When Rose Diseases Aren't

When Rose Diseases Aren't

by Mark Whitelaw


When is a rose disease not a disease? When a malady is mistaken for a disease because the rose exhibits the symptoms of a disease.

Misidentified symptoms?

Rose Wilt was long thought to be a suspected viral disease caused by grafting scions onto imported root stocks from the U.K., Canada and Australia. The American Rose Society's Consulting Rosarian Manual still lists the symptoms as "downward leafroll, vein-clearing, premature abscission, [and] shoot proliferation from single buds."

In fact, these characteristics commonly occur in a variety of rose classes and may be indications of grafting one rose class onto the root stock of another class. They could just as easily be a deficiency or overabundance of specific minerals needed to produce healthy, "normal" roses.

The fact is, rose wilt, as described by the U.S. Department of Agriculture, does not exist. They have since declared themselves in error by repealing the import restrictions placed on roses grafted outside the United States.

Deficiencies and Excesses

To new rosarians, soil deficiencies may appear as a disease. The most common among these soil maladies is calcium deficiency which often manifests as brown, curling leaves at the top of the plant. Often the leaves shrivel and die.

A soil malady often mistaken as a viral disease is boron deficiency. This shows up as malformed, mottled yellow new leaves, appearing to grow excessively close to one another near the new-growth area at the shrub's top.

Still another, potassium deficiency, frequently fools the new rosarian. This soil malady appears as brown, scorched older leaves near the bottom of the plant, usually at the leaflet tips. The symptoms translocate up the rose until the entire shrub looks as if it has been systematically killed by a viral infection.

"Vein clearing," where veins appear lighter in color than the surrounding leaf tissue and mentioned as a symptom of the fictitious rose wilt above, is a symptom of excessive boron in the soil. But it is also a symptom of too little oxygen in the soil, and may mean soil drainage is inappropriate for good rose growing conditions.

Specific Replant Disease

Also called "sick soil syndrome," this malady manifests when roses are planted into soil previously occupied by another rose. The new rose appears to "sulk" for a year or two after planting. Growth is stunted and bloom production minimized regardless of how well the rose is cared for and how often the plant is fertilized.

There are two schools of thought regarding this "disease": some think it doesn't exist and some swear it does. Of those rosarians who believe it exists, the "camp" is further divided into those who believe the problem is an alleopathic chemical [1] exuded by the previously planted rose, and those who believe the soil is deficient in microorganic activity normally present to assist in plant development (micorrhizal fungi [2], for example).

In my experience, specific replant disease appears to affect only certain rose classes, and apparently depends on the root stock of the previously grown rose. If an alleopathic chemical is involved, it appears to exude primarily from R. wichuraiana and R. fortuniana stocks, or stocks with R. canina in their heritage. Many of the modern roses in commerce today are grafted onto these stocks. Of those roses classes observed, hybrid teas, grandifloras, hybrid perpetuals, teas and chinas appear to be most affected, while floribundas, rugosas and polyanthas appear to be least affected. These observations make a strong case for the "alleopathic chemical camp."

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There is anecdotal evidence, however, that the incorporation of micorrhizal fungi inoculants, humates or activated compost (the latter of which stimulate and/or contain microorganic activity) may counter the effects of specific replant disease a plus in the column of the "microorganic deficiency camp."

For the day-to-day rosarian, the best counter for this disease is to change out the soil when a new rose is planted in the same location of a previously planted rose. Since this malady does not affect non-rose family plants and trees, moving the old soil to another location in the garden and relocating that soil back into the newly planted rose's location is merely a matter of muscle power and a wheel barrow.

The other option is to let the soil remain fallow for one to two years before replanting with a new rose. This procedure is used by many commercial rose growers and some home gardeners, who fill the empty space with a cover crop or companion plantings.

Next time, we'll look more closely into soil deficiencies, pH, and how these conditions affect roses.


[1] Many plants exude a chemical which inhibits the growth of surrounding plant life. These chemicals may come from the leaves and stems or from the roots. Chemicals of this nature are said to be "alleopathic."

[2] Micorrhizal fungi attach themselves to plant roots growing in highly organic soils. They are believed to help the plant in the uptake of moisture and nutrients, thus reducing seasonal stresses. When a plant is removed by the roots, the fungi leave with it, and the new plant must develop new micorrhizal fungi.

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