Rust Disease on Roses

Like blackspot and mildew diseases of roses, rust diseases of all plants are caused by parasitic fungus growths. Fungi are members of the plant kingdom but unlike most other plants contain no chlorophyll and hence cannot synthesize their own food. Like the animal kingdom they must depend upon chlorophyll containing plants for the origin of their food supply.

The actively growing parts of the fungi, called mycelia, penetrate the plant tissues and absorb 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 formation of reproduction bodies called spores. Summer spores of rose rust are reddish-orange in color, masses of these are the symptom by which gardeners know rust.

Rust fungi are among the most costly disease pests of many agricultural crops. Millions of dollars annually are lost to rust in wheat, oats, barley, rye, other grains and also certain pines and apples. Under favorable conditions rusts also infect many other plants including beans, peas, raspberries, asparagus, carnations, snapdragons, hollyhocks, chrysanthemums and many others. There are hundreds of species of rust fungus, each is highly specialized with regard to what host plant it can exist upon. Each species of rust fungus can usually attack only one species of 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 leaf rust of roses 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 certain areas of the Midwest and Northeast but has also been reported to be a summer problem on roses in certain areas of Maine.

Like blackspot and powdery mildew, the existence of rust as a problem to roses is determined by temperature and humidity. Probably all rose gardens have an exposure to transported or dormant rust spores on frequent occasions, but a rather exacting set of weather conditions involving moderate temperatures and high moisture must exist for a considerable length of time to allow these spores to multiply to a damaging degree.

Life Cycle

Common leaf rust of roses grows through several different stages during its life cycle. The form in which it is most commonly known is the summer stage, in this the obvious infection is composed of characteristic red-orange powdery masses of spores attached to underside of leaves. The fungous mycelia is constantly feeding upon the leaf and producing 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 plant, growth and bloom production are thus affected.

This same common leaf rust occurs in four spore stages at different times of year. These are different in appearance and may not always be recognized by gardeners as the same disease. The “black rust” on roses in the autumn is a different spore form of the same disease. With the coming of 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 newly infected leaves show small orange or yellow spots, masses of “spring spores.” The infected leaves are dwarfed and malformed.

With continued mild and humid weather the spring spores give way to a continuous reproduction of the more destructive summer form. 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 of temperature and moisture is necessary for continued existence of the 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° Fahrenheit and the presence of leaf moisture for four hours. Temperatures over 80° F. 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 the continuous rust production, only certain areas of the West Coast provide such conditions. Only in areas having extended mild periods with high moisture can rose rust thrive. In areas of very cold winters much of the overwintering spore population is killed by the severe cold.

Control

A combination of clean garden culture and a fungicide spray will hold common leaf rust in check. A major aid is 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 at 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 best chemical contact with fungous mycelia. Not many chemicals are known for effective control of rose rust. Check with other local rose growers for what works best in your area.

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