Managing foliar fungal diseases: Timing is everything
While robins may be a herald of spring, a sure sign that summer has arrived is the barrage of visits to my office from alarmed tree owners. They arrive with plastic bags of crumpled leaves and concerned looks on their faces. Almost to a person, they have two questions on their mind: "What is killing my tree?" and "What do I spray?" These calls continue through the summer and into fall only to vanish, along with the robins, as winter approaches. Generally, the culprit is one of several fungal leaf diseases that begin to express symptoms as the growing season progresses Fungal leaf diseases of ornamental trees often are frustrating problems. This may seem surprising because foliar diseases are not usually life-threatening. Most otherwise-healthy trees can withstand several seasons of partial defoliation without suffering significant injury. Trees produce an abundance of leaves, so some loss of foliage, in itself, is not a major cause for concern. The loss of many leaves early in the growing season, however, may result in depletion of food reserves as the tree attempts to refoliate. This depletion, particularly if repeated for several years, may leave the tree vulnerable, lacking the energy to defend itself and the reserves to survive additional stresses. Fortunately, heavy and repeated early season defoliation is more the exception than the rule for fungal foliage diseases.
While defoliation may not be a threat to the tree's life, it can be a threat to your job. Most ornamental trees can survive 20 to 40 percent defoliation with little or no significant injury, but the tree's owner may notice as little as 5 percent defoliation. Leaves are one of the most visible features of a tree, and people quickly notice any change in appearance. Ten-percent defoliation or discoloration may be cause for alarm for the owner and a call to action. Thus, you are managing foliar diseases primarily as aesthetic problems rather than life-threatening ones.
Another difficulty with managing foliar disease is timing. Once the symptoms are visible, it is usually too late for effective treatment in the current season. This is a never-ending source of frustration to tree owners. I have dealt with many unhappy homeowners who were upset that, while I could identify the disease as a rust or anthracnose, I could not do anything about it now. Managing the problem next year was not soon enough.
One additional source of difficulty with managing foliar diseases is that they typically present you with symptoms rather than signs. Symptoms are changes in the normal appearance of the plant (abnormally small or yellowing leaves, for example) while a sign is physical evidence of the disease agent itself (in the case of fungi, mushrooms, conks or some vegetative portion-hyphae-of the fungus). Signs are the "smoking gun" for disease identification. Once you have identified the sign, you know the disease. Unfortunately, signs are not always visible or available. Bacteria, viruses and other organisms are too small to see even with the aid of a hand lens. Fungi, while larger, still produce few visible signs other than fruiting structures, and these may not be present at the time of identification.
Fungi are among the most common foliar-disease agents, but they are not the only disease organism or disorder to cause leaf symptoms. So how do you determine that the symptoms are due to a fungus? Again, this is not always an easy task, but don't make the mistake of "picture-booking" your way through the diagnostic process and then try to spray your way out of the problem. There is a saying in the medical profession that applies to the tree-care profession as well: "Prognosis without diagnosis is malpractice." Always make sure you have identified the problem before you have identified the treatment!
How do you decide if the symptoms are due to a fungus? You have several means of separating symptoms of fungal leaf diseases from those of bacteria. Fungal leaf diseases tend to develop circular or halo patterns on the leaves with the dead interior area a tan color and the surrounding dying tissue a yellow or red color. The affected tissue often has a dry, papery feel. Bacterial leaf diseases often have a more slimy texture, and the disease appears as small, angular spots. Viruses, another possible disease agent, often show up as vein clearing with surrounding tissue remaining green or developing a mosaic-a patchwork of green and yellow. They may also appear as ring spots or flecks. These are only the general patterns of symptoms for each of the major disease organisms and numerous exceptions occur.
Foliar symptoms may also originate from disorders, which are non-living or abiotic factors. Many disorders produce foliar symptoms that mimic those associated with several fungal diseases. Thus, people often blame leaf fungal diseases for problems that have their origin elsewhere. Just because the symptoms appear on the leaves does not mean that this is where the basic problem resides. Browning leaves may be due to a leaf fungus or perhaps a fungus in another part of the tree. The symptoms might even be due to drought, construction within the rooting area, de-icing salts or herbicides, among many other possible agents. With disorders, you'll never see any signs because there is no pathogen. You must examine the pattern of the symptoms. Uniform injury, either on the leaf or within the canopy, is usually an indication of a disorder. Air pollution and drought injury on conifers, for example, will often show symptoms of needle tip burn with a distinctive boundary between living and dead tissue. Fungi and other living organisms produce a more random pattern of symptoms. Only scattered needles will be affected and to various lengths. The boundary between living and dead tissue will also be less distinct.
Sometimes, leaf curl or discoloration is not due to either a disease or disorder. These are called false symptoms-particular species or cultivar traits that may appear unusual. For example, it's easy to confuse the deep-yellow variegation of a 'Rainbow' dogwood (Cornus florida 'Rainbow') leaf with a potential problem if you are not familiar with the cultivar's ornamental characteristics. Finally, always remember that problems are usually multidimensional rather than caused by a single factor. A tree may have discolored leaves due to a scab and air pollution (and you only can manage one of these).
Know the life cycles Managing fungal leaf diseases not only requires correct identification but also some knowledge about the life cycle. Too often, applicators correctly identify the disease but apply the fungicide treatments at the wrong time. Many fungicides are protectants that provide a protective shield to prevent the fungus from penetrating the plant tissue. Protectants have little or no effect after the fungus enters the plant and becomes established. Therefore, to know when to start fungicide applications and how many to apply, you need to understand the leaf-disease cycle.
The typical life cycle begins in the spring. Spores produced from fruiting structures on dead, fallen leaves or dead twigs still in the canopy become air-borne and infect young, expanding leaves-the most susceptible foliage. The spores germinate and the fungus infects the new leaf and sometimes the young, succulent twigs. Later in the spring and early summer, fruiting structures form on the infected leaves and new infections occur, particularly if cool, moist conditions persist. In late summer or fall, the infected leaves fall and the cycle repeats the following year. If fungicide treatments begin in the summer when the symptoms appear, it's too late. In addition, if treatments are limited to only one or two sprays in the spring and cool, moist conditions persist into the summer, the disease may still develop. Treating fungus diseases requires good timing and persistence.
While fungi are responsible for many of our leaf problems, the problems are different throughout the country. However, several common types of fungal leaf diseases occur widely and understanding the symptoms and management of these types will be helpful when you are working on a specific disease.
Common fungal leaf diseases of deciduous trees * Anthracnose. Anthracnose diseases probably are the best-known foliar fungal diseases of deciduous trees. They affect many ornamental trees including major shade-tree genera such as sycamore (Platanus spp.), oak (Quercus spp.), maple (Acer spp.), elm (Ulmus spp.) and ash (Fraxinus spp.). Anthracnose actually is a general term describing symptoms such as dead irregular areas that form along and between the main vein of the leaf. The leaves may also become curled and distorted and twigs may die back. The fungus overwinters in infected twigs and the petioles of fallen leaves, and the spores disseminate in the spring by wind and splashing rain. The disease, while unsightly, rarely results in the tree's death. Sycamores and other trees often withstand many years of partial defoliation. However, one anthracnose disease is more serious.
Dogwood anthracnose (Discula destructiva) is a relatively recent landscaping problem. Pathologists first identified it in the late 1970s, and it has since become a major disease problem on flowering dogwood (C. florida) and Pacific dogwood (C. nuttallii). While Chinese dogwood (C. kousa) is supposedly resistant, some pathologists have reported infections of this species as well. Dogwood anthracnose differs from other anthracnose diseases in that it can kill trees, rather than merely disfigure them. The disease begins with the leaves developing brown spots surrounded by reddish-brown halos. The flower bracts may develop similar symptoms. The disease also spreads to the twigs with the tips of infected shoots turning gray-tan and then dying. These declining branches often support an abundance of water sprouts. Eventually the tree may die from the disease.
*Leaf blisters result in the blistering, curling and puckering of leaf tissue. Oak leaf blister (Taphrina caerulescens) is a common blister disease of oaks, particularly the red-oak subgenus, which includes red oak (Quercus rubra) and pin oak (Q. palustris) among others. The symptoms begin as a slight yellowing of the infected leaf followed by round, raised blisters. These turn brown, and the infected leaves fall prematurely. This fungus overwinters as spores on the buds.
*Leaf spot is another common fungal leaf disease. The general symptoms include dead spots with a defined boundary between living and dead tissue. The dead tissue often separates from the surrounding living tissue creating a "shot-hole" appearance on the infected leaves.
*Tar spot (Rhytisma spp.) is a leaf disease with initial symptoms similar to leaf spot. The disease is most common on red (Acer rubrum) and silver maple (A. saccharinum), but it can occur on a wide range of maple species from sugar (A. saccharum) and Norway (A. platanoides) to bigleaf maple (A. macrophyllum). The symptoms begin in the spring as small greenish-yellow spots on the upper leaf surface that, by mid-summer, progress to black tar-like spots about 0.5 inch in size. The disease is not fatal to the tree, but the appearance of the tar spots alarms some tree owners. A major outbreak in New York about 10 years ago left many maples completely defoliated by mid-August.
* Powdery mildew fungi infect most species of deciduous woody plants. The typical symptoms of this disease are small dusty-white or gray patches that develop by mid-summer. These patches continue to enlarge during the summer, and the entire leaf may eventually appear white. By late summer, tiny brown to black fruiting structures usually develop in these patches. The disease overwinters on the fallen leaves or as mycelium in infected buds. In spring, the fruiting structures release wind- and rain-dispersed spores from the leaves. This fungus grows best in warm, moist conditions. The mycelium, unlike most other foliar diseases, grows on the surface of the leaves rather than within the leaf. Infected leaves may have less photosynthetic capability, but the problem is generally aesthetic. Although this disease rarely results in sufficient injury to warrant treatments, its high visibility is a frequent cause of concern to tree owners.
*Rusts are unusual fungi in that they may require two unrelated hosts to complete their life cycle. Rust fungi produce fruiting bodies on host species "A," which releases spores that only infect host species "B." The fungi in "B" produces spores that only infect "A," and the cycle repeats. The fungi do cause some injury in both hosts, but often only one of the two species has any ornamental value. The rust disease that affects an ornamental tree may have an alternate host that is just an incidental plant in the landscape, or even a crop or a weed. Obviously, a simple solution would be the elimination of the alternate host. While some have attempted this strategy, for example the attempted eradication of Ribes spp. for control of white pine blister rust (Cronartium ribicola), it usually is an impractical undertaking.
Removal of the alternate host also is common recommendation for a rust that affects one of our most popular flowering trees, the crabapple (Malus spp.). However, here again, it generally proves to be impractical. Cedar-apple rust (Gymnosporangium juniperi-virginianae) is a problem familiar to most people that have a crabapple in their yard. Although many tree owners are aware that the disease must spend part of its life on cedar, to break the cycle you would not only need to remove the redcedars in your yard but also all those within a radius of up to a mile-obviously an impractical task.
* Sooty mold is a common concern to tree owners but it is not an infectious disease. The black soot that appears on leaves is a fungus that lives on the honeydew secreted by aphid and scale insects. The fungus does not live in the leaf, it lives on it. Any other symptoms such as yellowing or curling leaves are due to the insects that produce the honeydew, not the mold. While heavy levels of sooty mold can block enough light to reduce photosynthesis by up to 70 percent, this primarily is an aesthetic problem
Common fungal needle diseases of conifers Foliage problems on conifers are a little more serious than on deciduous trees. Conifers can not refoliate during the same season so the impact can be great. And unlike deciduous trees, the needles are important sites of food reserves for conifers. Fortunately most foliar diseases infect either the older needles or the new ones, so some foliage remains, and the trees rarely die from these diseases. However, an infected tree can look so bad that you may hope that it dies! Needle diseases are of three types: needle cast, needle blight and needle rust.
* Needle casts are a common problem with many conifers throughout the country. These diseases affect only the needles on newly formed shoots, but the symptoms are not evident until the following spring. The infected needles develop spots that turn tan to reddish-brown. The fungal fruiting structures emerge on these needles and are usually large enough to be visible to the eye. Rhizosphaera needle cast (Rhizosphaera kalkhoffii) is probably one of the most common cast diseases because it infects one of the most ubiquitous trees in the landscape, Colorado blue spruce (Picea pungens). A season after infection, the needles turn reddish brown to purple, with the fruiting structures appearing as rows of small dots running lengthwise along the needles. These infected needles are cast (dropped) in the fall. Trees infected with this disease for many years may only have the current year's needles remaining rather than the 5- to 8-year complement of needles a healthy spruce maintains.
* Sphaeropsis blight (Sphaeropsis sapinea), formerly called diplodia twig blight, is one of the most common needle blights, thought it is more a shoot blight. It infects the young, succulent shoots and needles on two-and three-needled pines with Austrian pine (Pinus nigra), ponderosa pine (P. ponderosa) and Scots pine (P. sylvestris) being the most susceptible. This disease typically does not show up until the trees reach maturity. Infected trees have stunted, twisted candle growth with the expanding needles becoming straw-colored and then brown. Sphaeropsis twig blight produces small black fruiting structures on the cone scales and at the base of the infected needles.
Several rust diseases affect conifers. Spruce needle rust (Chrysomixa spp.) and pine needle rust (Coleosporum asterum) to name two. Fortunately, with the exception of white pine blister rust-really a canker rust rather than a needle rust-these diseases are not a major problem in ornamental landscapes.
Cultural control measures for common fungus problems Foliage must remain wet for a period of several hours or more for spores to germinate, so persistently cool, moist weather is ideal for fungal leaf-disease development. While you can do nothing about the weather, irrigating in the early evening and maintaining dense plantings can achieve the same environmental conditions that favor disease. Therefore, always maintain good air circulation and never irrigate leaves in the evening-they may not dry completely until morning. Pruning can thin the plant to allow better air circulation. (Pruning also is important for removing dead, infected twigs that, for some diseases, are a source of spores in the spring. However, in most instances, pruning is more cosmetic than disease control.)
Raking fallen leaves is a common recommendation for removing the primary source of next year's infections. While this may be effective in certain instances, it is nearly impossible to eliminate all the infected leaves from a property, let alone the fallen leaves from adjacent properties. In addition, many fungal leaf diseases overwinter in the buds, twigs and cones remaining in the tree, so raking may have a minimal effect on the level of spore production.
Deciding if chemical treatments are needed Fungal leaf diseases generally do not require chemical treatments because they usually are not life-threatening. Here are some of the questions you should consider before deciding to apply a treatment.
*Have you correctly identified the disease agent? *Has the tree withstood several years of partial or complete defoliation due to this disease? *Has the tree recently experienced heavy stress from other pests or disorders? *Is the disease life-threatening to the tree? *Is the host tree so valuable or visible that the loss of some foliage will concern the client? * Is the client aware that several treatments may be necessary this growing season and that the disease may come back again next year? If you answered "no" to any of these questions, it may not be appropriate for you to treat-at least without additional information.
Chemical treatments for common fungus problems Treatment of many fungal leaf diseases is with protectants (see the tables on page XX for a list of treatments and fungicide sources). This requires maintaining a chemical barrier between the leaf and the fungus, which means repeated applications at fixed intervals-usually 7 to 10 days-beginning with bud break for deciduous trees or the formation of the candle for conifers. You typically need to make three applications, though more may be necessary if cool, moist conditions persist into the summer.
Several systemic fungicides also are available for treatment of fungal leaf diseases. You can trunk-inject these products into the plant, which then translocates them to the leaves and shoots. Because systemics are not subject to weathering, they can provide a longer period of treatment than other fungicides. Adjacent homeowners appreciate these products because they produce no drift. However, be sure to use the proper techniques to minimize wounding to the tree. And remember, you are wounding the tree when you inject, so the benefits should always outweigh the risks. Also, timing for systemic injections may be different than for the typical foliar application. In some instances, the most effective time to treat is fall, as with thiabendazole to manage sycamore anthracnose.
Fungal leaf diseases are a challenge to identify and manage. Avoid the temptation to quickly reach for the sprayer at the first appearance of leaf symptoms. First, identify the agent or agents responsible for the symptoms. Then, if it is a fungal leaf disease, determine the potential damage to the tree's health and appearance. And finally, remember that it's too late to do much right now, but it's just about right to begin planning for spring treatments.
Dr. John Ball is associate professor of forestry at South Dakota State University (Brookings, S.D.).
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