for reducing the amount of synthetic chemicals used. However, the disease also affects cultivated roses. One should be extremely cautious and good neighbor-minded when it comes to rose rosette disease.

    Multiflora rose growing at the edge of the woods. It would be great if this virus killed multiflora rose without harming our desirable garden roses, but that does not seem to be the case.

    Multiflora rose growing at the edge of the woods. It would be great if this virus killed multiflora rose without harming our desirable garden roses, but that does not seem to be the case.

Weed Control in the Vegetable Garden

With July 4th approaching many Delawareans are anxiously awaiting the first home-grown tomato from the vegetable garden.  You probably have most of your summer crops planted, so now is the time to focus on weed control.  Weeds steal moisture, nutrients, sunlight and growing space from crop plants.  Their presence can reduce crop growth, quality and yield.  They can also make harvest difficult and they provide cover for diseases, insects and animal pests.

The first line of defense is to plant crops in combination, using sprawling crops like squash, pumpkins, cucumbers, potatoes, etc. to shade the ground around taller crops like corn, pole beans, peppers and tomatoes.  This is the concept between the native American planting of the three sisters—corn, beans and squash.  Corn provides structure for beans to climb.  Beans fix nitrogen from the air and put it back into the soil and squash covers the ground to reduce weeds.
Fall planted cover crops that you till in prior to spring planting, like winter wheat, will reduce winter annual weed germination and as a bonus, improve soil by adding compost.

Use mulch to suppress weeds in the vegetable garden.  Many annual weeds require light to germinate, so a relatively thin covering a mulch can reduce many weed problems.  For tougher weeds, use a thicker layer of mulch (3-4 inches).  Organic mulches, like grass clippings, yard waste, shredded newspaper or straw provide some flexibility because they can be raked around existing plants.  Do not use straw with weed seeds.  Salt hay is a good alternative that will not contain seeds.  If you use grass clippings, know their source and make sure the grass has not been treated recently with an herbicide.

Landscape fabric that allows water penetration is an excellent inorganic mulch to use in the vegetable garden.  Install the sheets of fabric at the time of planting.  Landscape fabric can be used for many years if you carefully remove it in the fall and store it indoors over the winter.  Be sure to by good quality, name brand landscape fabric. You can also use black plastic to cover areas of the garden you are not currently using.  Don’t put black plastic around growing crops because it will exclude water.  If you have a fallow section of the garden, even for a few weeks prior to the next successional crop, black plastic can kill existing weeds by excluding light and raising the soil temperature, so that section of the garden is ready for planting when you are ready to use it.

Tilling is another important method of vegetable garden weed control.   It is easier to kill weeds by tilling when they are young.  Seedlings can be dug up and their roots exposed to air with a hoe.  Once weeds get to be 3 inches tall, they cannot be easily tilled and should be hand pulled, using a trowel to get the entire root system.  Once you have hand weeded an area of the garden, cover it quickly with mulch to reduce the need to continue weeding all summer long.

Herbicides can be used with great care in a vegetable garden.  Before a crop is present, glyphosate (commonly sold as Roundup or other generics) can be applied to kill all the present green growth.  Once you have crops in the ground, avoid using glyphosate in your garden.  Treflan can be used to control germinating weed seedling before the crop emerges.  Poast is a grass herbicide that will kill grass plants without harming broadleaf crops.

Weeds are best controlled preventatively in the garden.  Once you have a lush stand of weeds, you will spend many hours removing them.  But, a few hours spent applying mulch or tilling seedling weeds will prevent problems later on.  Never allow weeds to go to seed in the garden.  That increases your weed pressure for later in the season or the following year.

Tomatoes and peppers planted with weed control fabric are completely weed free.

Tomatoes and peppers planted with weed control fabric are completely weed free.

Enjoy those first ripe, red tomatoes!