PHOTO: © Joseph G. Strauch, Jr.

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Rust

IDENTIFICATION — You’ll spot powdery rust, orange or brown spots, like these on a snapdragon, on either side of the leaf. It’s most common in summer and fall, but can appear at any time. 

DAMAGE — Rust spores are carried by wind and water to infect plants during humid conditions. Affected plants include geraniums, asters, mums, snapdragons and peonies and also surrounding plants. Not only is it unsightly, rust weakens the plant, reducing flower and fruit production.

CONTROL — Remove and destroy infected leaves. Try to keep the foliage dry when you water and improve air circulation around the plant by removing dense vegetation nearby. Also, read descriptions to find resistant cultivars for your garden. If you still want to grow a susceptible plant, check with your local garden center for a fungicide you can spray in early summer as a preventative measure.

Photo courtesy of Iowa State University

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Crown rot

IDENTIFICATION — Crown rot affects hosta, bleeding heart, phlox, daylily and many other perennials. Yellowing foliage and mushy stems appear just above the soil line. It can start any time during the growing season, but it’s most prevalent during hot, humid periods. Once called Southern blight, this fatal rot is now spreading into much of North America.

DAMAGE — Crown rot organisms live near the surface of the soil, waiting to enter a plant and destroy the tissue. It kills the plant if it’s not stopped, and can become a permanent problem in your garden.

CONTROL — Dig infected plants and remove the top 12 in. of soil. Bury everything in a hole several feet deep to keep the spores from spreading. Never add infected plants to your compost pile. One fungicide, Terrachlor^®, may save infected plants if it’s applied before the entire crown has rotted.

PHOTO: © Joseph G. Strauch, Jr.

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Cedar-apple rust

IDENTIFICATION — In fall, you’ll spot hard brown galls on twigs of Eastern red cedar. In spring the galls swell, turn orange, as in the photo, and release spores.

DAMAGE — When those spores find the foliage of an apple tree in late spring, spots of orange rust, like the ones in the small photo, grow on the leaves during the summer. Infected leaves drop off in midsummer, leaving the tree unsightly. As they ripen, spores from the apple are then blown back to the cedar to continue the cycle next year.

CONTROL — Grow rust-resistant apple cultivars and don’t plant Eastern red cedars and apple trees in the same garden. If you have susceptible apple trees, spray them with a fungicide as a preventative when the flower buds begin to open in spring.

Photo courtesy Chris Wilson, Hallson Gardens

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Hosta Virus X

IDENTIFICATION — Unusual hosta leaves, like the one in the photo, could be symptoms of hosta virus X (HVX), a disease that’s spread rapidly in recent years.

DAMAGE — Though the virus doesn’t kill plants, the pigment and texture of the leaves may be distorted. (Markings can be mottled or have an “ink-bleed” look.) The biggest threat is how quickly the virus spreads to other hosta plants.

CONTROL — Healthy plants are infected when their sap comes in contact with the sap of an infected plant. So sterilize pruners or spades with a 10-percent bleach solution between plants as a precaution. To avoid rusty tools, dry them off afterward. There’s no cure for HVX so destroy infected hostas. For more information, go to www.hostalibrary.org

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Blackspot on roses

IDENTIFICATION — In spring small black spots appear on the leaves, and occasionally the stems, of roses. The spots look like soot, but can’t be rubbed off.

DAMAGE — By summer the spots grow larger, the leaves turn yellow, fall off and are replaced by new leaves. Repeating this process weakens the plant, making it more susceptible to other diseases, insects and winter injury. Plus, it’s just plain unsightly.

Fungus spores overwinter on infected leaves and stems left lying on the ground. Splashing water transfers the fungus to young leaves from spring through fall.

CONTROL — Search out resistant cultivars and grow roses where air is not blocked by surrounding plants. Avoid wetting the leaves, especially late in the day, and remove infected leaves as you spot them. Fungicides from the garden center will help, but spray before you see symptoms.

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Powdery mildew

IDENTIFICATION — The white granular patches look like dust on the stems, the tops and bottoms of leaves and occasionally on flowers and fruit of phlox, lilac, bee balm, aster, rose, lungwort, zinnia (in the photo), crabapple and many others.

DAMAGE — Spores of powdery mildew move by air. Powdery mildew won’t kill a plant; it only affects the appearance. You’ll find it any time during the growing season wherever susceptible plants are grown.

PREVENTION — Plant resistant varieties. Keep plants healthy and well watered to avoid stress. Always provide good air circulation around susceptible plants. Apply potassium bicarbonate fungicides and horticultural oils before you find mildew or spray them to keep the problem from spreading to more leaves or other plants.

PHOTO: © Neil Soderstrom

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Anthracnose

IDENTIFICATION — When anthracnose strikes after a cool, wet spring, you’ll find irregular dark blotches on leaves. Sometimes buds or small twigs will also be infected — symptoms vary with the host plant. Infected parts, especially leaves, eventually drop off.

DAMAGE — Anthracnose, caused by several types of fungi, attacks mainly shade trees, such as sycamore, ash and maple; also some small ornamental trees, like the dogwood in this photo. Spores overwinter on twigs, move through the air and begin to grow in cool, wet weather. Anthracnose rarely kills the tree, but repeated defoliation weakens the plant and makes it unsightly.

PREVENTION — Plant trees in sites with good air circulation and prune dense growth to improve air movement. Keep trees fertilized and vigorous so they can grow new foliage quickly. When choosing a new tree, read tags or ask for resistant cultivars.

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Botrytis gray mold

IDENTIFICATION — Botrytis gray mold usually appears during cool, wet weather in spring as small, wet-looking spots on tender new flower buds and blossoms. But it can spread to leaves and stems, as it has on these tulips. Even fruits can be damaged. The spots grow larger and are often covered with gray fuzz.

DAMAGE — Almost any plant can be stricken with botrytis gray mold. Flower buds don’t open, fruits are spoiled and the plant is weakened by the disease.

CONTROL — Wind and rain spread the spores. So grow plants where air moves freely and don’t wet flowers or foliage when you water. Bury infected plant parts 18 to 24 in. deep — don’t put them in your compost pile. Several fungicides, including Maneb and Dicloran, are effective against botrytis gray mold.

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Fire blight

IDENTIFICATION — This aptly named infectious disease makes a tree look scorched, with leaves and stems withered to brown or black. Many stems curl over in a distinctive “shepherd’s crook.” And infected leaves stay on a tree long after healthy leaves have fallen off in autumn. A sure sign of fire blight: Diseased wood under the bark is wet and rust-colored.

When it moves into larger limbs, fire blight forms dark sunken cankers, which ooze a creamy to amber-colored liquid filled with millions of bacteria. The bacteria overwinter in the cankers and spread quickly as temperatures rise and rainfall increases in spring.

DAMAGE — Fire blight, caused by the bacterium Erwinia amylovora, is extremely infectious. It affects more than 75 species, including pears, apples, hawthorns and roses. The disease is spread by insects, birds, wind, rain and humans. Bacteria enter through any opening, from blooming flowers to the smallest of cuts. It moves quickly, searing portions of a plant within weeks. And cankers in main limbs can girdle and eventually kill a plant

CONTROL — Fire blight occurs most often in new growth. To prevent this, remove unwanted suckers and waterspouts quickly, and don’t overfertilize or overprune susceptible plants.

When the weather is dry in spring, you can spray plants with streptomycin (Agri-mycin®) or other bactericides to help prevent infection. Apply copper compounds (like lime sulfur) during the plant’s dormant period.

As soon as you see the symptoms, prune the diseased areas off the plant. Cut 10 to 15 in. below the base of infection. Make sure you disinfect your pruners after each cut, using a 10 percent chlorine bleach solution (1 part bleach to 9 parts water). Burn or bury infected branches and leaves when you’re done. If the damage has spread to the trunk or main stems, it might just be easier (and cheaper) to remove the infected plant and replace it with a more resistant cultivar.

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Aster yellows on coneflower

IDENTIFICATION — One disease that can infect purple coneflower is aster yellows. It’s caused by a phytoplasma, a disease-causing organism smaller than bacteria but larger than a virus.

DAMAGE — Leafhoppers carry the disease by piercing leaves to feed on plant juices. Infected plants produce underdeveloped and deformed flowers, as shown in the photo.

CONTROL — There’s no cure, but you can stop the spread of the disease by pulling up all infected plants and destroying them. The phytoplasma needs a living host, so skip a season after the cleanup before planting more coneflowers.

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Daylily rust

IDENTIFICATION — Like other rust diseases, daylily rust causes orange spots and streaks on the leaves. If you see suspicious spots, rub a tissue along the leaves — rust will leave an orange smear. Infected leaves will turn brown and die, but rust won’t kill the plant.

CONTROL — There are some resistant cultivars available, but even those aren’t bullet-proof. To slow the spread, cut back, clean up and destroy infected foliage in the fall. Disinfect your hands and pruners between plants. Avoid overhead watering and make sure plants have good air circulation.

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Septoria leaf spot

IDENTIFICATION — Septoria leaf spot is a common fungal disease that can affect black-eyed Susans. It begins with brown-black spots on the leaves then progresses to where the leaves may turn completely brown and fall off. Symptoms usually appear on lower leaves first.

CONTROL — This disease is tricky to get rid of but won’t kill your plants. Septoria spores are spread by water droplets, so watering at the base of the plant will reduce the spread. Rake up diseased leaves and burn or bury them. Don’t put them in the compost pile.

You can apply fungicides before any symptoms appear, but keeping the area clean will keep the plants looking better next year.

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Blossom-end rot on peppers

IDENTIFICATION — Blossom-end rot is a common problem with peppers, as well as tomatoes, melons and squash. It starts out as a round, sunken, soft spot at the end of the pepper and grows steadily larger. Blossom-end rot is caused by a calcium deficiency in the developing fruit. The calcium deficiency is caused by fluctuating moisture and too much nitrogen that stress the plants. Deep cultivation and salt burn can also cause stress that may result in blossom-end rot.

CONTROL — To keep your fruits and vegetables healthy, mulch around them to keep the soil evenly moist and cool. Feed the plants with a low-nitrogen fertilizer at planting and give them about an inch of water per week. If you still have problems, spray your plants with a calcium chloride solution that you can find at your local garden center. Start spraying when the fruits begin to form and continue on a weekly basis for four weeks. Be sure to wash the fruit as you normally would before eating.

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Botrytis on peonies

IDENTIFICATION — Peony bushes can suffer from a variety of fungal diseases and botrytis is a big one. Fungal diseases thrive in warm, moist conditions like those in excessively rainy springs or humid summers. Small brown spots on the leaves will eventually spread to cover the entire leaf surface. Infected buds may shrivel up and fail to bloom.

CONTROL — Copper-based fungicidal soap sprays, such as Soap-Shield, may help. You may not need to treat the disease though; this fungus rarely kills the plant. As a preventative measure, cut the plants back to the ground after frost. Don’t compost infected material; discard it to get rid of fungus spores.

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Verticillium wilt

IDENTIFICATION — If the leaves of your annual flowers and vegetable plants seem to be yellowing and curling, your garden may be infected with verticillium wilt. Older, lower leaves fall off first, as younger, higher leaves become discolored and distorted. Only a few healthy leaves will remain at the top of the plant until it eventually dies.

To make sure wilt is the culprit, cut open an infected plant stem horizontally, near the soil line. Look for a narrow tan ring between the center and stem wall, or dark streaks shooting up the middle of the stalk.

CONTROL — Plants infected with verticillium wilt should be destroyed promptly because there is no control. The disease-causing fungus can live in the soil many years after the plant has died, so rotate your crop every four to six years. And make sure to keep it far away from your compost pile.

Protect your garden from wilt by buying resistant plants. Look for kinds that are labeled “VFN resistant.” These plants are less susceptible to verticillium wilt (V), fusarium wilt (F) and nematodes (N), which are all common pests.

Verticillium wilt also attacks some trees and shrubs. In large woody plants, the fungus causes leaves to yellow and curl and branches to die suddenly. If you suspect verticillium wilt has infected a tree or shrub, cut a twig from the infected area. Look for the ring in the cross section; there may also be streaking along the length of the twig if you peel back the bark. Before removing a valuable tree, consult with a plant pathologist to be sure of your diagnosis.

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Aster yellows

IDENTIFICATION — Deformed flowers, yellow foliage and stunted growth are classic signs of this disease. Though symptoms vary according to the temperature and the age and type of plant, the first thing most gardeners notice is green, twisted flowers with clusters of extra leaf growth in the center. However, if you’re really paying attention, yellowing of new leaves, including the veins, usually happens first. (If the veins remain green, the plant probably has a nutrient deficiency). Growth may also be lopsided, with one side of the plant looking normal while the other side is stunted.

DAMAGE — Aster yellows is caused by a tiny phytoplasma, an organism smaller than a bacteria but larger than a virus, that infects the food-conducting cells of the plant. Hundreds of different plants are susceptible, including aster, zinnia, coneflower, canna, carrot, lettuce, tomato and many others. Weeds, such as dandelion, thistle and plantain, can also carry this persistent disease.

The phytoplasma is carried from plant to plant by hungry leafhoppers, known as six-spotted leafhoppers (Macrosteles quanrilineatus). These tiny, winged insects live on the underside of leaves and feed on the sap, spreading the disease as they travel from plant to plant. Cool, wet summer weather favors both leaphopper and phytoplasma spread.

CONTROL — There is no cure for aster yellows, but it rarely kills a plant outright. It’s important to remove and destroy any infected ones. If you leave these plants in the garden, leafhoppers will continue to visit them and spread the disease. To prevent aster yellows, keep your garden clear of dead foliage and debris in late fall to eliminate overwintering sites for leafhopper adults and eggs. You can also spray the tops and undersides of leaves with insecticidal soap or insecticides like pyrethrin or neem to get rid of leafhoppers.

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Fireblight

IDENTIFICATION – This aptly named infectious disease makes a tree look scorched, with leaves and stems withered to brown or black. Many stems curl over in a distinctive “shepherd’s crook.” And infected leaves stay on a tree long after healthy leaves have fallen off in autumn. A sure sign of fireblight: Diseased wood under the bark is wet and rust-colored. When it moves into larger limbs, fireblight forms dark sunken cankers, which ooze a creamy to amber-colored liquid filled with millions of bacteria. The bacteria overwinter in the cankers and spread quickly as temperatures rise and rainfall increases in spring.

DAMAGE – Fireblight, caused by the bacterium Erwinia amylovora, is extremely infectious. It affects more than 75 species, including pears, apples, hawthorns and roses. Bacteria enter through any opening, from blooming flowers to the smallest of cuts. The disease is spread by insects, birds, wind, rain and humans. It moves quickly, searing portions of a plant within weeks. And cankers in main limbs can girdle and eventually kill a plant.

CONTROL – Fireblight occurs most often in new growth. To prevent this, remove unwanted suckers and waterspouts quickly, and don’t overfertilize or overprune susceptible plants. When the weather is dry in spring, you can spray plants with streptomycin (Agri-mycin®) or other bactericides to help prevent infection. Apply copper compounds (like lime sulfur) during the plant’s dormant period. If a plant is showing symptoms, prune diseased areas off right away. Cut 10 to 15 in. below the base of infection. Make sure you disinfect your pruners after each cut, using a 10 percent chlorine bleach solution (1 part bleach to 9 parts water). Burn or bury infected branches and leaves when you’re done. If the damage has spread to the trunk or main stems, it might just be easier (and cheaper) to remove the infected plant and replace it with a more resistant cultivar.

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Blackspot on roses
Diplocarpon rosae

IDENTIFICATION – The name says it all. This fungal disease causes black spots with fringed margins and a yellow halo on leaves. Infected leaves drop early, often leaving the plant almost bare. Blackspot doesn’t kill roses, but it weakens them, making them susceptible to insects, cold and other diseases. Warm, humid summers make blackspot worse.

CONTROL – The best control is prevention: Plant blackspot-resistant varieties. Ask at your local garden center which ones are best. Make sure roses have good air circulation and full sun, and water with a soaker hose. If you use overhead watering, water in the morning so the leaves dry quickly. Rake out fallen leaves around the plant to get rid of as many spores as possible. You can’t get rid of the spots already on leaves, but you can prevent more from forming. Spray your roses with a mix of 1 tablespoon baking soda and 1 tablespoon horticultural oil in 1 gallon of water once a week if you see signs of blackspot. Shake the sprayer to keep the mixture stirred and spray until all the foliage is coated. Try this on a few leaves first, to make sure it doesn’t burn them. If it does, use less baking soda. Neem oil is also effective. Spray on cloudy days or early morning, not in full sun, to avoid burning the leaves.

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Crown gall

IDENTIFICATION – A good word to describe crown gall on roses and other ornamental plants is “yuck.” The lumpy growths around the base of the plant are unattractive, to say the least. This disease, caused by the bacterium Agrobacterium tumefaciens, is found on a wide range of plants, including roses, euonymus, junipers, fruit trees and willows, as well as annuals like impatiens and begonias.

Usually, you’ll notice rough, woody growths (galls) on the stems and branches of the plant near the soil line. The galls form on the roots of some species. On trees, they may even form among the branches. There’s often one big swelling, with several smaller ones nearby.

Galls are green at first, but become rougher and browner as they grow. Galls eventually rot and crumble away, with new gall tissue forming underneath. On small plants and shrubs, the galls are usually 2 or 3 in. across, but they can be much larger on trees, weighing as much as 50 lbs. Plants can live for a long time with crown gall, but they may become weak and stunted as the gall interferes with the uptake of water and nutrients. If the gall completely encircles the stem or trunk, the plant may die.

CONTROL – The bacteria, which can live in the soil for several years, enters through wounds near the plant’s base. So the best control is prevention — don’t damage plants close to the soil line, and protect them from damage by mowers, insects and animals.

Unfortunately, once the galls are present, the only real control is to remove the plant and as much soil around it as possible. As the galls decay, they shed bacteria into the soil, so it’s best not to leave the plant in place. When you prune plants, especially near the base, sterilize your pruners or saw with a solution of 1 part bleach to 4 parts water. Check for galls on new plants before you put them in your garden. And if you’ve had problems with crown gall, don’t plant susceptible species in the same area for three to five years.

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Powdery mildew

IDENTIFICATION – In mid- to late summer, you may notice a gray-white film on a plant, and the plant may begin to lose leaves. The culprit is powdery mildew. You’re most likely to see this problem on roses, phlox, zinnias, bee balm and lilacs (in the illustration).

This condition won’t kill the plant, but it’s not very attractive. Symptoms look the same, but specific fungi cause powdery mildew on different plants. In other words, the fungus that causes these symptoms on roses won’t cause the same disease on zinnias.

FAVORITE CONDITIONS – Humid weather, with warm days and cool nights, encourages powdery mildew.

CONTROL – As with many plant diseases, the easiest way to control powdery mildew is to plant resistant cultivars. But there are a few things you can do to help your existing plants. The spores survive the winter in leaves and plant debris, so clean up around infected plants to get rid of as many spores as possible. Make sure your plants have enough sun and good air circulation, as this will cut down on powdery mildew.

Fungicides are very effective for preventing this fungus. Read the label to make sure the fungicide controls powdery mildew, not downy mildew, a different disease. You’ll probably need to apply it more than once, starting before you see symptoms. You can’t “cure” powdery mildew — prevention is key. Spraying horticultural oil on plants until the leaves are covered, top and bottom, cuts down on the problem. Test this on a few leaves first, to make sure the leaves don’t burn. Wait for a cloudy day to spray anything on your plants — the combination of sun and sprays can damage foliage.