Black Spots on Leaves
Not surprisingly, the disease, black spot (Diplocarpon rosae) produces black spots on the leaves of infected plants. These spots are surrounded by yellow. Eventually, leaves turn yellowish pink and fall off. In severe cases, this disease can defoliate a rose bush by midsummer.
Black spot prefers roses with light-colored blossoms; red roses are less susceptible. It thrives on moist environments and is most common in northeastern and southeastern states, and in some Midwestern states where summers are warm and humid. Black spots begin to appear when the air temperatures approach 65°F and rain is abundant or humidity high. Infection begins low to the ground. Young leaves, 6 to 14 days old, are the first to go.
To control this disease, prune and destroy all the affected leaves immediately and begin a weekly application of a sulfur spray, continuing applications throughout the season.
Brown Edges on Blossoms
If you see brown edges on your rose blossoms, and the buds only partially open or don’t open at all, suspect thrips. These insects attack buds in their early stages, working among unfurled petals. The buds become deformed and fail to open properly, and the damaged petals turn brown and dry. New growth also can be damaged, in the same way.
Roses are vulnerable to flower thrips (Frankliniella tritici) and tobacco thrips (Formica fusca). The adults are tiny, slender insects, 1/15 inch long, and may be pale yellow, black or brown. They have four narrow wings fringed with long hairs, and their legs are very short.
Since thrips burrow deeply between the petals, early identification and control is important. Set out yellow sticky traps about 4 weeks after the last frost. As soon as you spot thrips on the traps, spray your roses with insecticidal soap every three days for 2 weeks. Commercially available predatory mites, lacewings, ladybugs are effective backup to the soap sprays. Thrips prefer a dry environment, so make sure plants are adequately watered.
Brown Spots on Leaves
The disease that causes brown spots on leaves is called leaf spot. In lilacs, several kinds of fungi may be responsible for causing leaf spot disease. These include members of the Cercospora spp., Macrophoma spp., Phyllosticta spp., and Pleospora spp. among others. The leaves of infected shrubs show brown or black spots. Spray plants with a copper fungicide 2 or 3 times, at weekly intervals. Prune and destroy severely diseased branches.
The spots often come together to form larger patches of dead tissue (blotches). Sometimes you will see flecks of black dots around the spots. These are the spore-bearing bodies of the fungus. Pick off and discard the infected leaves, and spray the plants every 7 to 10 days with a sulfur spray. Avoid wetting the foliage while watering your vines. Mulching around plants helps prevent fungi from being splashed up from the ground by rain or when you are watering.
Dark Blotches on Leaves, Leaf miners
The larvae of lilac leaf miners (Gracilaria syringella) burrow between the upper and lower surfaces of leaves, forming unsightly blotches in the leaves. Heavily infested shrubs look scorched. From 3 to 8 larvae inhabit one leaf. Once they are fully grown, they emerge, spin webs around the leaves, and continue to feed until the leaves are skeletonized. Lilac leafminers make cocoons, which they cover with plant debris. They spend the winter in these cocoons on the ground under plants. Two or three generations occur a year, the last one in September. The adult moth of the lilac leafminer has brown forewings mottled with the silver, and two silvery bands across its middle. It spends winter in debris-covered cocoons under the plants. The moths usually emerge during May, and the larvae mature in July.
Control of Leafminers
To control the leafminers, remove any rolled leaves and burn them. If the infestation is very heavy and much damage has been done, cut back branches to healthy growth. In July, before the insect grows out of the larval stage, spray the entire shrub with a strong insecticidal soap solution.
The boxwood leafminer (Monarthropalpus buxi) is the most serious insect enemy of boxwoods in the United States. This pest weakens shrubs by feeding on leaves. Foliage turns yellowish, new growth is stunted, and twigs may die. It’s the larvae that are to fear. These 1/8-inch-long yellowish maggots feed on plant tissues. Oval, water-soaked swellings appear on the undersides of leaves.
Trim and destroy the infested leaves as soon as you discover them. Rake up and destroy any fallen leaves.
Holes in Leaves, Weevils
If you find holes along the margins of your arborvitae leaves, your plants are probably infested with weevils. These insects are small and black. Their body and wings are covered with metallic green scales and fine short hairs. The larvae are white to pink, with brown heads. They feed on the roots of arborvitae and may attack anytime from June or July to midwinter or the following spring.
Adult weevils emerge from the soil to feed on above-ground plant parts from May to July. The adults usually feed at night and hide in soil and trash during the day. Adult weevils will play dead when disturbed, folding their legs and dropping off plants to the ground. Turn this trait to advantage when fighting this pest. Gently beat the branches of an infested arborvitae and catch the startled insects in a drop cloth spread beneath the shrub. Apply a sticky substance such as Tanglefoot to the trunks of the shrub to prevent adults from climbing up and eating the leaves.
Another means of control is to spray shrubs once a week with a solution of pyrethrum mixed with isopropyl alcohol. Combine 1 tablespoon alcohol with 1 pint pyrethrum mix and apply at night, when weevils are active, making sure to cover all leaf surfaces.
Leaf Margins Look Burned, Mites
Two-spotted spider mites (Tetranychus telarius) cause damage resembling sunscald. The leaves look burned, especially along their margins. These insects are about 1/50 inch long, barely visible to the unaided eye. They may be yellow, green, red, or brown. As soon as you notice symptoms of mite damage, spray your shrubs vigorously with water to knock them from the leaves. Do this in the morning, once every day for 3 days. This should completely eliminate a light infestation. If mites are still present, spray with insecticidal soap every 3 to 5 days for 2 weeks. In late winter or early spring, before the leaves begin to come out, spray shrubs with dormant oil to destroy any overwintering mites.
Brown Spots on Leaves
Shoestring Root Rot is so named because dark, shoestring-like strands cover the bark of roots of infected shrubs. In addition, large, white “fans” fungal mycelium develop between the bark and the hardwood of the crown and larger roots. The most obvious sign of root rot is the growth of mushrooms around the base of infected shrubs in late fall or early winter. The guilty fungus is Honey fungus, Armillaria mellea.
Once the fungus becomes established, it is difficult to control. Expose the crown of the infected shrubs to air. The fungus cannot exist under dry conditions. Because the fungus thrives in wet, heavy soil, improve drainage and avoid over-watering. Plant resistant azaleas. Dig out and destroy seriously infected shrubs and discard the surrounding soil.
Rings on Leaves, Ring-Spot Virus
If you see concentric rings on your climbing hydrangea’s leaves, alternating in color from dark green to light green, your plants have hydrangea ring-spot virus. You also may find small spots of dead tissue on the foliage. There is no cure for plant viruses, so remove infected plants promptly and destroy them. If the infection is mild and doesn’t seem to be harming the overall performance of the plant, you can simply learn to live with annual leaf spots. Do not take cuttings from infected plants, for they will be infected, and disinfect pruning shears after each cut to avoid spreading the virus.
Shoots Blackened, Flowers Blasted, Blight
Bacterial blight caused by Pseudomonas syringae affects every part of the shrub it infects. Brown to black spots form on leaves, which eventually turn completely black and dry on the branches. Shoots develop black stripes or turn black at the ends. Flowers become limp and turn dark brown. Blight is most severe during moist, mild weather, and infects the developing young shoots. The bacteria may enter twigs directly or through blighted leaves. They overwinter in diseased twigs. White-flowered plants seem most susceptible to this disease.
The fungus Phytophthora cactorum causes a similar disease in lilacs. Terminal buds and leaves turn brown instead of black, roll up and droop. Stems are often killed to the ground. This fungus also causes dieback of azaleas, so if you grow both of these plants, place them far apart.
For bacterial blight, prune and destroy diseased shoots, sterilizing your tools between each cut. Spray the entire bush with a copper fungicide. To prevent this disease, avoid overfeeding your plants and promote good air circulation by proper pruning.
For fungal blight, prune the diseased tips well below the infection. In spring, as new leaves emerge, spray the entire shrub with Bordeaux mixture or another copper fungicide. Spray again 2 weeks later.
Small Bumps on Leaves, Scale
Yellow spotting of the leaves on hollies is a symptom of scale infestation. In addition, you might find sooty mold growing on leaves where honeydew was secreted by the scale. Shrubs eventually lose their vigor. If you notice these symptoms, look for the pests themselves. Holly scale (Dynaspidiotus britannicus) usually gather on leaf undersides, but will also infest twigs and berries. They resemble small bumps, having a rounded covering which protects them as they feed. The covering is oval, light brown, to tan in color, and extremely small. The insect itself and its eggs are lemon yellow. These scale overwinter in a partially grown condition. They begin to feed in late March or early April and lay eggs in June and July. Only 1 generation occurs a year. Although holly scale is most common, 11 other species of scale insects attack hollies. These include black, California red, greedy, holly, lecanium, oleander, peach, pit-making, soft, and tea scale.
If you catch these pests before too many of them have gathered on your shrubs, simply scrape them off plant surfaces with a cotton swab dipped in rubbing alcohol. Spray heavily infested plants with a mixture of alcohol and insecticidal soap every 3 days for 2 weeks. Make this mixture by combining 1 cup isopropyl alcohol and 1 tablespoon commercial insecticidal soap concentrate with 1 quart water. If you are using insecticidal soap already mixed with water, add 1 tablespoon alcohol to a pint of the diluted soap. If scale insects have consistently been a problem, try spraying hollies in late winter or early spring with a 3% dormant oil to smother overwintering scale and keep them from getting a good start.
Spots or Blotches on Flowers, Buds (Blight)
Several different types of fungi produce blights in azaleas and rhododendrons: Ovulinia spp., Briosia spp., Pestalotia spp., Monilinea spp., and others. The flowers, buds and growing shoots of infected shrubs become spotted, blotched and discolored. In addition, they may be covered with powdery fungal spores or a thin mat of fungal strands. Azalea petal blight (Ovulinia azalea), also known as azalea flower spot and Ovulinia flower blight, is a common disease of azaleas in the southeastern states. It first appears as small pale spots on the inner surfaces of the petals of colored flowers and as brown spots on white flowers. These spots rapidly enlarge until the whole flower collapses. Small dark resting bodies of the fungus overwinter on dead flowers and in the soil.
Prune and destroy infected branch tips, leaves, and flowers. The fungi that cause blight overwinter on the ground in dead flowers and other debris, so gather and destroy diseased plant material promptly. To prevent the disease from spreading, avoid overhead watering while the plants are in flower. Plant azaleas or rhododendrons where they will be protected from sunscald and winter injury, which makes shrubs more susceptible to blight.
Stems Break Off Near Ground (Beetles)
The pitted ambrosia beetle (Corthylus punctatissimus) is about 1/8 inch long, dark reddish-brown, and marked with several tiny pits. Its small white larvae eat galleries in woody azaleas or rhododendron stems, causing them to wilt, die and break. They usually infect stems near the ground. Adults overwinter in the chambers created by the larvae and emerge to feed on fungi and mulch.
Pick pitted ambrosia beetles off your shrubs and introduce beneficial nematodes to the soil. For long-term control, apply milky spore disease (Bacillus popilliae) to the soil. When spring arrives, carefully cultivate the soil around the azaleas and rhododendrons, without hurting the roots, to expose the beetle eggs, larvae, and pupae to the weather and to predator birds.
Stems Swollen and Split (Nematodes)
Hydrangeas are vulnerable to both stem nematodes and root nematodes. When infested with stem nematodes (Ditylenchus dipsaci) the stems become swollen and split, and the leaves drop off. Root knot nematodes (Meloidogyne incognita and M. hapla) attack the roots, of course. The entire shrub becomes sickly, wilted, and stunted, with yellow or bronze foliage. It declines slowly and dies. If you look at the roots, you’ll find that they’re poorly developed and may be even partially decayed.
Nematodes are not insects, but slender, unsegmented roundworms. Most live in the soil and are 1/20 to 1/10 inch long. Add lots of compost, especially leaf mold, to the soil around infested hydrangeas to encourage beneficial fungi. Fertilize with liquid fish emulsion, pouring it into the soil as a drench; it repels nematodes.
Sunken Spots on Leaves, Anthracnose, Twig Blight
If you find moist, sunken spots on the leaves of your shrubs, on which fruiting bodies are growing, anthracnose (Colletotrichum spp. or Gloeosporium spp.) has struck. These leaf spots may run together, resembling a blotch or blight. The dead areas follow the veins or are bounded by larger veins. This disease may progress down to terminal shoots to several inches below the buds. Pustules containing pinkish spores appear on the lesions. In severe cases, dieback and defoliation may occur.
In privet, anthracnose and twig blight are caused by the fungus Glomerella cingulata. The fungus causes leaf spot and eventually the entire leaf dries and hangs from the stem. Twigs are killed, and cankers spotted with pinkish pustules from at the base of the stems. Resistant varieties include amur, California, ibota, and regal privet.
How to Control Anthracnose
Gather and destroy diseased leaves when they fall and prune infected branches. Spray with a copper fungicide such as a Bordeaux mixture. Two or three applications at weekly intervals should take care of the problem. Maintain the vigor of your shrubs by feeding and watering well, especially during droughts.
Image Credits: hpgruesen