by Mark Whitelaw
In any Integrated Pest Management program, proper identification of the problem is Step 1. Rose diseases are no different. They generally fall into one of three categories - bacterial, viral or fungal.
Infection can occur in most any garden. The pathogen enters through wounds to the plant caused by improper and unsanitary pruning, grafting, mechanical injury during cultivation, or failure to prune away broken stems and roots during transplanting. Bacteria can survive in the soil for at least two years. Besides spreading via infected pruning equipment, insects and water are also known vectors.
Crown Gall - Unlike the galls formed by stinging insects which are normally harmless to the tree or plant, Crown Gall is caused by a bacteria. These galls begin with tumor-like cell growth at or just below the soil's surface, near the base of the plant and commonly on bud unions. Galls usually begin as green, pliable tissue; then develop into dark, crusty growths. Following gall formation, shrub growth is stunted, foliage is sparse, and bloom production is reduced.
Root Gall- Like Crown Gall above, but damage is confined to roots. Commonly caused by improper root pruning during transplant.
Stem Gall - Like Crown Gall above, but damage is confined to stems and canes above the soil's surface.
Control of bacterial diseases is achieved by checking existing and newly introduced plants for contamination. Use caution when cultivating fertilizers, organic materials or mulches around the rose's root zone. If galls are discovered, the plants should be removed as soon as possible. Replace contaminated soil with well-draining soil high in organic matter and beneficial microbial activity. When planting bare-root roses, prune broken or damaged roots and stems with disinfected pruning equipment. Dip pruning tools with a 10% liquid chlorine bleach and water solution. Disinfect tools between each plant when pruning non-infected plants; between each cut when pruning infected plants. (Note: This may discolor metal parts. After disinfecting, scrub discolored areas with steel wool, sharpen cutting edges, and oil metal surfaces to restore them.) Some rosarians also report success by pruning away galls and spraying the infected area with an anti-bacterial solution like Agri-strep or a copper compound like Kocide.
Viruses are smaller than bacteria. As such, they rely on living organisms for dispersal in the garden. In roses, their spread is chiefly caused by grafting infected scions, buds and/or root stocks; although some can be vectored by parasitizing, chewing or sucking insects.
There are currently no chemical solutions for curing roses infected with a virus. The best control is to remove the rose, replace the surrounding soil, and inform the nursery or mailorder firm where you obtained the plant.
Some of the more common viral diseases are:
Rose Mosaic - The most common of the viral diseases, Rose Mosaic symptoms are yellow, staggered patterns on the leaf. These patterns frequently look like yellow electrical streaks or veins. Some authors suggest Rose Mosaic may not be fatal to the plant, and that most of the roses in the U.S. were infected with the disease and showed few detrimental effects as of 1970. Other authors, however, document Rose Mosaic as a cause for bloom distortion, reduced flower production and size, reduced plant vigor, early defoliation, and susceptibility to winter damage.
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Except where the rare, naturally-occurring root graft may occur, Rose Mosaic will not pass from one plant to another in the garden. Removing the plant may not be necessary unless the rosarian is overly concerned about its exhibition form.
Temporary control may be achieved by pruning away stems where infected leaves appear. The virus is not spread via mechanical means like pruning. Because the disease is systemic, it can only be spread by grafting infected plant tissue onto uninfected tissue. Heat therapy programs for root stocks are effective in controlling this virus.
Rose Rosette - Although rare, this disease is passed via infected root stocks, commonly R. multiflora. Rose Rosette is characterized by three stages of development, occurring over two or three seasons. It is most probably caused by a virus which is 100% deadly to the rose. The first stage is rapid, vigorous growth characterized by unusually dense formation of prickles on stems and canes. Canes appear overly large and purple or deep red in color. New leaves appear distorted and crinkled, often purple or deep red. Petioles (leaf stems) may appear flattened and phylloid (leaf-like). The second stage is the development of lateral growth having closely spaced internodes (the space between petioles). When leaf buds open, they are distorted or fail to fully open, giving an appearance of rosettes. The third stage is characterized by spindly, chlorotic stem growth. The rose will most likely die the following winter.
Proliferation - Although not positively identified as a viral disease, Proliferation has all the characteristics of being one. Bloom buds may form within blooms showing a tightly knotted, malformed appearance. Control is best achieved by removing the bloom stem.
Rose Spring Growth - Another suspected viral disease, new leaves appear as rosettes or tightly compacted balls. The disease is passed by grafting infected tissues onto non-infected tissues.
Rose Streak - Brown bands appear on either side of leaf ribs, sometimes forming brown or green rings. Leaves may drop prematurely. Like most of the viruses, this disease is passed by grafting infected tissues.
Rose Leaf Curl - Another probable viral disease, manifesting as small leaves which may have yellow flecks along leaf ribs in the spring. Leaves may drop prematurely.
Rings - These could indicate any one of three diseases: Strawberry Latent Ringspot, Rose X, or Rose Ring Pattern. These diseases all manifest as random ring patterns on leaves. The accompanying symptoms are what distinguish them, one from another. Strawberry Latent Ringspot will be accompanied by short petioles and smaller-than-normal leaves. It is spread not only by grafting, but also by mechanical means and root-knot nematodes. Rose X's rings are accompanied by severe stunting with mottled green leaves which may have very fine, spiderweb-like line patterns. Rose Ring Pattern will show up as random rings amidst random line patterns on the leaves. Leaflets may appear yellow and blotched.
Next time, we'll take a closer look at fungal leaf diseases.