James A. Chatfield
Nancy J. Taylor
Joseph F. Boggs
Erik A. Draper
Jane C. Martin
Gary Y. Gao
Pamela J. Bennett
Amy K. Stone
Randall A. Zondag
Curtis E. Young
Barb Bloetscher
The 2000 growing season was quite wet for much of Ohio, with an extreme of more than 10 inches above normal rainfall for some areas of northeastern Ohio. This was in great contrast to 1999, in which much of Ohio had considerable drought problems. With wet and cloudy weather, especially in the springtime and early summer, infectious diseases of ornamental plants were quite prevalent this year, from apple scab on crabapple to many different anthracnose diseases, to rose black spot and white mold on Rudbeckia.
As the season progressed there was also a high incidence and severity of a number of different powdery mildew diseases, with dogwood powdery mildew seemingly worse than ever before. In addition, as a possible legacy from 1999, in which many plant root systems were drought-stressed, Verticillium wilt disease was common on many susceptible ornamentals, such as Japanese maple. The Verticillium fungus, which enters plants through root systems then spreads through the plant's vascular system, is thought to cause greater disease in years following such root-stressing droughts.
The 2000 season was a grim reminder of the effects of weather on incidence of apple scab (Venturia inaequalis). In 1999, we had a dry spring and early summer, and past seasons of apple scab on crabapples seemed but a faint memory. The 2000 season was different - wet weather during leaf emergence and expansion resulted in considerable scab. That included severe "sheet scab," resulting in coverage of crabapple leaves with the scab fungus, rapid leaf discoloration, wilt, and leaf drop. In such massive infestations, even leaf petioles were affected, resulting in leaf twisting. The overall rapid development of leaf wilting and defoliation was often misdiagnosed as other maladies, including bacterial fireblight, root decline, and winter injury.
In the National Crabapple Evaluation Plot at Ohio State University's Secrest Arboretum in Wooster, Ohio, 17 crabapple taxa had substantial defoliation and negative aesthetic impact from scab in 2000. 'Prairifire' had never before exhibited scab in our trials at Secrest but did have some lesions in 2000. 'Red Jewel' and 'Candymint,' two cultivars that had also shown very little scab in past years, had a trace of scab in 2000. Nevertheless, even with this heavy scab pressure, 25 of the 63 crabapple types in the crabapple evaluation plot showed no evidence of apple scab in 2000 and a total of 33 never received a rating that exceeded 1 (no aesthetic impact) on any evaluation date. Check out the crabapples with excellent scab resistance in the accompanying article on apple scab in this circular.
Plants commonly affected with anthracnose diseases in 2000 included sycamore, maple, ash, and yellowwood (Cladrastis kentukea). Sycamore anthracnose and ash anthracnose are common in most years, though were far worse in a wet year like 2000. Maple anthracnose is less common than ash and sycamore anthracnose typically, but was severe early this year. Yellowwood anthracnose is something we are not accustomed to seeing in Ohio - except for this year - causing considerable discoloration along the veins of leaves throughout the yellowwood trees.
Another anthracnose disease, anthracnose of flowering dogwood, has caused considerable concern in the past decade. In 2000, Margery Daughtery of Cornell University gave a fine seminar on dogwood anthracnose at an Ohio State University Department of Plant Pathology seminar. Here are some key points she made relative to this significant disease of dogwoods:
One fascinating research study looked at photosynthetically active radiation (PAR) values for dogwoods. A value of 33% was considered optimal for dogwood growth. In understories, the level was at 1%. In open sites, the value was 42%, but even in open sites, the value was 2% in the internal canopy of the tree. These values correspond to just the situations in which dogwood anthracnose is greatest - in densely shaded sites and in the interior portions of the tree.
Thus, controls of dogwood anthracnose should focus on proper siting (partial shade), avoidance of drought stress, and the use of protective fungicides, e.g., Banner (propiconazole), where warranted to protect valuable trees.
Powdery mildew diseases occurred on a wide range of plants in 2000. Diagnosis was sometimes a challenge. Most are familiar with the many powdery mildew diseases in the landscape, and the familiar sight of the "signs" of the powdery white to grayish growth of the mildew fungal pathogens on such hosts as lilac, monarda, rose, and turfgrasses.
Somewhat less commonly recognized, however, are some of the additional "symptoms" that infections by powdery mildew fungi can cause on the leaves of some plants. For example, the tendency for leaf reddening/purpling that occurs on plants such as flowering dogwood, deciduous azaleas, and magnolia, and the leaf distortion that is common on flowering dogwood, sycamores, and London planetree.
If you think powdery mildew disease is just about the white powdery growth, look closer as the disease develops and the foliage responds to the infection. Learning this pattern is part of the all-important "disease syndrome" which helps greatly in proper disease diagnosis. Failure to recognize the leaf purpling and distortion as due to powdery mildew on flowering dogwood fooled many of us a few years ago when dogwood powdery mildew became more evident and severe in Ohio and other Eastern, Southern, and Midwestern states.
Verticillium wilt is a common vascular wilt of many landscape trees, shrubs, and herbaceous crops, ranging from ornamentals to fruits, vegetables, and agronomic crops. The Verticillium fungus infects plant roots, then spreads upward and outward through the vascular system. Damage to the plant is then due to reduced availability of water to the stems and leaves and associated toxic effects of the fungus on plant tissue. As with many diseases, the disease process is a complicated one, making some of our common generalizations about Verticillium wilt a little oversimplified. Two key aspects of Verticillium wilt - vascular discoloration and disease incidence and severity - are discussed here.
Vascular Discoloration
We often say that Verticillium wilt causes vascular streaking in the conducting system of the plant's stem. True enough in general, but it is not always the case. For example, Japanese maples often do not show streaking in tissue in early stages of infection. Extension agent Jane Martin ran into this anomaly in 2000 when symptoms of leaf wilt and browning and branch dieback otherwise looked typical of Verticillium wilt, but there was no streaking in the stem. Laboratory isolations of the fungus, however, proved that Verticillium was nevertheless present in the non-streaked stem tissue. Just relying on vascular discoloration for Verticillium diagnosis would have produced a false negative in this case.
Conversely, sometimes isolations from areas of vascular streaking due to a Verticillium infection may not be positive for the presence of the fungus, especially if the isolations were attempted from the apex of the streaked area. As it turns out, exuded toxins produced by the Verticillium fungus can cause damage to the plant tissue and subsequent tissue discoloration in advance of the actual growth of the fungus in the vascular system. In this case, assuming that a failure to isolate the fungus means that the disease is not present would be another type of false negative.
The most common type of false negative, of course, is simply to check for streaking in only one or two spots, not find it, and then assume that Verticillium wilt is not the cause of the problem. It is important to be diligent in sampling, since infection by the fungus is not detectable or present on vascular tissue throughout the plant.
It is also important to realize that there are ample opportunities to make false positive diagnostic errors with Verticillium wilt when relying only on external plant symptoms or on any type of vascular discoloration. Verticillium wilt causes many acute and chronic symptoms, ranging from leaf wilting, leaf discoloration, leaf browning and death, and branch dieback to overall foliar sparseness, small leaf size, and heavy seed crops. But it is important to realize that all of these symptoms can also be caused by other factors as well. So external plant symptoms must be complemented by also checking for vascular discoloration.
The perils relative to false negatives were discussed earlier, but there are corresponding risks of false positives associated with vascular discoloration as well. This is because there are many things that can cause vascular discoloration that are not due to Verticillium infection. These include other vascular pathogens, many wood-staining fungi not involved at all in vascular disease, simple death and discoloration of vascular tissue from other causes, and even naturally healthy coloration of stem tissue on many plants, such as cherry and smoketree.
Furthermore, Verticillium infections cause different discolorations on different host plants. For example, Sinclair, Lyon, and Johnson's Diseases of Trees and Shrubs notes of Verticillium-induced discoloration: "Light tan in ash trees, dark reddish brown in black locust. Yellowish brown in cherry and smoke tree (distinct when you get the hang of it from the natural reddening of the stem tissue), and greenish to nearly black in maples."
So, like everything else in horticulture, it takes experience, a discerning eye, and willingness to keep learning to become good at diagnosing what the discoloration means - and a willingness to link your observations to laboratory confirmations of your diagnoses.
Disease Incidence and Severity
Factors affecting Verticillium wilt incidence and severity include all three angles of the disease triangle:
The Verticillium fungus infects a broad host range of plants, including many common woody ornamentals. A partial list of common susceptible hosts includes ash, azalea, barberry, catalpa, cherry and other stone fruits, daphne, elm, honeysuckle, lilac, magnolia, maple, Russian olive, peony, privet, redbud, rose, serviceberry, smoketree, spiraea, tree-of-heaven, tuliptree, viburnum, and yellowwood.
Plants with good resistance include all gymnosperms (pine, spruce, fir, hemlock, yew, ginkgo, etc.) and all monocots (grasses, sedges, lilies, palms, etc.). A partial list of other resistant plants includes mountainash, beech, birch, crabapple, dogwood, sweetgum, hackberry, hawthorn, hickory, holly, linden, honeylocust, oak, pawpaw, pear, planetrees, rhododendron, walnut, and willow.
The Verticillium fungus infects plants through the roots, moving upward by means of the vascular system. Environmental factors that affect Verticillium infection and subsequent disease development include moisture stress, root injury, damage from salts, transplant injury, and other factors that affect root health.
Effects of infection are not always the same. Plants may exhibit acute or chronic symptoms. As catalogued in Diseases of Trees and Shrubs by Sinclair, Lyon, and Johnson (Cornell University Press), acute symptoms include "curling, drying or abnormal red or yellow color of leaves or areas between leaf veins; defoliation; wilting; dieback; and death," due to infection of the current season's sapwood of a branch or the entire plant. Chronic symptoms may also develop due to these infections. These symptoms include "slow growth, sparse foliage, stunted leaves and twigs, leaf scorch, abnormally heavy seed crops, and dieback." These chronic symptoms develop from previous sapwood infections.
There are a wide range of scenarios relative to the progression of Verticillium wilt within a plant. Sometimes plants wall off infections, compartmentalizing the affected plant tissue; acute and chronic symptoms develop but may not recur unless new root infections occur in subsequent years. Sometimes a plant dies in a year, sometimes after many years of infections and their effects. Sometimes both acute and chronic symptoms are observed on the same plant due to several years of infections. Furthermore, there may be differential virulence of Verticillium strains that affect how severe the disease is in a particular case. Why does all this matter?
It matters because it means that recommendations relative to control are not simple. If a plant is diagnosed with Verticillium wilt, should it be removed? Not necessarily. Verticillium wilt is not necessarily a death sentence, at least for woody plants. If the strain is not too virulent, if environmental conditions are generally good, if the plant was vigorous enough to wall off an infection and subsequent infections in future years do not occur, perhaps the tree will recover. This scenario might occur, for example, if drought stress was not repeated, and all other factors were positive for the tree.
Another common question relates to whether or not a Verticillium-susceptible plant species should be planted near an area where a plant was previously diagnosed with Verticillium wilt. The usual answer to this is "better to be safe than sorry," but this is far from an absolute. We know that the Verticillium fungus is widespread in soils, but whether or not a plant dies of Verticillium wilt depends on many factors.
As noted previously, an occurrence of drought stress and the extent to which that might be modified by horticultural practices, the level of susceptibility of the host plant, and whether or not the first plant was infected before transplanting or at transplanting because of excessive transplant shock, which is again perhaps modifiable by horticultural practices. Planting impatiens back into a site where last year's impatiens crop also had Verticillium wilt, planting Japanese maples back into a site where the last two died from Verticillium wilt - do not do it. But having said that, there are many other situations where it is not so simple. Such is nature.
Sphaeropsis blight of pine is a common problem on Austrian, Scotch, red, and mugo pines in Ohio - and the 2000 season was no exception. Here are two useful plant pathologist perspectives for consideration relative to this fungal disease.
Symptoms and Plant Age
John Hartman of the University of Kentucky presented information from their work on Austrian pine at an ornamental workshop in Crossnore, North Carolina, this fall. He talked about the symptoms of the disease, typically peaking at plant age of 15-25 years - tip blight, stem cankers, cone infections, resin production on twigs and branches.
He also highlighted the fact that they readily find Sphaeropsis sapinea in asymptomatic pine tissue (needles, cones, bark, the phloem of stems). This of course raises many questions about these "latent infections" and how long they are present before various stresses trigger obvious symptom-producing disease development.
Hartman speculated on why the disease typically develops on 15-years or older trees - is it due to allocation of resources to cone production with subsequent lessened resources for preventing disease development? These kinds of questions perhaps relate to the difficulties of successfully using fungicides for pine tip blight disease.
Microscopes and Spores
Many of us are accustomed to talking about Diplodia tip blight of pine, though for many years plant pathologists have argued that it should more appropriately be called Sphaeropsis blight. How come?
Jim Blodgett, a new post-doctoral research scientist working at Ohio State University with OSU-Plant Pathology's Enrico Bonello, recently provided the explanation.
First of all, the name change relates to differences in "conidial ontology," something you no doubt would rather not be bothered with. Second, the spores of the fungus that cause this disease are mostly single-celled, not two-celled. This is something you might have noticed if you had been at any of the OSU diagnostic workshops when we used microscopes.
Some of the dark, cylindrical spores dug out of the black pycnidal pimples on pine needles were pinched into two cells (presumably the "Di" in Diplodia) but most were not. Ergo, "di"-spores are the exception rather than the rule, and the mycological meritocracy has decreed: It is Sphaeropsis sapinea - not Diplodia pinea. They have spoken - will we listen?
It was a good year, so to speak, for fireblight (Erwinia amylovora), with warm weather during bloom of many susceptible plants contributing to blossom infections. Fireblight causes foliar browning and shoot dieback, which on some cultivars spreads well back into the wood and can even kill young trees. There are no practical sprays for controlling fireblight in the landscape, and the best recommended control is pruning out affected shoots a foot or more back behind the obviously blighted shoot tissue.
At the 32nd Ohio-Kentucky Plant Diagnostic Workshop, there were samples of fireblight on crabapple and on chokeberry (Aronia spp.). Both of these plants are in the rose family (Rosaceae), and fireblight is a disease that occurs only on plants in this family.
On crabapples, there are only a few cultivars susceptible enough to develop serious problems in the landscape, but we are certainly seeing more than we had hoped for on an otherwise outstanding crabapple, 'Golden Raindrops.' This crabapple, also named 'Schmidt's Cutleaf,' has great resistance to apple scab, has snowy white blooms, tiny yellow fall fruits, a pleasing spreading upright growth habit, and an unusual trifoliate foliage with great autumnal effect. But apparently, in certain years - as seen in the Secrest Arboreum crabapple evaluation plots, a few garden centers this spring, and in some nurseries - it has had a problem with fireblight.
Samples of oak wilt (Ceratocystis fagacearum) were confirmed again in some northern Ohio areas in 2000. Oak wilt is a systemic fungal disease that infects the vascular tissue of oak trees. The fungus, Ceratocystis fagacearum, attacks most oaks, but especially those in the red oak group. Susceptible trees die within a few weeks while those with some resistance may decline slowly for two to three years or may recover.
With a name like oak wilt, it is safe to assume that there will be some wilting or drought-like symptoms occurring. In fact, at the top of the tree, leaves turn brown along tips and margins, wilt or flag, and begin to fall while there is still some green color left in them. This wilt then progresses down the tree. Twigs and branches die and often have brown streaks in the outer sapwood. When the ends of twigs are cut, the outermost annual ring may be completely brown. A fungal mat develops under the bark and erupts through the bark in the spring.
What can be done about this fungus? First, obtain a positive diagnosis that oak wilt is the cause of the wilting and defoliation. Next, to slow the spread to nearby oaks, root grafts to surrounding oaks should be broken before removing an infected tree. This can be done by trenching midway between oaks that are within 50 feet of the infected oak, to a depth of three feet.
After root grafts are disrupted, remove infected trees. Bury, burn, or debark the logs and stump. Do not stack or transport any wood from the tree if it has bark firmly attached because oak bark beetles and/or sap beetles may carry the fungus to other oaks. Do not prune oaks in the late spring or summer because the fresh wounds are attractive to insects that may be carrying fungal spores. Prune only during November through mid-April.
The disease in trees with less than 30% of the crown affected may be put into temporary remission by injecting the fungicide, Alamo. This fungicide will not kill the fungus that is already in the tree's roots or reverse the damage already done. Therefore, root grafts between this tree and neighboring oaks must be disrupted even if the tree is injected with fungicide. It is reported that oaks in high-risk areas, but that are not yet infected, can be protected by using a fungicide once every two years. Unfortunately, even doing all of this will not ensure that trees will not be infected.
One of the difficult adjustments for those who start to grow, sell, or maintain perennials is learning all the different infectious diseases of this wide range of plants. There are several books that are quite helpful. Two of these are published by Ball Publishing.
Phytophthora Diseases
In Ohio State's Plant and Pest Diagnostic Clinic (PPDC), these were confirmed on a wide range of plants, including English ivy cuttings, lavender cotton rooted cuttings, 'Miss Kim' lilac, holly and rhododendrons from landscapes, boxwood rooted cuttings, caryopteris, and petunia.
Phytophthora diseases were common in landscapes in 2000 as might be expected with the wet conditions, but they are also always a challenge in propagation systems, especially when sanitation is not ideal, since conditions are typically deliberately moist.
White Mold on Rudbeckia
Nancy Taylor in the PPDC reported a sample of Sclerotinia white mold disease on black-eyed Susan (Rudbeckia). This is a common problem on many herbaceous perennials, especially on plants in the Asteraceae (composite) family. The Sclerotinia fungus causes cankering, wilting, and collapse of affected stems. Accompanying white moldy fungal growth can be seen on and inside of the stems.
Eventually, there will be the development of blackish mouse poop-like hardened fungal sclerotia in the pith areas of the stem. Roguing out affected plants and providing adequate plant spacing to reduce high humidity and moisture levels on the stems are recommended.
Black Root Rot on Katsuratree
Nancy Taylor notes that this black root rot (Thielaviopsis basicola) diagnosis was possible only because of the inclusion of feeder roots in the sample that the PPDC received.
Phomopsis Tip Dieback of Spruce
A recommended control for this shoot dieback is to prune out the cankered tips that have died back; prune during dry conditions.
Septoria Leaf Spot of Black-Eyed Susan
Nancy Taylor reported a new disease for the PPDC - Septoria leaf spot on black-eyed Susan (Rudbeckia). Symptoms included angular purple leaf lesions delimited by the major leaf veins.
Alternaria Leaf Spot of Marigold
This was found to have caused a multitude of circular purplish spots. This disease is typically more of a problem in plantings with overhead irrigation.
A common diagnostic misconception is that galls on plants are due only to insects, or that all galls are bacterial crown gall disease. The fact is that galls may be caused by many different factors. Webster's defines galls as: "A tumor on plant tissue caused by irritation due to fungi, insects, or bacteria." That is a good start, though not complete.
Insects, indeed, do cause many galls, for example, reportedly causing more than 800 characteristic galls by different insects (mostly species of wasps and flies) on oak alone. Most of these oak galls are of little concern to the health of these affected oaks, but some, such as the horned oak gall and gouty oak gall, are more serious because they occur on stems rather than leaves. Insect galls commonly occur as a plant response to egg-laying by the insects. Obligingly, the plant produces the gall that then becomes the home for the developing insect egg and larvae.
The best-known plant gall is bacterial crown gall, caused by the bacterium Agrobacterium tumefaciens, which reportedly occurs on more than 600 species in over 90 plant families (most prominently for horticulturists on rose, euonymus, and the stone fruits). This bacterium typically enters plant tissue through root wounds, then transmits a little chunk of bacterial DNA to the host plant root cell. This bacterial DNA then causes the plant to produce too many cells and cells that are too large, and voila - the tumor develops, potentially disrupting the orderly vascular flow in the area where the gall develops.
But these well-known galls are only the beginning. Many fungi also produce galled tissue, such as the fairly unusual roundish, half-inch diameter, wart-like growths of Phomopsis fungal gall on forsythia, and the very common blackish galled areas on the stems of plums and cherries caused by the black knot fungus (Dibotryon morbosa).
And common galls that even Webster ignored are those caused by eriophyid mites, such as the maple bladder gall mite, the maple spindle gall mite, and the mite that causes ash flower galls. There are even "galls" that appear to have no relationship to infectious disease organisms or insect or mite incitement. Hemispheric burls in wood and other stem-swellings may be due, in the words of Sinclair, Lyon, and Johnson's Diseases of Trees and Shrubs, to simply the plant response to "environmental insults" such as freeze injury. So, the next 20 times you see plant galls, try to learn from peers, books, samples to the PPDC, and other sources, which gall it might be.
During each week of the Buckeye Yard and Garden Line (BYGL) electronic newsletter, the lead BYGL writer selects a horticultural quote to his or her taste. Here are some of the best from 2000 to add to your list of phytophilosophies:
Nothing is rich but the inexhaustible wealth of nature. She shows us only
surfaces, but she is a million fathoms deep.
- Ralph Waldo Emerson
The dandelion's pallid tube astonishes the grass, And winter instantly
becomes an infinite alas.
- Emily Dickinson
A weed is a plant that is not only in the wrong place, but intends to
stay.
- Sara Stein
The great wonder, in gardening, is that so many plants live.
- Christopher Lloyd
The planting and care of flowers and trees, an art and science upon our
knees, is both to man and womanese, as bracing and graceful as nature's breeze.
- Pellinor of Buckland
We must never get too busy to take time to sharpen the saw, never too busy
driving to take time to get gas.
- Stephen R. Covey
One must first seek to love plants and nature, and then to cultivate that
happy peace of mind which is satisfied with little. He will be happier if he has
no rigid and arbitrary ideals, for gardens are coquettish, particularly with the
novice.
- Liberty Hyde Bailey
And so the seasons went rolling on into summer, as one rambles into higher
and higher grass.
- Henry David Thoreau
How sweet I roamed from field to field, And tasted all the summer's pride.
- William Blake
It is better to know some of the questions than all of the answers.
- James Thurber
I have a rock garden. Last week three of them died.
- Richard
Diran
What gets us into trouble is not what we don't know. It's what we know for
sure that just ain't so.
- Yogi Berra
Just living is not enough. ... One must have sunshine, freedom, and a
little flower.
- Hans Christian Anderson
Listen! the wind is rising, and the air is wild with leaves, We have had
our summer evenings, now for October eves!
- Humbert Wolfe
Autumn is a second spring, when every leaf is a flower.
-
Albert Camus
What do you get if you divide the circumference of a pumpkin by its
diameter? Pumpkin pi.
- Anonymous
A list of useful disease-control and plant-health-management publications and resources follows:
Disease Control in the Landscape OSU Extension Bulletin 614
This publication contains much useful information including discussions of
plant-health management, disease diagnosis, lists of disease-resistant plants,
fungicides for ornamental plants, disease descriptions, and
plant-health-management options for common diseases. It contains more than 100
color pictures. It can be ordered through any OSU Extension county office and
also is available through:
Extension Media Distribution Office
385 Kottman Hall
2021 Coffey Road
Columbus, Ohio 43210
614-292-1617
614-292-2270
Buckeye Yard and Garden Line
This weekly (April-October) update from OSU Extension provides timely disease, pest, and overall plant-health management information throughout the growing season. It is available in a Web version, by e-mail, and by a fax subscription.
Web site: http://bygl.osu.edu/
This version is connected through the Horticulture in Virtual Perspective website to more than 23,000 fact sheets and 7,000 plant and pest images.
E-mail version: Simply request addition to the BYGL e-mail list by e-mailing chatfield.1@osu.edu
Fax subscriptions: Get a list of OSU Extension BYGL fax centers by contacting Jim Chatfield at the e-mail address given earlier or by calling 330-263-3831.
Diseases of Trees and Shrubs
Wayne A. Sinclair, Howard H. Lyon, and Warren T. Johnson
This is a reference photo compendium accompanied by extensive information about each disease.
Cornell University Press
124 Roberts Place
Ithaca, NY 14850
Common Insect Pests and Diseases of Ornamental Trees and Shrubs
This resource includes a 160-slide set of woody ornamental diseases and accompanying fact sheets and slide scripts.
Ohio Nursery and Landscape Association
800-825-5062
Pest Resistant Ornamental Plants
Deborah C. Smith-Fiola
Lists of ornamentals with resistance to specific diseases and insects.
Rutgers Cooperative Extension
1623 Whitesville Road
Toms River, NJ 08755-9720
908-349-1246
Ohio State University's C. Wayne Ellett Plant and Pest Diagnostic Clinic
The place to send your samples for diagnostic verification. Sample forms are available at all OSU Extension county offices.
C. Wayne Ellett Plant and Pest Diagnostic Clinic
The Ohio State University
110 Kottman Hall
2021 Coffey Road
Columbus, Ohio 43210
614-292-5006
614-292-4455 Fax