There is a point in summer when gardeners can’t show fear. The growing season fills all of our spare time – we’re running full tilt, harvesting, weeding and watering; there’s blooms to pick, mulch to replenish and insects to fight off. At a certain point I wonder why I always smell like plants, but it’s not a bad scent and soon after washing I’m back in the plants again, getting covered in sap.
The rush and frenzy can get overwhelming, so for stress relief, take time to stop and smell the roses.
Our roses don’t have many problems here in Colorado compared to other places, but a few problems crop up each year for gardeners. Leaf cutter bees are a nuisance, cutting circular pieces out of the leaves to use in building nests. These small, mild mannered bees are grey with white stripes and, though the leaves with holes look alarming, the bees are good pollinators and harmless.
Spider mites can also be problematic, feeding on sap and reducing the vigor of plants. Thriving in hot, dry weather, their damage can cause leaves to look bronzed and drought stressed. Look closely at the plants for tell-tale webbing and if you have spider mites, drive them off by increasing humidity around the plant with mulch to hold moisture.
Powdery Mildew, one of the most prevalent rose diseases, can show up in wet or dry weather. Conditions that encourage the fungus to develop are daytime temperatures near 80 degrees with higher relative humidity and nighttime temperatures around 60 degrees. Early signs are raised bumps on the upper surface of leaves. Later the fungus will distort leaves and buds and cover them with a whitish, talcum-like powder. Untreated plants eventually become stunted, and their leaves will curl and drop. To contain the problem, remove and destroy all infected canes and shoots. Spraying with sulfur dust is helpful but should not be used in hot weather since they can cause leaf damage. Rake up and destroy all leaves at the end of the season to prevent the fungus from spreading to other plants.
Rust fungus can infect roses when moisture levels are continuously high for two to three hours at temperatures between 65 and 70 degrees. First appearing as orange, powdery pustule-like growths on the underside of leaves, the rust becomes visible on the upper leaf surface as orange-brown spots; eventually leaves turn black. Remove infected leaves and canes
at the first sign of rust.
Black Spot shows up on leaves as circular dark brown spots with jagged edges. Leaves infected with black spot will turn yellow and drop. Wet weather and temperatures between 65 and 75 degrees promote the disease’s development. Black spot spores move easily and can be spread by splashing water, pruning shears, and gardeners’ hands. Since the fungus overwinters on fallen leaves and in lesions on canes, remove and destroy all infected foliage, particularly at the end of the season. To avoid transmitting black spot to healthy roses, rinse the pruning shears in a bleach solution after working on an infected rose.
By Carol A. O’Meara. Carol is an Extension Agent – Horticulture Entomology at Colorado State University Extension Boulder County. For more information call 303.678.6377, e-mail [email protected] or visit ext.colostate.edu/boulder.