Gardening in Larimer County

At this time of year, my lilac and sweet pea leaves are covered with a white, powdery substance that looks and feels like talcum powder. The culprit, a fungus called powdery mildew, is a widespread and easily recognized plant disease. Grains, vegetables, trees, shrubs, ornamentals, and even weeds are susceptible.

Although there are many different types of powdery mildew, the symptoms of the disease are similar. Typically, the fungus grows on the upper surface of the plant leaves. In severe cases, it can affect the bottom of leaves, stems, buds, flowers, and fruit. Succulent, new tissue is more susceptible to the disease. The white to gray powdery growth is the fungal body, or mycelium. Haustoria, which invade host plant tissues, function like roots and absorb nutrients.

With a closer look, you might see round structures that look like black pepper. These are the reproductive bodies, or cleistothecia, of the fungus. These fruiting bodies produce spores that spread the disease with the help of wind, insects, and rain. Cleistothecia can over winter in fallen leaves and other plant debris. Composting infected leaves is not recommended.

The severity of the disease depends on variety of the host plant, age and condition of the plant and weather conditions during the growing season. The good news is that powdery mildew is host specific. Therefore, the variety of mildew affecting your lilac bush can not spread to your rose bush. The bad news is that this disease does not need surface moisture in which to spread between like hosts. All that is required is high relative humidity. Plants in shady, crowded, poorly ventilated areas are especially prone to cases of powdery mildew.

Roses, Kentucky bluegrass, vegetables, and other plants are being developed with resistance or tolerance to the disease. Planting these varieties can help reduce or prevent cases of powdery mildew in your yard. If you have susceptible plants, avoid overhead watering to help reduce humidity. Prune trees and shrubs to increase air circulation, and remove infected leaves or other plant parts. Avoid late summer applications of nitrogen fertilizer to limit the production of succulent growth.

If these cultural practices do not prevent severe powdery mildew outbreaks in your yard, several fungicides are available. Chlorothalonil, sulfur, or triforinel products can be sprayed in seven to fourteen day intervals. As with all chemical control products, read the label carefully for recommended rates and uses.

For more information, see Fact Sheet # 2.902 - Powdery Mildews.  You can also call Planttalk Colorado toll free at 1-888-666-3063 and request recording #1415 on Powdery Mildew.

Question: I have a daylily that is about five years old. It is very big and has always had beautiful blooms. This year, it does not have as many blooms as usual. What is wrong?

Answer: Many perennials must be divided after a number of years growing. Daylilies should be divided every three to five years. This can be done when the plant is through blooming, preferably in the fall. If the plant is really big, it will take two people with digging forks or shovels to get it out of the ground. Divide the clump any way that seems to work. You can put two digging forks back-to-back and pry the clump apart or you can use a sharp knife to cut the fleshy roots apart. Divide it into pieces, each with a nice-sized section of roots and sword-like foliage. Don't divide the clump into too many pieces as it will take them a long time before they bloom. Just get some good pieces and you will be able to have a few new daylilies to plant or to give away.

Question: I would like to fill in a perennial area with a few annuals for color. Is it too late to plant them?

Answer: The temperature has been hot and planting during this time is risky. However, plants purchased from garden centers have roots which can do well if the plants are properly watered and mulched.

Question: My columbine grew tall and flowered early this year. Is it okay to cut them back?

Answer: Cutting foliage and removing spent flowers is routine maintenance for perennials. Foliage will grow and you may enjoy a second flowering in the fall.

Question: Some of my tomatoes have brown spots on the bottom. Is it a disease?

Answer: What you are describing sounds like blossom end-rot, a symptom that occurs when tomatoes are not evenly watered. With our dry conditions, be sure and keep the plants moist with regular irrigation. Place an organic mulch over the root system to hold moisture and prevent weeds.

Harvest lavender blooms now to keep plants tidy and increase blooming.

Salt deposits can build up in soil causing leaves to burn. Flush out this white, crusty  buildup with water. This occurs in both indoor and outdoor plants.

Cut sprigs of rosemary and freeze whole for future use.

Cut back thyme growth by two thirds to encourage new growth.

Turn and dampen compost pile often. Flies breed in moist clippings if not turned regularly.

The authors have received training through Colorado State University Cooperative Extension's Master Gardener program and is a Master Gardener volunteer for Larimer County.

Gardening and Insect Fact Sheets are available on-line by clicking HERE.

Return to Master Gardener Articles

This page updated:  March 17, 2003