Be on the lookout for two prominent plant diseases: downey mildew on impatiens and rose rosette disease on our favorite rose bush, the Knock Outs.

One of our favorite summer landscape annuals, impatiens, is often infected with a disease called downey mildew. The mildew disease became prominent last year on the traditional impatiens and is likely to be the culprit this year as well.

Downey mildew was likely introduced into the landscape by purchased plants infected with a fungus-like organism in commercial greenhouses. The pathogen overwinters in the soil.

Downey mildew reproduces and spreads in mild, wet conditions. High humidity and wet leaves foster development of this disease.

Impatiens’ downey mildew symptoms can be hard to spot before the plant is plagued with rapid defoliation, leaf discoloration, downward cupping of leaves and white, powdery mildew on the undersides of the leaves. Eventually the plant will look like a bare stem and will die.

Fungicides are not effective in eliminating downey mildew.

The UGA Cooperative Extension Office in Hall County recommends if the symptoms are present on impatiens, then gardeners need to remove them, seal them in a plastic bag and dispose of them. Replace diseased impatiens with a different type of plant this season and the next. Torrenia or Wishbone plant is an excellent alternative to impatiens. Also, the New Guinea variety of impatiens is not affected by downey mildew.

Northeast Georgia will likely see this disease on traditional impatiens for a while, so avoid purchasing them. Growers are not expected to market traditional impatiens for a few seasons.

Another disease which is viral in nature is the rose rosette virus.

Knock Out roses are by far one of the most popular plants in the area. But as this rose has been planted in such mass quantities, it is very easy for a pest or disease to decimate the over planted (referred to as monoculture) flower, as in the case of the famous red tips (Photinia) planted in millions of landscapes years back. Red Tips began to suffer from a fungal leaf spot known as Entomosporium. When monoculture occurs as in the case of Knock Outs, the causal agent is known as rose rosette disease.

The culprit is a tiny microscopic mite. Mites transmit the virus from one rose to another through wind and other insects and birds. The mites feed on plant sap from the leaf and stems. If the mites are carriers of the Rosette disease, plants will show signs typically within one to three months.

Infected roses exhibit reddened thick growth on infected branches and the stems become thick and succulent. Stems exhibit an abnormally high amount of thorns, which are green or red. Lateral branches on the roses grow excessively and the new growth has a witches’ broom appearance.

Symptoms of the virus are evident in late spring and summer and progress through the growing season. By late summer, the plant will have a large amount of abnormal, gnarly growth.

Unfortunately, there is no cure for rose rosette disease. To avoid spread of the virus, nearby roses can be treated with an insecticide spray containing bifenthrin or a horticultural oil every two weeks during the growing season through September. But once the virus in detected, it may have spread to adjacent roses.

To lessen the chance of the disease, buy clean stock roses. Don’t be tempted to get that killer deal at the end of the season from the garden center. Inspect the plants before buying.

When pruning Knock Outs, work cleanly. Sterilize pruners between cuts and individual plants. Pruning can spread the disease from one plant to another.

If you observe rose rosette disease, dig the Knock Outs up and bag the infected plant. Avoid over planting the roses in the future.

Using fresh soil in containers, allowing space between plantings for good air circulation and changing locations may increase chances of survival. Good sanitation practices are also important.

If you are concerned your Knock Outs or impatiens are affected, call the extension office or bring in a sample for an accurate diagnosis.

Wanda Cannon serves as Master Gardener coordinator and horticulture assistant for the UGA Cooperative Extension office in Hall County. Contact her at 770-535-8293. Her column appears biweekly and on gainesvilletimes.com/life.