Ornamental Plants

Annual Reports and Research Summaries

Special Circular 152-96

Landscape Ornamental Disease Problems in Ohio: 1995

James A. Chatfield, Ohio State University Extension-Northeast District/Horticulture and Crop Science; Nancy Taylor, Ohio State University Extension/Department of Plant Pathology; Randall H. Zondag, Ohio State University Extension-Lake County; Mary Ann Rose, Ohio Agricultural Research and Development Center/Ohio State University Extension/Horticulture and Crop Science; Gary Yu Gao, Ohio State University Extension-Clermont County; Daniel Balser, Ohio Department of Natural Resources Division of Forestry; and David J. Shetlar, Ohio Agricultural Research and Development Center/Ohio State University Extension/Entomology

A compilation of noteworthy disease problems affecting landscape ornamental plants in Ohio during the 1995 season is presented here. These observations are drawn from information in The Ohio State University's Buckeye Yard and Garden Line (BYGL) reports, Plant and Pest Diagnostic Clinic (PPDC) samples, Dave Shetlar's P.E.S.T. Newsletter, reports of the Ohio Department of Natural Resources Division of Forestry, and other observations.

Apple Scab

Apple scab on crabapple (caused by Venturia inaequalis), is a yearly occurrence on susceptible crabapple species and cultivars. Some years are worse than others, depending on environmental conditions. The spring of 1995 was just such a year in much of Ohio.

This spring was characterized by prolonged humid, wet, moderate-temperature weather, great conditions for primary infections (from spores produced on infected leaves on the ground from the 1994 season) and cycles of secondary infections (from spores produced on leaves infected this season).

In some cases, symptoms were atypical -- instead of roundish, scabby, discreet gray to olive-green spots on leaves, entire leaves quickly were covered by grayish fungal growth, with defoliation soon to follow. Fruit infections were also severe in some cases. On 'Candied Apple' crabapple at some Ohio locations, tiny fruits were covered with velvety green fungal growth by early June, robbing this cultivar of its namesake shiny red fruit color for the entire season.

The "sheet scab" this year on leaves was very difficult to prevent, simply because infection periods were back to back and protective sprays were difficult to apply during periods of near-constant rain. The best control for scab, of course, remains the planting of crabapples that have excellent to good resistance to the disease.

Anthracnose Diseases

These diseases typically cause brownish, reddish-brown, or tannish-brown blotches along leaf veins. More serious twig and branch dieback can occur with two common anthracnose diseases -- sycamore anthracnose and dogwood anthracnose.

• Sycamore anthracnose (Caused by Apiognomonia veneta). In both 1994 and 1995, cool wet conditions during leaf emergence in mid-spring resulted in predictably severe anthracnose on sycamore and to a lesser extent on London planetree. These trees do tend to re-leaf by the end of June, but heavy infestations in successive years may trigger stress problems such as invasion by ambrosia beetles.

• Dogwood anthracnose (Caused by Discula destructiva.) This disease is a serious problem in heavily wooded forested areas in the southeastern and eastern United States, especially in upland sites, and in some landscape situations with dense shade and poor air movement.

  Dogwood anthracnose does occur in Ohio; for example, the Ohio Department of Natural Resources confirms reports of it from nine counties in Ohio in 1995. In most Ohio landscapes, however, especially where flowering dogwood was planted in sites with partial shade, good air movement, moderate fertility, and proper mulching, dogwood anthracnose was not a major problem.

• Ash anthracnose (Caused by Apiognomonia errabunda.) Ash anthracnose was also quite prevalent in 1994 and 1995, thriving in the wet conditions of late May in many areas, resulting in considerable water soaked leaf spotting and leaf drop and tannish leaf blotching on leaves infected later. Generally, trees recovered well from ash anthracnose, although a number of other problems, such as ash decline, were widespread on ashes.

  Several other relatively uncommon anthracnose diseases identified in Ohio in 1995 were elm anthracnose (pathogen unidentified) and beech anthracnose (Discula sp.).

Powdery Mildew of Flowering Dogwood

Every dog has its day, but the increasing incidence of powdery mildew on flowering dogwood (caused by Microsphaera sp.) in Ohio, Kentucky, and other Midwestern states during the past few years is a great surprise to many plant pathologists and horticulturists. The symptoms of this powdery mildew disease are often diagnostic foolers at first to those unfamiliar with the disease.

Although typical powdery white colonies of mildew do develop eventually, other symptoms are often more noticeable, especially early in the infestations. As John Hartman of the University of Kentucky outlined in an early July 1995 Kentucky Pest Newsletter, dogwood powdery mildew is often characterized by symptoms of:

• Small dark red splotches on upper leaf surfaces which develop into brown dead areas.

• Yellow mottling and distortion of new leaves.

• Yellowing of older leaves.

The intensely humid weather this season may have contributed to repeated infections by this powdery mildew fungus.

Fireblight

(Caused by Erwinia amylovora.) Bacterial fireblight is a common problem on plants in the rose family, such as crabapple, Callery pear, pyracantha, cotoneaster, mountain ash, and others, causing shoot dieback (and plant death on highly susceptible hosts). The 1995 season was no exception, as fireblight was widespread.

Incidence of fireblight depends to a great extent upon host susceptibility. For example, certain crabapples are much more susceptible than others. Severity also varies widely. For example, many Callery pears become infected, but the disease typically tends to spread only a matter of inches, rather than feet, back along the branch on this host.

Environmental conditions also play a role in fireblight infections. For example, the most common type of infection is blossom infections, although shoots, fruitlets, and spurs may also be infected. Blossom infection is favored by warm (over 60 degrees F) humid conditions during flowering. This spring at Secrest Arboretum in Ohio these conditions were common during bloom of 'Harvest Gold' crabapple.

At Secrest, two 'Harvest Gold' replicates in the National Crabapple Evaluation plots flowered heavily while one did not. This crabapple has a tendency for alternate year blooming. The ones that flowered had literally hundreds of small blighted shoots from these blossom infections, while the non-flowering 'Harvest Gold' exhibited no fireblight this year.

Diplodia Tip Blight

This disease (caused by Sphaeropsis sapinea) continues to be a significant problem on stressed Scots, Austrian, mugo, red, and many other two- and three-needled pines, with occasional occurrences on white pine, spruce, and certain other conifers. Infections occur during development of new growth in spring, resulting in dieback of the new season's growth, and progressive branch dieback, typically moving upward in the tree from lower branches over the years.

Preventive fungicide controls are used in spring as new growth starts to develop. Cultural controls such as proper pruning, fertilization, proper site selection, and improved drainage are year-round propositions.

Diseases of Herbaceous Annuals and Perennials

A greater awareness of these diseases is evident in the past few years, especially as perennials become more popular. In addition, the past two seasons have brought out the worst in terms of some of these diseases.

The occurrence of Botrytis gray mold (caused by Botrytis cinerea) on geranium and other annuals was severe in 1995 due to extended humid periods in much of the Midwest. Plants looked especially bad where deadheading was not used, as infected senescent blossoms fell onto leaves, resulting in messy infections of leaf tissues as well.

Verticillium wilt (caused by Verticillium dahliae) was identified in the Ohio State University Plant and Pest Diagnostic Clinic on several geranium samples.

Southern blight of hosta (caused by Sclerotium rolfsii) has caused problems in some Ohio landscapes in recent years of heavy moisture and humidity. This disease is evidenced by a fan-like white fungal mycelial growth over the infected leaf and crown tissue and eventually development of "mustard-seed" sized reddish brown hardened sclerotia of the fungus. This is fairly unusual in northern states on hosta, although a common problem in northern landscapes on ajuga, and very common on many ornamental plants in more southerly states.

New Disease Reports for Ohio

Nancy Taylor of the Plant and Pest Diagnostic Clinic and Department of Plant Pathology Professor Emeritus Wayne Ellett reported several firsts for Ohio, now to be included in the Ohio Plant Disease Index by Dr. Ellett. These were:

• Venturia blight of maple (Caused by Venturia acerina.) Following is a description of this disease from Sinclair, Johnson, and Lyon's Diseases of Trees and Shrubs:

  "Necrotic lesions up to [one inch] in diameter develop in mid-summer. The lesions are more or less round except where they are bounded by major veins or the leaf edge. On the upper surface they have deep reddish-brown centers and dark brown edges with diffuse margins. The lower surface is grayish green to grayish tan. Coalescing lesions kill large areas of leaf blades. Severe infection leads to premature reddening and casting of red maple leaves."

  As indicated in Diseases of Trees and Shrubs, this is typically an "inconsequential leaf blight of maple," but as noted in Buckeye Yard and Garden Line 95-24, in this case it was "significant enough to apparently contribute to some early leaf drop for the past two years."

• Verticillium Wilt of Ash (Caused by Verticillium dahliae.) Although its presence has been suspected in Ohio, Verticillium wilt of ash had not been confirmed in Ohio until 1995. Initial symptoms included scorch of leaflets and leaf drop. On ash, the typical dark streaking of vascular tissue often associated with Verticillium wilt diseases is not present and is not a diagnostic feature.

• Rust on Switchgrass (Caused by Puccinia sp.) Although ornamental grasses are often described as being disease-free, this rust disease was identified in Ohio in 1995.

Unidentified Problems

Two problems that continue to baffle diagnosticians are the blackening of arborvitae foliage and the unusual distortion of the foliage on oaks in springtime. On the arborvitaes, speculation ranges from horticultural oil injury or damage from fertilizer contamination of oil applications to various root problems and winter injury symptoms. None of these seem to make sense for all of the instances of the foliar blackening.

With the oaks, speculations range from frost injury to growth-regulator herbicide injury to various insect problems. Again, none of these suspicions was confirmed or seemed to explain all instances of the problem.

BYGLSpeak

At the conclusion of each week's BYGL in 1995 is a quote, sometimes inspirational, sometimes silly, generally related to horticulture. To conclude, here are a few BYGLSpeak entries for the past year.

First, from Mark Twain, a reminder that, though every year is unique, there are similarities. Sort of a "It's not one darn thing after another, it's the same darn thing over and over again" philosophy. As Samuel Clemens puts it: "The past may not repeat itself, but it sure does rhyme."

Second, from a modern day Twain, or perhaps a Jack Kerouac of the 90s, this from Ohio State University Extension-Lorain County's Charles Behnke in BYGL 95-10 on June 8:

"In Lorain County, cottonwood fluff is blowing in the wind. Cottony fuzz, tree lint for Mother Earth's navel. Sod webworm adults are fleeing from the Behnke mower/bug shredder. Spittlebugs are spitting, maple petiole borers are at work. That's it from the Behnke Estate."