Black spot is the most prevalent rose disease. It is characterized by circular black spots with irregular margins on leaves. Control with fungicides is difficult. Mulching around plants can reduce disease occurrence. The leaf spots may increase in size and number with time. Infection will cause leaves to turn yellow and drop. Severely infected plants may be completely defoliated by midsummer.
Black spot disease is unsightly. More importantly, the disease reduces leaf function, which weakens the entire plant. The result is a plant that grows very slowly and is more susceptible to other diseases and winter injury. Black spot disease causes discoloration and leaf drop.
Powdery mildew is a rose disease that stunts and distorts young, tender growth of buds, leaves and canes. Older leaves may also be diseased, but damage is not as severe. The fungus is a prolific spore producer. These spores form on the surface of the tissue, which gives it a “powdered” appearance – hence, its name. Fungicide application to new growth can reduce disease incidence and spread. Disease development requires damp, cool nights, which can occur any time during the growing season. Some varieties of climbing roses and small-flowering ramblers are most susceptible to this disease. Mildew is severe on stems, buds and leaves during damp, cool weather.
Botrytis blossom blight is severe on some varieties of hybrid teas, particularly the whites and pinks. This disease affects the buds and partially opened flowers. It is recognized by its grayish-brown fungus growth on the buds and petals under humid, wet conditions. The fungus overwinters on infected plant parts, especially winter-killed stems. Pick off and destroy faded and infected blooms. Fungicides are effective in controlling the disease.
Crown gall, caused by a soil-borne bacterium, severely damages roses. The bacterium usually gains entry into the plant via wounds or natural openings. This disease causes large, corky growths or galls to form on the crown or roots. Sometimes galls develop on above-ground portions of the plant. These galls first appear as small swellings that slowly increase in size. Diseased plants show a loss in vigor and produce smaller blossoms and leaves. This effect on the plant becomes evident only when the galls have reached a diameter of several inches. Do not confuse crown gall with normal enlargement of the graft union of plants (Figure 10). Infection may occur during grafting; therefore, inspect nursery stock before buying. Burn or destroy all infected plants. Chemical control of this disease is not effective.
The large, dark- to flesh-colored gall is characteristic of Crown Gall disease. Galls can also occur on shallow roots.
Stem cankers. The three most common stem canker diseases are caused by fungi that invade the plant through wounds caused by pruning, cultivation or winter injury. Hybrid tea varieties are more susceptible than others. Failure to control leaf diseases with fungicides increases susceptibility to stem cankers.
The fungi overwinter in cankers and continue to spread during the season. If left untreated, the infection can girdle the cane, causing death or poor growth above these infected areas. Symptoms of stem canker, notice the difference between the dark, infected tissue and the lighter, healthy tissue.
To control stem canker infections, prune and discard all infected tissues. Be sure to make cuts well below the blighted areas. Disinfect pruning shears between cuts with a surface disinfectant such as isopropyl rubbing alcohol.
There are several viral diseases of roses. Most of these maladies are transmitted to non-infected plants through vegetative propagation processes used by commercial rose producers or through pruning. There is some evidence that at least two of these viral diseases are transmitted to other plants by insects or spider mites. Viral infected roses tend to be less vigorous, are more susceptible to other diseases, and are less likely to tolerate environmental stresses as healthy plants do.
There are at least six known viral diseases of roses. Yellow and green mosaic patterns, leaf distortion and ring spots are possible symptoms of virus infections. (Figure 12) Others that exhibit viral-like symptoms have not been linked to a particular virus. It is not unusual to encounter plants infected with more than one virus. Also, such occurrences usually cause problems in positive identification.
Some chemical toxicities and nutrient deficiencies express symptoms similar to those caused by viral diseases. Removing severely infected plants may be the only solution. Preventive measures to prohibit the spread of viruses include disinfecting pruning shears when moving from one plant to another, and a good insect control program is a must. Also, buy plants that are apparently healthy and exhibit no peculiar foliar symptoms.
Symptoms of the rosette virus, note the increased numbers of thorns; clusters of deformed, crinkled leaves; and the many small leaves originating from one point (a condition called “witches’ broom”). Symptoms sometimes mimic those associated with herbicide injury.
Insects and Mites
Aphids. Several species of aphids damage roses by removing large quantities of plant juices and by secreting a sticky substance over the surface of infested plant parts. Heavy infestation can cause abnormal growth of plants.
Aphids are small, approximately ? inch long, and vary in color from yellow to green to black. Their life cycle is completed in as few as four days under favorable environmental conditions. Remove and destroy heavily infested plant parts. Then treat plants with an insecticide registered for aphid control on roses. Inspect roses for aphids two to three days after application. Due to their short life cycle, aphids can rapidly build back up in number; therefore, several applications of insecticide may be necessary to bring the population under control.
Thrips. Thrips are another troublesome pest on roses. Thrips enter developing flower buds to feed and cause petals to become flecked and discolored. Heavy infestations cause deformities in new growth and can be responsible for buds not opening properly. Adult thrips are tiny, slender, brown to yellow insects with feather-like wings. They are most easily seen by shaking new growth or flower buds over a white cloth.
Thrips have repeated generations during spring through fall. In hot, dry weather they complete a life cycle in less than two weeks. Cutting off and disposing of old blooms lessens infestation. Do not leave old blooms on the ground or thrips will quickly leave them and reinfest plants. Treat infested plants with an insecticide registered for thrips control on roses. Complete coverage of infested plants with the insecticide is important to your control program.
Spider Mites. Mites are such small pests that they often cause damage before they are noticed. Mites are found on the underside of leaves, there they feed by sucking plant juices. Infested leaves will have many tiny chlorotic spots. Heavy infestations cause leaves to turn brown and die.
Mites have a very rapid life cycle and can complete development in less than a week under hot, dry conditions. A control program for mites begins with the removal of weeds and other plants serving as alternate hosts for mites. Frequently inspect roses for the presence of mites. A magnifying glass is useful for seeing mites. When mites are found, treat plants with a registered miticide and reinspect plants for mites three to four days later.
Other Insect Pests. Rose chafers, scale insects and Japanese beetles are occasionally found damaging roses. Rose chafers are buff-colored beetles 1/2 inch long with long, spiny legs. They feed on the rose blossoms. Japanese beetles are bright green, about ? inch long; they feed on the foliage . Scale insects are small, stationary insects found on the stems and leaves of roses. They damage roses by removing plant juices with their sucking mouth parts.
Nematodes. Nematodes are microscopic worms that feed on plant roots. They may stunt young rose plants and can cause a slow decline in older ones. If you suspect nematodes have entered your rose bed, take a soil sample from the root zone of the plants involved. Place a pint of this soil in a plastic bag. Be sure the soil is moist. Take this sample to your county extension agent, who will have the soil examined for nematodes.
For more info, contact:
D.A. Compound, Elliptical Rd., Diliman,Quezon City
Tel. Nos. (632) 929-6065 to 67 / 920-3991 / 928-1134
source: pubs.caes.uga.edu, photo from cherryvalleynursery.com