Tuesday, July 10, 2018

What's Bugging my Roses? by Donna Duffy

It’s that time in early summer when roses come into full bloom. Their beauty and fragrance make them the superstars of the early summer garden. Undeservedly, roses have a reputation for being difficult to grow. In fact, very few rose diseases are found in typical Colorado growing conditions, primarily due to our high altitude and dry conditions. Even so, your roses may become afflicted with a rose pest or disease. Here are four common rose problems and their controls, courtesy of the Denver Rose Society.

Small Carpenter Bees
Look for holes in the center of the rose canes, and sawdust-like material. You might also notice wilting, dieback and death of a cane. This is often caused by small carpenter bees that lay eggs in the cane where it has been pruned. They tunnel into the pith and damage healthy tissue. What to do? Prune infested canes below the damage and seal the pruning cut (Elmer’s glue will work). Be sure to throw away all of the damaged prunings.

Leafcutter Bees
Look for smooth, semicircular cuts or notches in leaves. You probably have adult leafcutter bees, though they are rarely seen. It takes the bee less than 10 seconds to cut out a section of the leaf, which is then taken away to the nest. What to do? The good news is that this damage doesn’t require any control. These bees are desirable pollinators and predators of other pests. Don’t worry about the little leaf holes!

Two-spotted Spider Mites
In the early stages, the leaves will have small stippled spotting. In later stages, you might notice desiccated, yellow/orange leaves and webbing. You can often see the mites as small specks on the underside of leaves. What to do?  Keep your roses well watered. As soon as you notice the mites, hit the undersides of leaves with a strong water spray daily. You can also treat with horticultural oil. Avoid the use of Sevin or Malathion.

Powdery Mildew Fungus
This fungus needs daytime temperatures near 80 degrees, nighttime temperatures around 60 degrees, and humidity between 40-70%. Look for distorted leaves and buds covered with a white powdery growth. What to do? First, choose disease-resistant varieties. Roses that receive morning sun to dry off petals and foliage are less likely to have this problem. Water roses early in the day to avoid having wet foliage all night long. Sulfur dust and horticultural oil may also be used as well as certain fungicides (read the label carefully).

Many rose problems can be managed with good cultural practices. Remove all dead leaves and cuttings, don’t overhead water, and plant roses in areas with good soil drainage and ventilation.

For more information about rose care or other gardening questions, contact the Jefferson County CSU Extension at 303-271-6620.