As promised in my Feb. 4 column, this article is devoted to pests and diseases to which roses in our area can fall victim. The extra effort it takes to keep your roses healthy will pay big dividends in beautiful flowers.
Let’s face it, no one grows a rose for its foliage, at least no one I know. However, if not a rose’s best friend, foliage is certainly a good friend, as it often provides the most obvious clues regarding the health of the plant.
Evidence of pest infestation and some diseases is visible above ground. However, rose lovers should be aware of what may be happening below the soil’s surface before your roses actually show signs of damage due to possible subsurface diseases.
Four pests can wreak havoc with roses: aphids, caterpillars, spider mites and thrips.
Aphids, which appear primarily in the spring and fall, are tiny, soft-bodied insects only about one-eighth of an inch long, which makes them sound harmless enough. The problem is that they arrive en masse, can totally cover new buds and leaves, and really like roses.
Aphids pierce new growth and literally suck the vitality from young buds and leaves, leaving them misshapen and discolored. In the process, they excrete a sticky substance called honeydew that can lead to sooty mold. Young foliage is particularly vulnerable.
There are a number of physical means of rescuing roses from aphids if you are not lucky enough to have any natural predators nearby. Some gardeners may derive satisfaction from squashing the aphids, although this can be a bit messy.
Spraying the aphids with a stream of water can remove them, at least temporarily, but damp foliage has its own set of problems. Creating a physical barrier with a lightweight fabric or locating a sticky trap a couple of inches above the rose can also help. Application of horticultural oil, insecticidal soap or neem oil can also be used.
Caterpillars do not limit their attack on roses to new growth. The safest way to protect roses from an array of different kinds of caterpillars (e.g., corn ear worms, army worms, rose slugs) is a regular chemical spray program, although they can be removed manually. I have something of an aversion to mashing anything, but I make an exception with caterpillars. I just close my eyes and squish.
Mottled, yellow foliage often covered in fine webbing is an indication that spider mites are feasting on your roses. These pests are so small that the fine webbing is the only clue to their presence. They thrive in hot, dry weather when stress is already an issue.
The worst thing you can do to eradicate spider mites is to use chemical pesticides, which kill predatory beneficial insects (e.g., big-eyed bugs, damsel bugs, lacewings, ladybugs) and may lead to even greater spider mite infestation. Keeping areas free of weeds and providing adequate moisture during dry spells will help deter spider mites. If promoting beneficial predatory insects is not possible and spider mites are a serious problem, spray the underside of the affected leaves with insecticidal soap or horticultural oil.
Slits and brown flecks on your rose’s petals are evidence of thrips. Buds damaged by these minute, slender insects fail to open properly. Old, infected blossoms should be removed from the plant and destroyed.
Numerous diseases can attack roses along the Gulf Coast: black spot, powdery mildew, crown gall, stem canker, cane blight and nematodes.
Black spot, a fungus, does more damage to roses than any other rose disease in our area. Black spots, often with a yellow halo, appear on the leaves, which then turn yellow and fall off. Keeping leaves free of standing water will help protect your roses against this disease, which occurs most often in mid-summer. (Remember the earlier column’s advice about watering your roses below the foliage.)
Powdery mildew is a fungus appearing in spring and fall. Leaves, shoots and buds may be covered with white, powdery masses of spores that are easily windborne and germinate in high humidity and temperatures below 80 degrees.
Both black spot and powdery mildew are best controlled by removal of diseased matter, mulch replacement and a season-long fungicide spray program. The spray program has the added benefit of protecting roses from stem canker and cane blight. However, once the diseases are present, it is too late for a fungicide to help, as it only provides protection to uninfected plants.
A variety of nematodes can attack the roots of roses, and the unwary gardener can bring what appear to be healthy plants home from the nursery only to discover the presence of nematodes when roots are revealed during planting. An investigation of the roots of roses exhibiting lack of vigor, premature leaf fall, yellow foliage, stunting, and twig and branch die-back will often reveal nematode damage. Root-knot nematodes cause galls, but there are other nematodes (lesion, spiral and stunt nematodes) that lack galls but result in roots that are rough, discolored and decayed.
The best defense against nematode infestation is to check the roots of your intended purchases for nematode damage before actually buying them. Bringing healthy roses to nematode-infested soil is equally detrimental, and you should seek information from your county agent regarding nematode control if you suspect your soil is infected. It is easy to mistakenly assume nematode-infested roses are suffering from lack of fertilizer or poor soil.
Above-ground damage to roses caused by crown gall mimics that caused by nematodes. Crown gall is a bacterium often transported by contaminated nursery stock and can live in soil for several years even in the absence of host plants. The bacterium can enter healthy plants that have been wounded during planting, cultivation or grafting, or chewed on by insects. The galls are rough, woody growths that can exceed six inches in diameter.
Removal of diseased plants followed by fumigation or solarization of soil will help control for crown gall. Application of root dips during planting can help to further protect gall-free transplants.
An extensive discussion of rose diseases and how to control them is available online in ANR-505: Diseases of Roses and Their Control at www.aces.edu. Especially helpful in this article is variety-specific information to help you purchase disease-resistant roses for your garden.
UPCOMING: (free and open to the public)
What: Plantasia! Spring Plant Sale
Where: Mobile Botanical Gardens
When: March 18-20, 9 a.m. to 4 p.m.
Friday and Saturday
1 p.m. to 4 p.m. on Sunday