Chapter 14- Plant Diseases

Author: Alan Windham, Retired Professor and Extension Specialist, University of Tennessee

Plant diseases have been around as long as there have been plants. It should come as no surprise that examples of plant diseases can be found in any lawn, garden, field, nursery, or greenhouse. In some cases, the damage caused by plant diseases is severe; in others, the damage is so slight that it is easily overlooked. A plant disease may be defined as plant damage or loss of yield caused by an infectious agent or stress that affects the metabolism of its host. Infectious agents that cause plant diseases are called pathogens.

Infamous Plant Diseases

Although there are a multitude of plant diseases, many known only to experts, a few have gained widespread notoriety because of the nature and extent of the damage they have caused in the past; for example:

• Late blight of potato
• Chestnut blight
• Dutch elm disease

The Irish potato famine of the 1840s and 1850s is legendary. A sizable proportion of the US population may even be descended from Irish immigrants who were displaced during the famine. The famine had many societal causes, but the dependence on potatoes for subsistence by the Irish populace was a primary one. Potato crops were destroyed by late blight caused by a fungus-like organism called Phytophthora infestans. It has been estimated that 1 million people died during the famine and another 1 million emigrated to other countries. Surprisingly, late blight is still a threat to potato and tomato plants. In 2009, a late blight pandemic in the northeastern US destroyed tomato crops in home gardens and organic farms.
The discipline and science of plant pathology (the study of plant diseases) arose primarily from the studies conducted during and after the potato famine of 1840.
Chestnut blight was first found in the US in New York in 1904 after infected Asian chestnut trees were brought into the country. This fungal disease causes a localized canker infection to the trunk and branches of infected trees. Trees are killed to the soil line. While the disease in effect wiped out American chestnuts in the eastern forests of the US, chestnut stumps with shoots can still be found in the forests of East Tennessee. Through the work of the American Chestnut Foundation and university researchers, a blight-resistant American chestnut is on the verge of being reintroduced to our forests.
Dutch elm disease is a vascular wilt disease of elm trees caused by a fungal pathogen. It was introduced to the US via elm logs from the Netherlands that were imported for veneer. Many US cities lost massive numbers of American elms that formed a canopy over residential streets. Management of this disease is complicated because the pathogen is transmitted by elm bark beetles. Examples of this disease may be seen each year in Tennessee as it attacks our native September elm and winged elm. Researchers were successful in breeding resistant elms, such as the Princeton elm, which is a common street tree in Tennessee cities.

Plant Diseases Making News Today

Plant diseases continue to be a threat to food, fiber, timber, and ornamental plants today. In our global society, plants and plant pathogens are moved unintentionally in the course of international commerce. Some examples of exotic plant pathogens (diseases) that have been introduced into the US recently are:

• Citrus greening: This bacterium-induced disease, vectored by an insect (the psyllid), has the potential to destroy the US citrus industry.
• Sudden oak death: This disease of shrubs and trees has killed millions of tanoak and oak species on the West Coast and causes a leaf blight of shrubs. Although occasionally found on nursery stock from the West Coast, this disease has not affected native oak species in the Southeast.
• Boxwood blight: This fungal disease was first found in the US in 2011 and in Tennessee in 2014. It is a threat to many boxwoods grown in gardens and nurseries. Boxwood blight can rapidly defoliate plants in gardens.
• Downy mildews: At least four organisms that cause downy mildew of garden impatiens, rudbeckia, basil, and coleus have been introduced to the US since 2005. Garden impatiens (Impatiens walleriana) was one of the top annual bedding plants in the US before the introduction of impatiens downy mildew.

Plant diseases in other parts of the world also affect the US. For example, coffee prices have spiked because of a rust disease introduced into South and Central America in the last decade. Coffee rust defoliates trees and drastically reduces the production of coffee beans.

Plant Disease Triangle

The plant disease triangle (Figure 1) illustrates the interaction of three components necessary for plant disease to occur: a susceptible host, a virulent pathogen, and favorable environmental conditions. Some common diseases illustrate how the disease triangle works.

• Early blight, a foliar disease of tomato, occurs when a susceptible host (tomato), a virulent pathogen (Alternaria), and favorable environment (warm, wet weather) are present.
• For black spot of rose, the host (rose), the pathogen (Diplocarpon), and the right environment (warm temperatures and water on leaves) are necessary for the disease to develop.
• For phytophthora root rot, the host (rhododendron), pathogen (Phytophthora), and environmental conditions (excessive water, poor drainage, and standing water) are needed for disease development.

The plant disease triangle is a useful concept to keep in mind when considering disease management options. Diseases can be addressed by targeting the pathogen, modifying the environment, or altering the physiology or genetics of the host plant.

Plant Disease Management

Plant diseases are managed by one or more of these strategies: exclusion, eradication, protection, and resistance.

Exclusion

Preventing the introduction of a pathogen to a specific geographic area is one way to avoid the occurrence of disease. This approach may be as simple as inspecting plants for signs or symptoms of disease before purchasing them at a garden center or as complex as the system that state and federal regulatory agencies have established for inspecting plants at a port of entry to the US or at a nursery that receives plants from out of state. Other exclusion strategies include producing pathogen-free seed and plants in areas of the country where disease prevalence is low.

Eradication

Destroying plants that are infected with a pathogen of economic or regulatory importance is the basis for the eradication approach. The process may be as simple as removing and destroying a single diseased plant in a garden or as complex as removing hundreds or thousands of diseased plants from a nursery, garden center, or field. Other eradication strategies include using steam or fumigants to kill soil-borne pathogens in a landscape bed, greenhouse, or field. Solarization — the use of clear plastic to cover soil during the heat of the summer to elevate the soil temperature for several weeks — has been used to reduce soil-borne pathogens such as root knot nematodes in gardens. (see the supplemental materials for more information on solarization.) Hot water has been used to eradicate smut fungi, nematodes, or other pathogens on seed, bulbs, or plants. Crop rotation (planting a nonhost crop in an area infested with a pathogen from a previous crop) can greatly reduce the presence of some soil-borne pathogens such as nematodes.

Protection

Applying a chemical control agent (such as a fungicide or bactericide) is one way to protect plants, but this plant protection strategy encompasses more. Adjusting soil pH, fertility, or soil physical properties to improve drainage can help protect plants from disease. For example, the pH of substrates used for plants susceptible to black root rot (Thielaviopsis) can be adjusted to reduce the pH below 6.2 to minimize disease severity. Lawns that have a history of dollar spot disease (Sclerotinia) are rarely affected if nitrogen levels are maintained at optimum levels. Poor drainage favors development of root rot diseases caused by water molds such as Pythium and Phytophthora. Adding soil amendments that improve drainage, raising bed height, and preventing standing water minimize the risk of these diseases.
Fungicides and bactericides are pesticides that help prevent diseases caused by fungi and bacteria, respectively. They are best used as preventive treatments, not as rescue or curative treatments. Remember the concept that fungicides protect healthy plants! Most fungicides work by providing a protective barrier that prevents spore germination or fungal growth. Fungicides that are partially systemic move upward in the plant. Fungicides that are fully systemic move up and down via the plant’s vascular system. Before applying a pesticide, always read the label.

Resistance

Disease resistance is the best strategy for managing diseases. Many vegetables, fruit trees, woody ornamentals, and trees are resistant to certain plant diseases. For specific information check Extension Service publications and seed catalogs.
For example, many tomato cultivars are resistant to one or more diseases. Check for disease resistance codes for tomatoes on the labeling of tomato bedding plants or seed packs. If the abbreviation VFN appears on the label, it indicates that the tomato cultivar is resistant to verticillium wilt, fusarium wilt, and root knot nematodes. For more information on tomato disease resistance codes, check seed catalogs and Extension Service publications that list disease-resistant vegetables.
Disease resistance has been used to combat many new diseases. The University of Tennessee AgResearch dogwood group has released the ‘Appalachian’ series of dogwood trees that are resistant to either dogwood anthracnose or powdery mildew. The USDA research group at the National Arboretum has released many ornamentals, including crape myrtles, resistant to powdery mildew. Trees and woody ornamentals are an investment worth researching before planting. Choose species and cultivars resistant to common diseases in the area.

Disease Diagnosis, Signs, and Symptoms

Plant disease problems can be identified and diagnosed by following a set of logical steps.

• Identify the host. Each plant species has a distinct group of diseases and their corresponding plant pathogens. It is important to identify the plant correctly or the process will be derailed from the start. Also, most references on plant diseases list the diseases by host. This sounds like a simple concept, but many novice gardeners confuse flower, shrub, and tree species.
• Imagine a healthy plant (to visualize what is “normal” for the species). Normal in this context means conforming to a standard, namely, the usual, typical, or expected shape, color, size, and other aspects of the species. It helps to know what a healthy specimen of the plant species or particular cultivar looks like. Is it normal for this plant to have nodules on roots? Variegated foliage? Flattened stems? Knowledge of plant structure and characteristics is helpful when considering plant disease problems. It pays to compare plants. Is there a healthy species or cultivar to compare with one that is less desirable because of its size, shape, or conformation?
• What plant parts are affected? Leaf spots and blights are limited to leaves. Canker diseases affect stems and branches. Root rot appears on roots, of course. Vascular wilt diseases damage the xylem tissue that conducts water to the leaves.
• Look for signs and symptoms of disease. A sign of disease is the visible portion of a plant pathogen, such as the white mycelium of fungi that cause powdery mildew, or the orange spores associated with rusts. A symptom refers to damage caused by the pathogen. Symptoms may include wilted leaves, yellow leaves, dead leaf tissue, discolored vascular tissue (xylem), stunted plants, ring spots, mosaic, witch’s broom (as shown in the photo on this page), and the like.
• Consider both living (biotic) and nonliving (abiotic) causes of plant disease problems. Biotic pathogens that cause plant disease include fungi (the most prevalent), bacteria, viruses, nematodes, and phytoplasmas. Abiotic causes of plant problems include excessive or deficient moisture, temperature extremes, pesticide injury, soil pH extremes, and nutrient deficiencies or excesses (high soluble salts).
• Document the signs and symptoms of the disease using a camera to capture the appearance. Record four to five images that “tell the story” of the problem. Suitable images could include a comparison of a healthy and unhealthy plant side by side; an image of a whole plant; an image of the root system with the substrate or soil washed away; and an image of leaf spots, mildew, or rust. These images can be forwarded to the local Extension office for review and analysis.
• Collect a specimen. If you decide you need an expert opinion to aid in diagnosing a plant problem, collect a specimen. For most herbaceous plants, this specimen would include the whole plant; for woody plants, collect the portion of the plant with signs or symptoms of disease.
• Carry a diagnostic kit with a few items to help in examining and collecting specimens. Include a small magnifying glass (with 3- to 4-power magnification). Use the magnifying glass to triage specimens by closely examining leaves and twigs for signs of disease. Carry a trowel to remove small plants, collect roots, or take a soil sample. Include hand pruners for removing twigs or branches for further examination. Always carry plastic bags for collecting specimens. Refrigerate specimens if they cannot be processed quickly.
• Search for disease diagnosis information on the Web. Using a search engine to search by keywords, such as the host and symptoms, can produce a wealth of helpful information. Google images are very helpful for diagnosing plant problems. For a leaf spot disease on flowering dogwood, simply use the keywords “dogwood” and “leaf spot.” Web pages associated with land-grant universities, state Extension Services, and botanical gardens are some of the best aids in diagnosing plant problems.
• For assistance with disease diagnosis, contact the county Extension office. For problems outside of the local agent’s experience, send samples to the University of Tennessee Soil, Plant, and Pest Center in Nashville, Tennessee.

UT AgResearch Dogwood Team Works to Save the Flowering Dogwood

Flowering dogwood, Cornus florida, is a staple of the nursery industry in Tennessee. More dogwoods are produced in Tennessee nurseries than anywhere else in the US. Dr. Michael Dirr has called flowering dogwood “the aristocrat of native flowering trees.” It is a great tree for Tennessee gardens but is not without problems. In recent years, dogwood anthracnose and powdery mildew, caused by Discula destructiva and Erysiphe pulchra, respectively, have threatened the health of this tree. Dogwood anthracnose was introduced simultaneously in Seattle and New York City in the mid-1970s and moved down the Appalachian Mountains, where it was discovered in the Cherokee National Forest, landscapes plantings on Lookout Mountain, and nurseries in Middle Tennessee by the late 1980s to early 1990s. It was particularly devastating to dogwoods growing as understory trees in forests. Powdery mildew was first reported on dogwood in the late 19th century, but it was not widespread on Tennessee dogwoods until the early 1990s. You can now find powdery mildew on dogwood in nearly every dogwood planting across the eastern US.
In response to these threats to a beloved landscape tree and valuable nursery commodity, a team of UT Institute of Agriculture research scientists formed a working group, the Dogwood Team. The group first targeted dogwood anthracnose, with short-term goals of studying the biology of the causal fungus, Discula destructiva, and management strategies for nurseries. The long-term goal was to identify disease-resistant dogwoods via plant selection or plant breeding. Scientists from the team traveled to Catoctin Mountain Park in Maryland to look for dogwoods that had survived intense disease pressure. Four trees were identified and tested for resistance to anthracnose. Of these four, a tree was identified with superior horticultural characteristics including resistance to anthracnose. After further investigations, Cornus florida ‘Appalachian Spring’, a white-flowering dogwood, was released to the nursery industry. Appalachian Spring proved popular with growers and was selected as a “Plant of the Year–Honorable Mention” by the Garden Clubs of America in 2003.
The threat from powdery mildew was actually more pronounced because the disease was not limited to the Appalachian region of Tennessee. Again, the team addressed short-term goals of studying the fungal biology of the causal pathogen, Erysiphe pulchra, and identifying fungicides to protect healthy trees in nurseries and landscapes. As powdery mildew was widespread in nurseries, scientists walked row after row of infected seedling dogwoods to look for seedlings not infected by the disease. Forty trees were identified for further study. After several years of evaluation, three new white-flowering, mildew-resistant cultivars were released for nursery production: ‘Appalachian Blush’, ‘Appalachian Snow’, and ‘Appalachian Mist’. A few years later, ‘Appalachian Joy’, another white-flowering, mildew-resistant cultivar was released.
The Appalachian series of disease-resistant dogwoods remain popular today and are grown by nurseries across the US. Trees may be found in garden centers, botanical gardens, and arboreta nationwide. The Dogwood Team remains active and has released two additional trees in recent years. Cornus kousa ‘Red Steeple’ and ‘Empire’ are two upright Korean dogwood cultivars suited for the small spaces found in many urban gardens today.

Plant Pathogens and the Diseases They Cause

Fungi

Fungi are usually filamentous and produce thread-like structures called hyphae. A mass of hyphae is called mycelium. Some fungal species produce overwintering structures, or resting bodies, called sclerotia, which are composed of tightly bound hyphae. Most but not all fungi produce spores that are important for dissemination and infection. Examples of fungal spores are conidia, ascospores, basidiospores, oospores, zygospores, urediniospores, teliospores, and others.
Some fungi complete their life exclusively on a living host. Others can grow on either a living host or decaying organic matter. Saprophytic fungi may live their whole lives on decaying organic matter. Water molds such as Pythium and Phytophthora are no longer considered fungi but have instead been grouped with brown algae.
Many plant pathogenic fungi are microscopic, visible only with the aid of a microscope. Others produce fruiting bodies, mushrooms, or conks (fruiting bodies of various shapes and colors often seen on tree trunks) that are visible to the naked eye. It has been estimated that only 10 percent of all fungi have been identified. Examples of fungal diseases include powdery mildew, downy mildew, rusts, smuts, leaf spots, blights (such as late blight and boxwood blight), anthracnose, root and stem rots, vascular wilt diseases, canker diseases, and fruit rots.

Bacteria

Most bacteria are microscopic, single-celled organisms having rigid cell walls, but another group also classified as bacteria are phytoplasmas. They, too, are microscopic, but they may take various shapes because they do not have a rigid cell wall. Phytoplasmas are associated with “yellows” diseases. Examples of bacterial diseases include crown gall, fire blight, bacterial blights, bacterial leaf spots, bacterial leaf scorch, soft rot of fruit, and bacterial wilt. Examples of phytoplasma diseases include aster yellows, elm yellows, ash yellows, and palm yellows.

Nematodes

Nematodes are nonsegmented, microscopic roundworms. Those that attack plants have a spear-like mouth part called a stylet. Only about 10 percent of nematodes are plant pathogens. Other nematode species are those that feed on soil microbes (called free-living nematodes) and those that feed on insects (entomopathogenic nematodes). The best way to determine if plant parasitic nematodes are a problem is to collect a soil sample for processing and microscopic examination at a plant disease clinic. Common nematodes in Tennessee include root knot nematodes, which cause stunted, galled roots (knots), and foliar nematodes (shown below), which cause angular lesions on many shade-loving perennials.

Viruses

Viruses are infectious, microscopic pathogens composed of nucleic acid and a protein coat. Viruses have the ability to replicate in host cells, and most can cause disease. Nearly 1,000 plant viruses have been identified. Plant viruses are often named for the host in which they were discovered — for example, tobacco mosaic virus, tomato spotted wilt virus, cucumber mosaic virus, and rose rosette virus. However, the name sometimes does not reflect the full range of plants the virus can infect. Tobacco mosaic virus infects many plant species in addition to tobacco. Cucumber mosaic virus attacks cucumber but may also attack many annual bedding plants. Other viruses are more specific in their hosts. Rose rosette virus has only been found on rose. Plant viruses may be vectored (transmitted to plants) via insects, nematodes, mites, fungi, and sap. Common symptoms caused by virus infections include ringspots, mosaics, mottling, rosette (witch’s broom), and line patterns.

Plant Diseases

Powdery Mildew

Powdery mildew is easily identified by the presence of white to gray mycelium on affected leaves or flowers. The first sign of disease is usually isolated colonies of white fungal growth. With time, whole leaves may be totally covered with fungal growth. On some plants, such as pin oak, mildew may be present only on the undersides of leaves. On dogwood, crape myrtle, and nandina, infected leaves may be curled, twisted, or otherwise distorted. Leaves may be abnormally red with little mycelium visible; on sedum lesions are scabby and brown.

Hosts

Amelanchier, azalea, begonia, columbine, crabapple, crape myrtle, dogwood, euonymus, hydrangea, lilac, magnolia, nandina, oak, phlox, rhododendron, rose, sedum, tulip tree, verbena, and zinnia.

Host Resistance

Use disease-resistant species or cultivars for crabapple, crape myrtle, dogwood, lilac, rose, and zinnia.

Downy Mildew

Downy mildew sounds similar to powdery mildew, but the diseases are very different and are caused by fungi from entirely different taxonomic classes. The fungi-like organisms that cause downy mildew are more closely related to Phytophthora and Pythium than the fungi that cause powdery mildew. Symptoms of downy mildew can range from leaf spots and defoliation to rapid blighting of diseased shoots. Angular leaf spots on rose may range from red to brown to black. Signs to look for include gray to white tufts of mycelium on the undersides of leaves, directly below chlorotic lesions. Look for mycelium early in the morning while the leaves are still wet.

Hosts

Alyssum, basil, brambles, coleus, grape, impatiens, pansy, rose, rudbeckia, salvia, snapdragon, tobacco, and viburnum

Host Resistance

For downy mildew of garden impatiens substitute begonias, coleus, New Guinea impatiens, SunPatiens, or Torenia.

Gray Mold

Gray mold may be found on herbaceous and woody ornamentals, usually during cloudy, cool, moist weather. Stems, leaves, and flowers may be attacked. Woody ornamentals in overwintering structures may become infected. Symptoms of infection are blighting of flowers, tan to brown leaf spots, shoot blights, and stem rot. A sign of infection is gray-brown mold on diseased plant parts.

Hosts

Almost any herbaceous or woody plant

Sanitation

In greenhouses and propagation areas, remove infected plant parts or plants.

Environmental

In greenhouses, use fans and vent to remove moist air at the end of the day. Minimize leaf wetness.

Rusts

Rusts cause bright yellow, orange, reddish-brown, or chocolate-brown raised pustules that are usually most visible on the undersides of leaves. Cedar-apple rust on eastern red cedar produces gelatinous orange tendrils of rust spores each spring from galls. Pine needle rust produces pustules on pine during spring. Early symptoms on leaves are yellow leaf spots. Rust galls may appear on stems of pine, red cedar, and hawthorn. Twig rust may cause branch dieback on plants as diverse as hawthorn and hemlock. Daylily rust was first found in the US in 2000 and in Tennessee in 2001.

Hosts

Amelanchier, apple, aster, azalea, cedar, crabapple, daylily, fuchsia, geranium, grasses, hawthorn, hemlock, hollyhock, iris, jack-in-the-pulpit, juniper, mayapple, oak, pear, pine, potentilla, quince, snapdragon, sunflower, and beans

Host Resistance

Cedar-rust-resistant cultivars are available for apple, crabapple, and hawthorn.

Virus Diseases

Virus diseases cause a variety of symptoms including mosaic, ringspots, stem lesions, rosette (witch’s broom), “oak-leaf” pattern, stem pitting, stunting, flower break, and other conditions. Hosta virus X (HVX) is fairly common on hosta. HVX is most easily diagnosed on gold hosta cultivars where abnormal green stripes appear parallel with leaf veins. Canna yellow mottle virus is very prevalent on the Tropicana series of canna. Symptoms include stunted plants, necrotic streaks in leaves, and muted variegation. Rose rosette is a viral disease transmitted by eriophyid mites. Virus diseases may be difficult to diagnose unless you are familiar with symptoms associated with these diseases.

Hosts

Canna yellow mottle virus: Tropicana series of canna lily
Hosta Virus X: Many common cultivars of hosta
Impatiens necrotic spot virus: More than 350 ornamental plants
Tomato ringspot virus: Dogwood, fringetree, peach, and cherry
Tomato spotted wilt virus: Perennial plants and tomato
Rose mosaic virus & Rose rosette: Rose
Cucumber mosaic: Many hosts
Tobacco mosaic: Many hosts

Sanitation and Cultural Control

Do not propagate plants with symptoms of a viral disease. In production, remove and discard infected plants. For impatiens necrotic spot, monitor and manage thrips populations in greenhouses.
Rose rosette: Remove roses with symptoms of rose rosette. Break up mass plantings by using nonhost plants as a barrier between smaller plantings. Remove multiflora rose near rose plantings. Pruning and control of the vector (eriophyid mites) are being evaluated.

Leaf Spot

Leaf spot diseases are usually caused by fungi, but a few may be caused by bacteria. These are among the most common plant diseases. Symptoms vary depending on the pathogen and host. Some common symptoms include frogeye or bull’s-eye spots marked with concentric rings; irregular, round tan spots with small black fruiting bodies; angular tan or black spots; black or tan spots surrounded by a yellow “halo”; oval-shaped leaf spots; and tan to gray spots with red or purple margins. Fungal leaf spot diseases are usually favored by wet seasons, high humidity, or frequent overhead irrigation. Many leaf spot diseases cause premature defoliation.

Hosts

Alternaria Leaf Spot: Aucuba, impatien, marigold, and zinnia
Black spot: Rose
Bull’s-eye leaf spot: Magnolia and maple
Cercospora leaf spot: Buckeye, crape myrtle, leucothoe, laurel, redbud, rose, and vegetables
Entomosporium leaf spot: Indian hawthorn, pear, and photinia
Leaf blotch: Buckeye and horse chestnut
Phyllosticta leaf spot: Holly, magnolia, maple, and witch hazel
Septoria leaf spot: Dogwood and tomato
Scab: Crabapple, apple, and pyracantha

Sanitation and Cultural Control

Rake and remove diseased leaves. Minimize leaf wetness; use drip irrigation.

Host Resistance

Choose disease-resistant cultivars of rose, crabapple, Indian hawthorn, buckeye, and horse chestnut

Shot Hole Diseases

Shot hole diseases cause infected leaves to be covered with circular “shot” holes on plants that shed diseased leaf tissue in response to fungal or bacterial infections. The holes result from diseased tissue falling out. Infected leaves may become chlorotic and drop prematurely. Shot hole diseases may be caused by fungi or bacteria. Damage from shot hole disease may be confused with insect feeding. Remember, shot hole disease occurs only on plants in the genus Prunus. Similar symptoms on other plants may be caused by insects.

Hosts

Almond, apricot, cherry, cherry laurel, peach, and plum (plants in the genus Prunus)

Sanitation and Cultural Control

Rake and remove fallen leaves. Minimize leaf wetness, especially for bacterial shot hole diseases of cherry laurel.

Anthracnose

Anthracnose diseases cause leaf, stem, or fruit lesions. These symptoms may appear as irregular leaf spots or lesions along leaf margins and across or between veins. Anthracnose may kill entire leaves, young shoots, and twigs, and it may cause premature defoliation. Diseased leaf tissue may fall out of leaf lesions. Stem cankers may form at the base of succulent shoots. Look for anthracnose diseases of ash, dogwood, maple, and sycamore during April and May.

Hosts

Ash, dogwood, euonymus, hosta, maple, oak, sycamore, pepper, and bean

Sanitation and Cultural Control

Rake and remove fallen leaves. Prune and remove cankered or dead branches. Space plants to increase air movement and minimize leaf wetness. Use drip irrigation to minimize leaf wetness.

Needle Blight and Cast of Conifers

Needle blight and cast of conifers is basically a leaf spot disease of conifers that leads to premature shedding of needles. During certain times of the year, distinct yellow to brown lesions are visible on pine needles. Infected needles turn brown and drop. Fungi that cause needle cast are generally weak pathogens that infect older needles in the interior of the tree’s canopy. Black fruiting bodies of various fungi may be observed in single or multiple rows along the length of infected needles. Conifers infected with needle cast have brown needles and thin canopies.
The most common tip blight diseases of juniper include phomopsis blight, which attacks new flushes of growth in late spring or early summer. Kabatina blight of juniper attacks trees injured from snow or ice in late winter through early spring. Tip blights rarely kill more than four inches of the terminal of juniper shoots.

Hosts

Cyclaneusma needle cast: Scots pine
Lophodermium needle cast: Eastern white pine
Ploioderma needle cast: Loblolly pine
Rhizosphaera needle cast, Stigmina needle cast: Spruce
Phomopsis blight: Juniper
Kabatina blight: Juniper and Leyland cypress
Passalora needle blight: Leyland cypress

Sanitation and Cultural Control

For field-grown plants, choose locations with good air movement. Space plants to minimize needle wetness. For needle blights such as Kabatina, prune if desired to remove diseased shoots.

Canker Diseases

Canker diseases are caused by fungi, which may live as endophytes on susceptible hosts. The term endophyte means that the fungus lives on or in host tissue but produces no signs of disease. If the plant comes under significant stress, however, dieback may not be far away.
Leaf death and twig dieback are some of the first symptoms of canker diseases. Cankers are usually found on branches but may infect trunks of young trees. The disease may be initially undetectable except by shaving the surface of a branch to reveal brown discoloration of bark or underlying sapwood.
As cankers enlarge, oval, sunken areas may develop on branches. Large cankers may girdle and kill branches and entire plants if they develop on the trunk or main stems of shrubs.
Gum production (gummosis) is often associated with fungal and bacterial cankers of cherry. Resin is often associated with canker diseases of junipers. Fire blight is a bacterial disease that may cause cankers. Fire blight is usually observed on plants in the Rosaceae family. Early symptoms may be blossom blight during bloom, followed by the shoot blight phase and signature symptoms such as shoots being killed rapidly in the shape of a “shepherd’s crook.” Water-soaked cankers may be found on the trunk and branches of infected plants.

Hosts

Botryosphaeria canker: Ash, crabapple, dogwood, juniper, laurel, Leyland cypress, redbud, maple, and rhododendron
Endothia canker: Pin oak
Fire blight: Apple, cotoneaster, crabapple, hawthorn, pear, pyracantha, and serviceberry
Nectria canker: Dogwood, pear, and contorted filbert
Phomopsis canker: Azalea, ash
Seiridium canker: Leyland cypress, Arizona cypress
Thyronectria canker: Honey locust
Eastern filbert blight: Contorted filbert

Sanitation and Cultural Control

For fungal canker diseases, keep plants irrigated during dry periods, remove diseased branches, and remove heavily cankered plants.
Fire blight: Streptomycin may be used during bloom and copper sprays afterward. Prune during dormancy to remove cankered branches. Apple, crabapple, and pear cultivars resistant to fire blight are available.

Leaf Galls

Leaf galls produce conspicuous white, yellow, red, or gray blisters or galls on leaves. Leaves may become puffy, puckered, thickened, or curled. Infected leaves may drop early. The most common leaf gall diseases are azalea leaf gall, peach leaf curl, and oak leaf blister. Symptoms may be confused with insect- or mite-induced galls, but the latter are more numerous.

Hosts

Leaf gall: Azalea, blueberry, camellia, and rhododendron
Leaf blister: Red oak, water oak, and willow oak
Leaf curl: Peach and plum

Sanitation and Cultural Control

Azalea leaf gall: Remove and destroy the galls.
Peach leaf curl: Apply two to three fungicide sprays, one at 50 percent leaf fall in late October and one to two dormant sprays in late winter before flower buds open.

Crown Gall

Crown gall produces rough-surfaced, hard or soft, spongy, swollen tumors or galls up to several inches in diameter on stems or roots. Galls may be flesh-colored, greenish, or dark. Galls are usually found near or below the soil line. Galls may form at wounds made during propagation. As galls continue to develop and enlarge, surface layers may become brown, woody, and roughened. Plants with crown gall usually become unthrifty and possibly stunted. Plant death may eventually occur.

Hosts

Apple, crabapple, euonymus, holly, maple, peach, plum, rhododendron, rose, willow, and wisteria

Sanitation and Cultural Control

Crown gall may be more severe in heavy soils where water stands. Avoid planting in locations with a history of crown gall. Destroy infected ornamental plants with crown gall.

Stem Rots: Southern Blight

Southern blight usually occurs in gardens, perennial borders, and nurseries during hot weather near midsummer. Symptoms include wilting and leaf scorch followed by plant death. Signs of disease include white mycelium on the stem of infected plants and tan to reddish-brown round, spherical resting structures of the fungus (sclerotia) on the stem and soil surface.

Hosts

Ajuga, apple, clematis, crabapple, forsythia, hosta, many annual and perennial flowers, vegetables, and rarely some turf species

Sanitation and Cultural Control

Remove infested plants plus soil near the stem so as to remove the sclerotia.

Stem Rots: Sclerotinia and Rhizoctinia

Sclerotinia stem rot, unlike southern blight, usually appears during mid-spring to early summer when conditions are cool and moist. Affected plants usually wilt and die. White mycelium may be visible on stems near the soil surface. Black, oblong sclerotia may be present on the outer surface of woody plants or in the stem pith of herbaceous plants. Diseased stems should be split lengthwise and examined for signs of sclerotia.
Rhizoctonia stem rot, or damping off, is often the cause of wilting and death of seedling plants. Seedling annual or perennial flowers or woody ornamentals may be killed by this fungus after it attacks the stem near the soil surface. Diseased seedlings often fall over and die. In the field, the fungus may move short distances down the row, killing several adjacent plants. In propagation beds or flats, diseased plants may be killed in circular areas as the fungus moves outward.

Hosts

Sclerotinia stem rot: Campanula, euonymus, several herbaceous flowers, and vegetables

Rhizoctonia stem rot: Many herbaceous plants and seedlings of woody plants, conifers, and vegetables

Sanitation and Cultural Control

For Sclerotinia, remove infested plants plus soil near the stem so as to remove the sclerotia.

Nematode Diseases

Nematode diseases are caused by microscopic roundworms that live in the soil. Millions of nematodes may live in a square meter of soil, but only a few are parasites of plants. Most plant parasitic nematodes attack plant roots; some attack foliage. Nematode damage can be difficult to diagnose as most of the damage occurs below ground. Plants damaged by nematodes may appear stunted, unthrifty, and discolored, and they may have discolored roots with lesions or galls. One sure way to identify nematode problems is to submit a soil or root sample for analysis at a plant diagnostic laboratory; submit symptomatic foliage where foliar nematodes are suspected.

Hosts

Root knot nematodes: Abelia, aucuba, begonia, boxwood, dogwood, gardenia, holly, hydrangea, impatiens, ligustrum, nandina, photinia, rose, and vegetables
Foliar nematodes: African violet, anemone, begonia, brunnera, hosta, and many shade-loving perennials
Lesion nematodes: Boxwood, juniper

Sanitation and Cultural Control

Avoid planting susceptible hosts into infested soil. Destroy infested plants. Do not propagate plants infested with foliar nematodes or root knot nematodes. Rotate crops. Use soil solarization. Choose species and cultivars resistant to root knot nematodes.

Wilt Diseases

Wilt diseases are usually responsible for the slow to moderate decline of trees and some shrubs. Individual branches may discolor and die. Some wilts may affect only one side of the plant. A common symptom associated with wilt diseases is vascular discoloration (discolored sapwood). Leaf scorch and a reduction in canopy size are additional symptoms. Wilt pathogens may be spread by insects (such as Dutch elm disease by elm bark beetles and bacterial leaf scorch by leaf hoppers). Bacterial leaf scorch is very common on pin oak and other oaks in the red oak family.

Hosts

Bacterial leaf scorch: Elm, red maple, mulberry, sycamore, pin oak, and shingle oak
Dutch elm disease: Elm
Verticillium wilt: Ash, barberry, boxwood, buckeye, catalpa, daphne, elm, lilac, euonymus, smoke tree, maple, and tomato
Fusarium wilt: Mum; more common on herbaceous plants including vegetables

Sanitation and Cultural Control

Planting diverse species prevents the loss of large numbers of street trees when disease strikes any one species.
Dutch elm disease: Remove and destroy infected trees to limit the spread of elm bark beetles. Trees of high value may be injected with fungicides by arborists.
Verticillium wilt: Do not plant susceptible maple into infested locales. Replace diseased shade trees with resistant species.
Vegetables: Use disease-resistant cultivars when available.

Root Rots

Root rots cause affected plants to be stunted, wilted, and generally unthrifty in appearance (mimicking nutrient deficiency), and they eventually die. Discolored, decayed roots are sure symptoms of root rot diseases. Poor drainage, standing water, improperly constructed landscape beds, the planting of infected plants, and excessive irrigation favor phytophthora and pythium root rots.

Hosts

Black root rot: Japanese holly, blue holly, inkberry, vinca, pansy, and petunia
Phytophthora root rot: Azalea, dogwood, forsythia, fir, holly, juniper pieris, rhododendron, and yew

Sanitation and Cultural Control

Check root health, if possible, before purchasing plants in containers.
Phytophthora root rot: Avoid planting susceptible plants into heavy, poorly drained soils. Avoid soil contamination of new pots and bark media. Place container plants on gravel or ground cloth. Irrigate from water sources free of phytophthora.
Black root rot: An alkaline soil pH favors disease development. Avoid soil contamination of flats, pots, and soilless media.

For more information on plant pathology:

• Essential Plant Pathology, 2nd ed. Schumann and D’Arcy, APS Press
• Advance of the Fungi, E. C. Large, APS Press
• UT Soil, Plant and Pest Center Facebook page: facebook.com/SoilPlantPestCenter
• UT Soil Plant and Pest Center website: soillab.tennessee.edu/