This article is drawn from reports in the Buckeye Yard and Garden Line throughout the year, diagnoses in the Ohio State University Plant and Pest Diagnostic Clinic (PPDC), and other observations in Ohio in 1996. A number of diseases were especially severe in springtime due to extended early wet, cool conditions.
Botrytis Diseases (Botrytis cinerea, Botrytis spp.). Botrytis grey mold disease on many hosts was a major problem this spring due to the extended wet weather. It was a problem in the greenhouse and in landscape plantings. Botrytis tulipa was found on tulips. Botrytis elliptica, an uncommon species, was found on Asiatic garden lilies in the spring of 1996.
Bacterial Blight of Lilac (Pseudomonas syringae pv. syringae). This disease was frequently reported this year, during extended wet, cool conditions in spring as new growth developed on lilacs. Infections resulted in blackened leaves, stems, and blooms. When lesions girdled succulent shoots, the branch end or flower cluster wilted. Fortunately, lesions rarely develop on woody stems, so dieback of major branches did not typcially occur, and plants generally recovered, even though the disease was more severe than usual in 1996. It is also believed that this same bacterium was involved in a bacterial blight in magnolia this spring in Ohio.
Peach Leaf Curl (Taphrina deformans). This fungal disease was common in the cool, wet spring of 1996 that followed a mild winter. Developing leaves become severely distorted (thickened and puckered) and have a reddish or purple cast. Later, as spores form on the leaf surface, the leaves become powdery gray in color. Those leaves will shortly turn yellow and drop.
If leaf curl does result in significant defoliation in the spring, the fruit on affected trees should be thinned to compensate for the loss of leaves. A single fungicide application (thoroughly covering the tree), after leaves drop in the fall or before buds swell in spring, will provide adequate control.
Phytophthora Root and Crown Rots (Phytophthora spp.). These diseases occurred in a number of sites with poor drainage conditions. Some common hosts included rhododendron, brambles, chrysanthemum, peony, snapdragon, and lilac. On lilac, Phytophthora root rot and Phytophthora stem dieback resulted in pale leaf coloration, branch dieback, and plant death. On chrysanthemum, samples exhibited a common symptom of many Phytophthora diseases - black discoloration of lower stem tissue up to the first set of leaves.
Diplodia (Sphaeropsis) Tip Blight of Pine (Sphaeopsis sapinea). Symptoms developed early on Austrian and Scotch pine samples with new growth discolored and drooping over from this spring's Diplodia infections. Tiny black, pimple-like fungal fruiting bodies breaking through the needle surface (especially noticeable if you pull back needle sheaths) are a good diagnostic clue for this disease. In some cases, there was subsequent decline of the tree from death of new growth each year. In other cases, especially in some Christmas tree plantations, infections caused canker development further back on the branch and part way down the new candle growth, resulting in the eventual collapse of new candles in the upper part of the tree.
Anthracnose Diseases (Discula spp., Gloeosporium spp.). A number of different anthracnose diseases were reported in 1996, with greatest severity noted on ash and on planetrees, especially sycamore. Other reports were on dogwood, maple, oak, hickory, beech (only the second Ohio report for beech anthracnose), and on unusual hosts such as spiderwort and hornbeam. On hornbeam, symptoms were not the classic blotchiness associated with major veins of leaves as seen with many anthracnose diseases, but rather a diffuse scorching that is very difficult to distinguish from non-infectious physiological leaf scorch.
Crabapple Diseases. Though early extended wet conditions in the spring resulted in concern that it would be a bad apple scab (Venturia inaequalis) year, scab incidence was close to the average for the past four years in one carefully monitored location - Secrest Arboretum in Wooster. Frogeye leaf spot (Botryosphaeria obtusa) was considerably more severe than in most years, with significant defoliation occurring on several cultivars. Incidence of fireblight (Erwinia amylovora) varied widely, as was also the case in many apple orchards. Side-by-side orchards often varied from heavy fireblight to light fireblight for reasons that did not seem to be due to past incidence, control strategies, susceptibility of cultivars, or environmental differences.
Black Root Rot (Thielaviopsis basicola). There were several reports of black root rot on blue hollies (Meserve hybrids). This disease causes stunting, foliar discoloration, and twig dieback from the death of fine roots. Black root rot also results in characteristic black root lesions which are visible if affected roots are washed. Over the past several years, this disease was diagnosed on blue hollies (Meserve hybrids) in both nursery and landscape situations in Ohio. The PPDC has also found black root rot on a number of other plants including petunia, snapdragon, and in past years on verbena, impatiens, fuchsia, and periwinkle. Black root rot was also identified in greenhouse pansy production in 1996.
Coccomyces Leaf Spot of Cherry (Blumeriella jaapii). This disease mainly affects the leaves, but lesions may also appear on fruit, petioles, and fruit stems. Small circular purple spots appear on the upper surface of the leaf and gradually enlarge to about 1/4" diameter and turn reddish-brown. The most conspicuous symptom, especially on sour cherries, is the golden yellowing of older leaves before they drop off. The homeowner can reduce the possibility of leaf spot by collecting and destroying the fallen cherry leaves in late autumn and by selecting a planting site that has good air circulation and drainage. Pruning to open the canopy to the sunlight can help as well.
Herpobasidium Blight of Honeysuckle (Herpobasidium deformans). This disease results in brownish, scorchy discoloration of the upper leaf surface, leaf curling, and a white fungal growth on the lower leaf surface. Looking at the leaf from above, the symptoms are similar to those seen with a severe spider mite infestation. From below, symptoms are suggestive of downy mildew disease. Infections can occur throughout the season during humid weather.
The Whole World Turned Upside Down. Reports this year included an incidence of Phomopsis disease occurring on spruce (Phomopsis sp.) and of Cytospora disease occurring on blue chip juniper Cytospora sp.). For those familiar with the much more common Phomopsis twig blight on juniper (Phomopsis juniperovora) and Cytospora canker on spruce (Leucostoma kunzei), the previous reports sound strangely reversed, but are true nevertheless. This is a reminder of the need for good laboratory confirmation of problems. For example, the Phomposis on spruce caused superficial symptoms of branch dieback including whitish resin exudate, which looked similar to the familiar Cytospora canker symptoms on spruce.
Pine-Pine Gall Rust (Endocronartium harknessii). Several samples of this Scotch pine disease in Christmas tree plantations were sent to the PPDC this year and to several OSU Extension county offices. This autoecious rust (no alternate host) is characterized by globose to pear-shaped woody stemmed galls. Reddish-brown in color, the galls are approximately the shape and size of toasted marshmallows. Sprays are not effective except as preventives, since by the time noticeable galls form, initial infections are probably at least a year old. Development of spore masses from the galls usually do not occur until two to four years after infection.
Verticillium Wilt (Verticillium spp.). Samples were common on certain maples, barberry, and Russian olive. On a sample of white ash, this disease was a fooler; the streaking that we typically expect to see in the sapwood from Verticillium colonization and degredation was not present. Leaves that look scorched and are dropping are signals for possible Verticillium wilt.
Suspect samples of ash sent to the PPDC for confirmation should include sections of affected branches about a half-inch in diameter and leaves that are exhibiting the scorch symptom. Leaf samples should be fresh, not crispy, and taken from within the tree, not gathered off the ground.
Downy Mildew of Viburnum (Plasmopara viburni). This disease was quite prominent this year in many Ohio locations on cranberrybush viburnums and leather leaf viburnum. In some cases it resulted in leaf scorching and eventual loss of green foliage on over half the plant by late summer. This fungus is not related to the powdery mildew fungi that cause many different powdery mildew diseases. This downy mildew fungus is, however, whitish in color, though typically a little more off-white than powdery mildew fungi. On cranberrybush viburnum, the fungal growth is seen on the lower leaf surfaces; on corresponding upper leaf surfaces there is a leaf scorch symptom, often in blocky, angular patterns.
Black Rot of Boston Ivy (Guignardia bidwellii f.sp. parthenocissi). This disease was quite noticeable this year, causing tan to brown roughly circular lesions on Boston ivy and Virginia creeper. Black, pimple-like fungal fruiting bodies develop in the upper surfaces of these lesions. Some leaf distortion also occured from infections as leaves were expanding. Leaf tattering symptoms also developed as the lesions aged in many plantings. The fungus causing this disease is closely related to the grape black rot pathogen.
Septoria Leaf Spot on River Birch (Septoria sp.). Yellowing of leaves on river birch is sometimes a symptom of moisture stress in mid and late summer. In some cases, however, it may also be associated with Septoria infections. With Septoria leaf spot, look for brown spots and black fungal fruiting bodies in the center of the spots.
Tubakia (Actinopelte) Leaf Spot (Tubakia dryina). This was frequently reported both for pin oak (associated with interveinal leaf yellowing and brownish speckling due to iron deficiency) and on red oak (larger brown spots on otherwise green leaves). In neither case does this leaf spot disease typically threaten plant health.
Phomposis Dieback of Buckthorn (Phomopsis sp.). This uncommon disease, found at one site in Ohio in 1996, was associated with heavy use of an overhead irrigation system.
A list of useful references for diagnosing infectious and noninfectious disease problems on ornamental plants in the landscape is included on the following pages. This list includes everything from long reference books to short fact sheets, to World Wide Web sites, to a new compact disc with herbicide injury symptoms, to how to get the Buckeye Yard and Garden Line on your computer.
The newest feature to the Web version of the Buckeye Yard and Garden Line (BYGL) is the incorporation of color slide images for some of the articles. You can also receive BYGL on your e-mail simply by requesting it from fischnich.1@osu.edu or by FAX from a number of OSU Extension offices.
For additional information on this CD, please see the list of references on page 31 of this publication.
APS Press, The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121-2097. 1-800-328-7560.
American Nurseryman Publishing Company
77 Washington Street, Suite 2100
Chicago, IL 60602-2904
1-800-621-5727
FAX: 312-782-3232