Roses tend to be hardy plants: even in less-than-ideal conditions, they’ll pay back a little attention with dozens of bright, cheery flowers that can come in a rainbow of hues. However, most modern roses are descended from a few successful hybrid lineages, and these strains have their weaknesses.
Many insects, from thrips to leaf-cutter ants, find their leaves quite tasty, and their foliage is susceptible to a number of debilitating diseases. One of the worst of these is Black Spot.
Black Spot is a serious fungal infection that kills leaves and deprives buds of the nutrients they need to develop. It’s the scourge of modern hybrid tea roses, especially the yellow strains, nearly all of which can trace their ancestry to the susceptible Austrian rose known as Rosa foetida.
Prior to 1900, yellow roses were almost unheard of; in that year, a French rose breeder managed to originate a reproducible yellow strain by introducing Rosa foetida into the modern rose lineage. Yellow roses soon experienced an explosion in popularity, leading rose breeders to trade hardiness for desirable color combinations.
As a result, any rose with yellow ancestors — from the buttery Sunbright to the multihued Joseph’s Coat — can expect to have recurring problems with Black Spot disease. Unfortunately, it’s not limited to yellow roses.
Recognizing Black Spot Disease
Black Spot, as its name suggests, is characterized by large black spots on the upper surfaces of the leaves of infected rose plants. Often surrounded by a yellowish area, the spots will eventually grow together and cause the leaf to turn yellow, wilt, and fall off. If this happens to enough leaves, the plant will be unable to use sunlight to synthesize the nutrients it needs to grow and blossom.
Treating Black Spot Disease
Because Black Spot is a fungal infection, it can generally be prevented by regular applications of fungicide spray. When you treat your roses in this way, make sure the fungicide is sprayed on the upper and lower surfaces of the leaves, as well as the canes; application every two weeks or so should be sufficient.
You can also help prevent fungicidal infection by limiting the rose plants’ exposure to excess water, by watering at the roots rather than from above, and by planting your rose bushes far enough apart that air can circulate easily between them. Air circulation can be enhanced if you prune away any dead or stunted limbs.
If Black Spot has already struck your roses, there’s only one effective remedy: severe pruning. You should remove all infected leaves as well as the branches on which these occur, even if this means leaving the plant denuded of foliage. It may be necessary to remove entire canes.
Black Spot reproduces by spores and, even though the disease doesn’t infect branches, a branch might still have spores on it that can infect later generations of leaves. Once you have finished removing all the diseased foliage, make sure you remove that foliage from the vicinity of the rose bushes.
Never leave it lying nearby; the spores from the Black Spot can easily re-infect your roses. Several days after pruning your roses, check again for any evidence of Black Spot. Remove any infected material as it appears.
When you’re done, your roses might look like little more than a spindly collection of thorny sticks. However, keep in mind the fact that roses grow very quickly; the red sprouts of new foliage will appear within days.
Flowers will follow as soon as the plant can afford the effort of growing them. The new flowers may start out somewhat small since there’s a lot less foliage to support them than before; however, by the end of the season, it’s probable that your roses will have leafed out completely, having fully recovered from the dreaded Black Spot.
But keep an eye open — there’s always next year, and Black Spot is nothing if not patient.