Shrub roses (Rosa spp.) in southwest Ohio are struggling with several issues. One of the most obvious problems is Rose Rust which is unusual for much of the state.
Rose rust is produced by fungi in the genus Phragmidium with P. mucronatum, and P. tuberculatum being two of the most common rusty rose fungi in North America. The rust fungi are autoecious meaning they can complete their development on one host. This is unlike other rust fungi such as the fungus behind cedar-quince rust that require two hosts (= heteroecious).
Environmental conditions that favor rose rust infections and disease development include cool temperatures (55 – 75 F.) coupled with lengthy periods of light rain or foggy conditions. Historically, rose rust has been rare in Ohio and more common in the coastal regions of the U.S.
Indeed, prior to this season, all of my rose rust images showed infections on wild roses growing among dense vegetation (= high humidity) in a park in southwest Ohio. However, this season, it’s hard to find shrub roses in the southwest part of the state that are free from infections.
Look for older leaves showing yellow to orange spots on the upper leaf surface. Of course, flipping the leaves over will reveal masses of rust pustules. Rusty-looking infections may also occur on stems and flower sepals.
Heavily infected leaves commonly curl and drop from the plant but not before releasing their infectious spores. Heavily infected stems will die back. Rose rust can kill entire plants; however, this dire outcome is rare in Ohio because new infections typically cease as our environmental conditions change during the summer.
Rose rust can be managed by removing and destroying diseased leaves and stems. Most infections occur on the lower portions of affected plants. Consequently, removing the older infected tissue will help to protect newer growth that can eventually cover up the damage.
Although we’re transitioning towards warmer, dryer conditions, it’s important to avoid overhead irrigation, particularly in the evening. The high moisture conditions coupled with cool nighttime temperatures could contribute to additional infections.
Rose rust is being joined by holey leaves produced by so-called “rose slug” or “roseslug” sawflies (order Hymenoptera). Two of the most common culprits are the Roseslug (Endelomyia aethiops), sometimes called the European rose slug, and the Bristly Roseslug Sawfly (Cladius difformis (= C. pectinicornis). Although both have “slug” in their common names, they don’t share the slug-like body plan of other “slug sawflies” and the larvae don’t cover themselves in slime.
Roseslugs feed on the upper leaf surface as skeletonizers. However, there is only one generation per year, so much of the damage is ending for this season.
The bristly roseslug sawfly has multiple generations per season with the damage starting in the spring and continuing into the fall. Look on the underside of the leaves for the sawfly larvae.
Early instar bristly roseslug larvae feed by removing the lower leaf surface and the mesophyll beneath. The corresponding epidermis on the upper leaf surface remains intact and turns white producing a characteristic "windowpane" effect. Eventually, the "windowpanes" drop out to produce holes.
Later instars feed between the main veins to directly produce holes in leaves. Heavy feeding damage by early and late instars may combine to produce "see-through" leaves. The expanding numbers with each new generation may produce heavy defoliation by the end of the season.
Control and prevention of further damage depend on the proper identification of the true culprit. Only the bristly roseslug is worthy of control measures because it continues to produce damage throughout the season.
Biorationals such as insecticidal soaps are effective, but direct contact is necessary. Products containing spinosad (e.g., Conserve, Entrust) are effective against sawfly larvae and will also have less impact on bio-control agents. Chlorantraniliprole (e.g., Acelepryn) is also effective and presents a minimal risk to pollinators. Soil drench applications of systemic insecticides such as imidacloprid (e.g., Merit) or dinotefuran (e.g., Safari) are effective and provide lengthy protection.
Keep in mind that roseslug larvae are related to wasps, bees, and ants (Hymenoptera). They may look like caterpillars, but they’re not related to moths or butterflies (Lepidoptera). Products based on strains of the bacterium Bacillus thuringiensis (Bt) kill moth caterpillars but do not affect sawfly larvae.
