Rust diseases are caused by parasitic fungus growths, similar to diseases like blackspot and mildew in roses. Fungi are part of the plant kingdom, but they are different from most plants because they don’t have chlorophyll. As a result, they can’t make their own food.
Just like animals, fungi rely on plants that have chlorophyll to get their food.

Fungi Penetrate Plant Tissues
The actively growing parts of the fungi, called mycelia, penetrate the plant tissues and absorb the nourishment; this alone causes the resulting plant injury from blackspot, powdery mildew, and rust infections on roses.
The mycelia of rust are seldom seen; rust fungi reproduce and spread by forming reproduction bodies called spores. Summer spores of rust in roses are reddish-orange; masses of these are the symptom by which gardeners know rust.
Rust fungi are among the most costly disease pests of many crops. Millions of dollars annually are lost to rust in wheat, oats, barley, rye, other grains, and certain pines and apples. Under favorable conditions, rusts also infect many other plants, including:
- Beans
- Peas
- Raspberries
- Asparagus
- Carnations
- Snapdragons
- Hollyhocks
- Chrysanthemums and more…
Highly Specialized Rust Fungi
There are hundreds of species of rust fungus; each is highly specialized in what host plant it can exist upon. Each species of rust fungus can usually attack only one species of the host plant.
The rust fungus which attacks the rose is named Phragmidium mucronatum. It causes the disease commonly known as “common leaf rust” and is known to attack most members of the genus Rosa and no other plants.
Common rust on rose leaves has been reported since 1665 and has been found in Europe, Asia, South and North America, South Africa, and Australia.
In the United States, it is a more extensive problem in the Pacific Coast states. It is a less severe problem in some regions of the Midwest and Northeast but has also been reported to be a summer problem on roses in some regions of Maine.
Like blackspot and powdery mildew, the existence of rust as a problem for roses is determined by temperature and humidity. All rose gardens have exposure to frequently transported or dormant rust spores.
Still, a rather strict set of weather conditions involving moderate temperatures and high moisture must exist for a considerable time to allow these spores to multiply to a damaging degree.
Rust Rose Life Cycle
Common leaf rust of roses grows through several stages during its life cycle. The form in which it is most commonly known is the summer stage; in this, the apparent infection is composed of characteristic red-orange powdery masses of spores attached to the underside of leaves.
The fungous mycelia constantly feed upon the leaf and produce new masses of spores; under favorable moisture and mild temperature conditions, this summer stage can reproduce itself each 10 to 14 days.
Severe infections cause discoloration, defoliation, and severe weakening of the rose plant; growth and bloom production is thus affected.
This common leaf rust occurs in four spore stages at different times. These are different in appearance and may not consistently be recognized by gardeners as the same disease. In the autumn, the “black rust” on roses is another spore form of the same condition.
With cool autumn weather, the red-orange summer spores give way but may appear together with the black-colored autumn spores.
The latter can withstand colder temperatures and the fungus overwinters as these spores.
In the spring, these black spores germinate, forming sporidia, and these infect the newly formed leaves. The freshly infected leaves show small orange or yellow spots, masses of “spring spores.” The infected leaves are dwarfed and malformed.
Continuous Reproduction In Humid Weather
The spring spores give way to a continuous reproduction of the more destructive summer form with continued mild and humid weather. In areas of cold winters, rose rust has these four seasonal forms; it passes the winter as black spores on hanging or defoliated leaves.
In mild climates, the reddish-orange stage may be present throughout the year, and the other stages of the disease may be absent.
A favorable relationship between temperature and moisture is necessary for the continued existence of rust infection. This accounts for the limited geographical distribution of rust as a garden problem.
Optimum spore germination and infection in the summer stage require a temperature of approximately 64°-70° degrees Fahrenheit and the presence of leaf moisture for four hours.
Temperatures over 80° degrees Fahrenheit and dry conditions in most areas of the United States are very unfavorable to germination and survival.
Therefore frequent rain, dew, and fog with continuous mild temperatures are necessary for constant rust production; only certain areas of the West Coast provide such conditions.
Rose rust thrives only in areas with extended calm periods with high moisture. In regions of frigid winters, much of the overwintering spore population is killed by the severe cold.
Control
A clean garden culture and a fungicide spray will hold common leaf rust in check. A major aid in the destruction of overwintering spores on leaves, those on the plant, and on the ground.
In areas of known rust infection, all leaves should be collected and destroyed in midwinter; all leaves clinging to the plants and tender foliage should be removed (this is also an aid to blackspot control).
Spraying or dusting should commence with the appearance of new foliage and continue at weekly or semi-monthly intervals for best control. A spray with a spreader sticker will give the best chemical contact with fungous mycelia.
Not many chemicals are known for the effective management of rose rust. Check with other local rose growers for what works best in your area.
Related: Pest Solutions For Rose Gall Disease