James A. Chatfield, Nancy J. Taylor, Joseph F. Boggs, Jane C. Martin, Gary Y. Gao, Erik A. Draper, Pamela J. Bennett, Amy K. Stone, Randall A. Zondag and David E. Dyke
As noted in this circular, the spring and early summer of 1999 were abnormally dry throughout Ohio, with these dry conditions continuing throughout the growing season in some areas of the state. This resulted in less disease pressure than in most years for many foliar diseases. This misled many to conclude that they had finally come up with an effective fungicide spray program, when actually the environmental conditions resulted in many plants "escaping" some of these diseases in 1999.
This was a classic reminder of the three factors involved in plant disease development. This article will detail these factors, put into perspective the season of 1999, discuss plant disease-management strategies, and list some useful phytopathological resources.
The first step in disease prevention and control is to have a good understanding of infectious disease. If you have ever taken an introductory course in plant pathology (from the Greek "phyto" meaning plant and "pathos" meaning disease), the instructors surely trotted out the concept of the disease triangle when they talked about infectious plant diseases. This simple concept is extremely powerful. On one level, it is simply a way to understand that there are three components necessary for a disease to occur:
On another level, the disease triangle concept is a creative way to remind yourself of the different ways to prevent disease, by thinking of all the different points at which you can attack the disease triangle. Break the links at any point and disease will not occur. Let’s look at some examples.
Example No. 1
If the apple scab fungus is abundant in a particular site and it is a very moist spring and there is a susceptible crabapple such as Malus ‘Radiant,’ all three components of the disease triangle are present, and significant scab disease will occur. However, if the cultivar is Malus ‘Prairifire,’ with excellent genetic resistance to scab disease, scab disease will not be a problem. The susceptible host was the missing component of the triangle in that case.
Example No. 2
If American elms are highly susceptible to Dutch elm disease (which they are), and environmental conditions are conducive to disease development (which they usually are), but there is no Ceratocystis ulmi fungus present in the United States (which there was not until the late 1920s), then there would be no Dutch elm disease in the United States. However, in the late 1920s, elm logs infested with this fungus were brought into this country. The disease triangle was completed with the introduction of the fungal pathogen (with the help of elm bark beetles which vectored the fungus from logs to trees), and Dutch elm disease has devastated American elms ever since.
Example No. 3
Like many fungal diseases, the number of hours of leaf wetness is the key environmental condition needed for infection by the pathogen involved in black spot of rose (Diplocarpon rosae). Imagine a case in which the pathogen is present in old leaves from the previous season on the ground in a rose planting. Further imagine that black-spot susceptible roses are being grown in the planting. Two components of the disease triangle are present. But what if there is dry weather for two months and irrigation water is kept off the foliage and good air movement is provided in the planting by pruning and proper plant siting? Without an environment conducive to disease, black spot will not be severe, because the key environmental element of leaf wetness for infection by this fungus is absent.
This key environmental factor of leaf wetness for rose black spot disease is even better exemplified by the story of greenhouse rose production. In the 1930s, black spot was a major problem in some greenhouse rose ranges; in recent decades it is almost nonexistent. What happened? The difference was an environmental change in production practices.
In the 1930s, spider mites were a major problem on greenhouse roses, and it used to be that good miticides were not available. One way to control them was literally to wash the mites off the foliage numerous times each day with a coarse stream of water. As you might imagine, this constant leaf wetness resulted in just the right environmental conditions for black spot disease.
In modern greenhouse rose production, effective miticides are available, and drip irrigation is used for the plants. There is virtually never a drop of water on the foliage of the plants, and there is virtually never any black spot on greenhouse-grown roses.
Again, in each case, all three components of the disease triangle must be present for disease to occur. Using the disease triangle to highlight different control strategies in the previously mentioned cases would involve use of scab-resistant crabapple varieties, use of quarantines to exclude the pathogen that causes Dutch elm disease, and use of cultural practices to modify the environment conducive to infection by the rose black spot disease fungus.
Imagine that someone calls you and asks how to control rose black spot disease. Don’t jump to the squirt-gun botany solution and simply suggest a fungicide you can buy at the garden center. Instead, think of the disease triangle. Check out which rose host plants have genetic resistance to black spot disease. Recommend sanitation practices that will exclude the pathogen, such as removing black-spotted rose leaves during the growing season and in the fall. Recommend cultural practices that reduce the length of leaf wetness, such as pruning surrounding vegetation, planting in sunny sites, and keeping irrigation water off the leaves. Focus your prevention efforts by creatively using all the components of the disease triangle as you consider the options.
As noted earlier, the droughty spring and early summer throughout Ohio (which continued through the summer in central and especially southern Ohio) had a beneficial side effect in terms of lower incidence of many common foliar diseases of ornamental plants. Apple scab on crabapple, rose black spot, sycamore anthracnose, ash anthracnose, oak anthracnose, and many other anthracnoses were far less serious than in previous years. In many plantings, it was actually difficult to find any Volutella leaf blight and stem canker of pachysandra. Some people were wondering why their walnuts still had leaves in September (little anthracnose this year). Buckeye and horsechestnut trees were not blow-torched by Guignardia leaf blotch by mid-summers as they commonly are in most years.
The relative lack of Guignardia leaf blotch provides some useful insight into a question often asked in recent years, namely whether the brownish leaf scorch on buckeyes and horsechestnuts is due primarily to Guignardia leaf-blotch disease or simple physiological leaf scorch during hot, dry weather. Since buckeye and horsechestnut tress almost uniformly looked better and showed far less leaf browning in the hot, dry conditions this year, it is not unreasonable to infer that, indeed, in most years, the main problem with leaf discoloration of buckeyes and horsechestnuts is Guignardia leaf-blotch disease, which, like many foliar diseases, thrives on early season wet conditions.
It was a fine year for plant lovers as they realized how beautiful sycamores, scab-susceptible crabapples and roses can really be, absent significant foliar disease problems.
So, foliar diseases of woody plants were down this year, but what does the droughty season of 1999 mean for the year 2000 and beyond? There are no certainties, but here is a bit of educated crystal-balling.
A positive is that for some diseases, overwintering fungal inoculum will be reduced. For example, with rose black spot, there will be less diseased leaves and canes that will harbor the fungus that will infect newly emerging rose leaves next spring. There was, of course, some black spot this year, so do not forget that good sanitation is still important – clean up any black-spotted leaves and canes before next year’s new growth. However, we should get off to a better start than usual next spring, though protective fungicides should still be considered where susceptible varieties and wet spring conditions exist.
On the other hand, if you used overhead irrigation on your roses all season, then everything said earlier may already be water over the dam. Where sprinkler irrigation on rose foliage replaced regular rainfall, rose black spot was as bad as ever this year. Nevertheless, with many foliar diseases, all other factors being equal, the potential for a bad disease year next year is somewhat reduced, though a wet spring can change things in a hurry.
There is a potential for increases in certain diseases next year and in the future due to this year’s drought. This is probable, for example, with certain fungal canker diseases, such as Cytospora canker of spruce. Cytospora and a number of other canker fungi are better able to infect host plants if the plant is stressed and less able to defend itself. There is also evidence that certain vascular wilt diseases on some plants, such as Verticillium wilt of maple, are more common in years following stressful, droughty conditions. It is thought that the stressed roots are more prone to successful infections by the Verticillium fungus. Another disease for which there is some concern is dogwood anthracnose. This anthracnose disease, which is considerably more severe than many other common ornamental anthracnoses, has also been shown to be more severe on stressed plantings of flowering dogwoods.
In addition to the infectious diseases noted earlier, it is certainly to be expected that noninfectious plant problems will occur next year as a result of drought stress this year. This will pose a diagnostic challenge in many cases, since many will not want to believe that plant decline in 2000 is due to environmental stresses all the way back in 1999. This may be especially true on many evergreens, both newly planted and established trees and shrubs, which may look green through the winter, but then decline once the new season begins.
It will be tempting for many to blame it on whoever was on the landscape last, rather than on the fact that the plants were simply showing the effects of the stressful season of 1999. These aspects of what urban forester Alan Siewert terms "cumulative stress syndrome" will, of course, worsen, if it is a harsh winter and if the drought continues into 2000.
An important fact to remember is that not all diseases need prevention. One of the ways of looking at this involves the concept of "treeage."
The medical community has long grappled with the dilemma of treating a wider range of problems with limited resources. Any-one who has watched TV shows such as "MASH" and "ER," centering on emergency medicine, has witnessed the solution: It is called "triage."
The concept is used by emergency medicine practitioners to place patients into one of three treatment groups (the "tri" in triage), based on the prognoses. Group 1 includes patients who will survive without immediate treatment. Group 2 involves patients who will not survive despite all efforts, and Group 3 includes patients who can be saved by immediate treatment. Of course, the physicians center their limited resources (including time) on patients in Group 3.
Triage becomes "Treeage" when the emergency medicine concept is applied to plant pest management. As with emergency medicine, the "patient" or plant is examined and placed in one of three groups, based on the prognosis. There are two differences between the triage of emergency medicine and the concept of plant treeage. First, with plant treeage, we are not talking about emergency plant care, but rather how to develop long-term plant health-management strategies. Second, with plant treeage for ornamental plants, the overall aesthetics of the plant may play a big role in such strategies along with the actual health or survivability of the plant.
Let’s look at the three treeage groups and how they apply to certain infectious plant diseases:
Group 1: Plants will survive, thrive, and retain their aesthetic appeal without any intervention.
Maple tar spot caused by the fungus, Rhytisma acerinum, is a good example of a Group 1 disease. It is a disease of certain species of maples that results in shiny black, 1/4 to one-inch diameter spots on the upper leaf surface. In their Cornell University Press book, Diseases of Trees and Shrubs, Sinclair, Lyon, and Johnson describe tar spots as "the most showy and least damaging [of] foliar diseases." Theoretically, someone could prevent tar spot of maple by using fungicides, but why bother? The plant does well year after year despite this disease, whether or not the disease is controlled. There are many examples of such diseases, from many of the powdery mildews to oak leaf blister, from Phyllosticta leaf spot of maple to many of the anthracnose diseases of ornamental plants.
Group 2: Plants will continue to decline and die whether or not intervention is attempted.
Dutch elm disease is an example of a Group 2 disease, once significant damage is noticed on a susceptible elm. Dutch elm disease is a fungal vascular disease in which elm bark beetles vector the fungus (Ceratocystis ulmi) to upper branches of the tree where the vascular system of the elm becomes infested with the fungus in the feeding galleries where fungus-infested beetle eggs were laid. The fungus then spreads in the vascular system, resulting in "flagging" of upper branches (leaf discoloration and wilt), followed by progressive leaf yellowing, browning, shriveling, wilt, and branch dieback throughout the canopy and eventual tree death, often in the first year of infection. The fungus spreads in the vascular system, eventually robbing the tree of needed water as the fungus plugs up the xylem, causes plant cells to produce gums and distort cells that also block water movement in the plant, and further damages plant tissue by the production of toxins.
It is true that if there is early detection of flagging of upper branches (less than 5% of the canopy affected), and the elm is considered valuable enough to warrant expensive treatments, injection of systemic fungicides can be valid treatment options. It is also true that in areas where surviving elms exist, sanitation treatments to remove all diseased elm wood are important when considering the overall population of elms in the area. However, for most individual elm trees, once the disease is present, treatment options other than removal are not warranted.
Group 3: Plants will potentially be improved by some form of intervention.
Apple scab is a good example of a Group 3 disease. Leaf infections occur on both leaf surfaces, with olive green spots often darkening to grayish and brown, roughly circular, scabs. In some years, when sustained spring moisture conditions occur, "sheet scab" develops in which leaves quickly become covered with velvety fungal growth, with leaves quickly browning and dropping. Yellowing of affected leaf tissue commonly occurs several weeks after infection, and leaf drop can be severe. For crabapples that lack disease resistance, leaves can be effectively protected in most years by making two to three springtime fungicidal applications. However, why not take a long-range view of dealing with scab by planting or recommending use of scab-resistant crabapples?
Emergency-room physicians use triage to prioritize where they should apply their skills. Likewise, the treeage concept can be used to plan and prioritize plant health-management efforts. For example, treeage can help avoid the problem of pursuing unnecessary Group 1 problems as well as pursuing heroic and costly efforts to "save" a Group 2 plant when resources can be better used targeting a Group 3 problem. Treeage is a pest management decision-making concept for selecting the best course of action. The "action" may be "no action," if nothing can be done to save the plant or if the pest problem does not cause significant harm.
There is one final note with regard to developing your own treeage categories for which diseases you want to concentrate on preventing. It is that disease is in the eye of the beholder. Albert Einstein would approve; which diseases matter to you is a relative thing. It would be easy to designate powdery mildew of lilac as a Group 1 disease. For many gardeners it comes every year, but never seems to hurt the plant; if they wanted to, they probably could not kill their lilacs. On the other hand, if you operate a garden center, your perspective might differ radically. Customers are likely to make you feel a bit ill if your lilacs have powdery mildew in the production area. You might consider it a Group 3 disease, that you help prevent with better air movement and a more sunny exposure in the holding area or that you prevent by selling more mildew-resistant lilacs or that you prevent with well-timed fungicides.
A list of useful disease control and plant health-management publications and resources follows:
Disease Control in the Landscape.
Ohio State University 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:
Media Distribution Office
Ohio State University Extension
385 Kottman Hall
2021 Coffey Road
Columbus, Ohio 43210
614-292-1617
Fax 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://www.hcs.ohio-state.edu/hcs/webgarden/BYGL
This version is connected through the Horticulture and Crop Science 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 Jim Chatfield: 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 by 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 by 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 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
Fax 614-292-4455