James A. Chatfield, Nancy J. Taylor, Joseph F. Boggs, Jane C. Martin, Gary Y. Gao, Pamela J. Bennett, Randall A. Zondag, and David E. Dyke
This summary was developed from observations by the authors throughout the 1998 season, published reports from Ohio State University Extension's Buckeye Yard and Garden Line (BYGL), and records from The Ohio State University's C. Wayne Ellett Plant and Pest Diagnostic Clinic (PPDC). It was a typical year for most disease problems, with wet spring weather resulting in many early problems, but dry mid-summer conditions moderating secondary infections for many diseases. We had all the usual suspects -- rust diseases, Verticillium wilts, powdery mildews, and Phytophthora root and crown rots in poorly drained sites. Some additional phytopathological comments on the season of 1998 follow.
New disease reports for Ohio, confirmed in 1998 by the PPDC include:
Like almost everything else this year, apple scab (Venturia inaequalis) got started earlier. Dr. C. Wayne Ellett, professor emeritus in plant pathology at The Ohio State University and the man for whom the PPDC is named, noted primary scab lesions on crabapple leaves in the central Ohio area in late April. Dr. Ellett reported that this was the earliest that he ever remembered seeing scab symptoms, and that typically early infections do not result in observable symptoms until the first weeks of May.
Primary scab is caused by sexual fungal spores that developed in the spring on overwintered leaves and fruits. Secondary scab then occurred and recurred throughout the spring and into the summer, from infections by asexual spores that developed in the early scab lesions.
For many areas of Ohio, relatively dry weather in late spring and in the summer resulted in a little less scab in 1998 than in most years. For information on scab incidence of various crabapple taxa in 1998, see the article on scab evaluations in this ornamentals special circular.
Frogeye leaf spot (Botryosphaeria obtusa) on crabapple was often mistaken for apple scab, especially when seen from afar when yellow leaves littered the ground. The symptoms of the two diseases are quite different, however. Instead of the olive-green fungal growth and later grayish to brown scab lesions on the leaves with apple scab, frogeye leaf spot starts with circular brown leaf spots with purple margins. Some spots gradually enlarge at some points along lesion margins, resulting in lobed lesions around the original spot, developing a concentric layered effect with brown areas around the now grayish original center. Voila -- to some people, this looks just like a frogeye.
The same fungus also causes a black rot of fruits on apple and crabapple and black rot cankers on woody stems of many plants. For control, keep plants as healthy as possible with good pruning, fertilization, mulching, and watering practices. Consider early fungicide sprays if the leaf spot disease gets out of hand on a particular crabapple cultivar over the years and check before you assume that leaf drop is due to scab on a reportedly scab-resistant crabapple.
For the third straight year, environmental conditions during leaf emergence on sycamores, ashes, dogwoods, and oaks were cool and wet, and this predictably resulted in considerable infections from the anthracnose fungi (Gloeosporium sp., Discula sp.) that infect these trees. Relative to sycamore anthracnose, Dr. Bobby Joyner of TruGreen-Chemlawn noted it was "the worst he has ever seen."
Despite the early severity of the various anthracnoses, fungicide applications for anthracnose on shade trees can be problematical for the following reasons:
Most trees infected this year re-leafed and appeared to recover well, with the exception of flowering dogwoods with the more serious dogwood anthracnose disease caused by Discula destructiva. This fungus moves back into stem tissue readily and can kill flowering dogwoods, especially if they grow in dense shade conditions.
This fungal disease (Diplodia pinea or Sphaeropsis sapinea) continues to be a major problem on Scots, Austrian, red, and mugo pines in Ohio. Here are some key things to keep in mind relative to this disease.
Rose black spot (Diplocarpon rosae) got off to a roaring start in the spring of 1998, but in most cases was not severe through the summer, due to dry conditions in much of Ohio.
There were exceptions to this, especially when droughty conditions pressured horticulturists into use of sprinkler irrigation. With frequent water on the leaves, the disease triangle of susceptible rose varieties, presence of the black spot pathogen, and an environment conducive to disease were all met, and in some cases disease incidence was phenomenal.
In early spring many pachysandra plantings exhibited considerable leaf scorching, especially at the tips of leaves. The problem appeared to be due to winter desiccation injury, but it was often misdiagnosed as Volutella blight (Volutella pachysandricola).
Further confusing the issue was that this type of desiccation injury to leaf tissue makes it more susceptible to infection from the Volutella fungus. Sure enough, as spring progressed, many pachysandra plantings developed considerable Volutella blight. Fortunately, with drier summer weather, this disease became less prevalent, and by fall it was actually difficult to find Volutella samples for plant diagnostic clinics.
Symptoms of Volutella blight and stem canker include tannish to greyish colored target-like leaf blotches, followed by stem blackening and the presence of salmon-colored sporulation of the fungus on dying stems during wet conditions.
Removal of badly infected and dead plants, limiting overhead irrigation, protective fungicide sprays during periods of active plant growth, proper fertilization, and hopefully drier weather are all key elements for control of this disease. High mowing of badly infected areas and removal of diseased tissue is sometimes the best way to renovate plantings.
There were numerous reports this year of Phomopsis dieback (Phomopsis sp.) on vinca. This fungal disease causes the shoot tips to turn dark brown, wilt, and die back to the soil surface. Some stems become black in color, and small, black fruiting bodies of the fungus may erupt through the stem's surface. Wet spring weather and overhead irrigation favor disease development.
As with other groundcover diseases, control involves debris removal in early spring, removal of infected plants, stand thinning, and proper fertilization. Fungicides labeled for control of this disease include copper (i.e., Phyton 27 or Kocide) and mancozeb (i.e., FORE). However, treatment should begin at bud break and continue once a month until plant growth slows in mid to late summer.
This common disease of zinnia (Xanthomonas nigromaculans f.sp. zinniae) first appears on zinnia leaves as diffuse, translucent, circular spots, 1-2 mm in diameter, surrounded by large chlorotic halos. Under wet conditions, the lesions slowly enlarge to about 5 mm across, becoming reddish-brown in the center and angular in shape. The lesions may coalesce into areas 0.5-1.0 cm long and develop into ragged patches of decaying tissue. When the humidity is very high, the disease may produce small brown spots on the flowers. If infection is severe, the flower heads are seriously disfigured and may decay completely. Diseased plants should be destroyed.
This fungal disease (Rhytisma spp.) seemed much more prominent in Ohio this year, especially in northern Ohio sites. It is often noted that this summer and fall foliar disease of maples is spectacular but relatively harmless to plant health, but the size (quarter size and larger) and frequency of the spots this year still caused concern. Symptoms include black, tarry spots on upper leaf surfaces, associated with yellowed areas on the foliage.
Often these spots are few and almost unnoticeable overall, except to phytopathological afficionados who are looking at individual leaves. At other times, the damage is much more prominent, noticeable to even the casual homeowner observer from afar.
The key to the more severe outbreaks of tar spot appear to be sheltered, moist sites with poor air movement, which favor survival and development of the Rhytisma fungus. Some speculated that the seemingly higher incidence of tar spot in 1998 was attributable to mild winter temperatures during the winter of 1997-1998, resulting in higher than usual winter survival of the fungus.
This is a common fungal disease (Apiosporina morbosa) on plums (other hosts include flowering almonds, cherries, and other members of the Prunus genus). This disease is characterized by black warty growths on the stems and can ultimately result in death of the plant if the disease is not controlled over the years.
On susceptible plants, infected stems should be pruned out at any time, and preventive fungicide sprays should be applied in the spring at flower bud break and continue through the early summer at seven- to 10-day intervals. Fungicides containing thiophanate-methyl, chlorothalonil, and captan can be used for black knot control.
The reddish-brown leaf blotches and leaf drop caused by the Guignardia fungus occurred on buckeyes and horsechestnuts throughout Ohio again during 1998, but damage from this disease was again confused with simple physiological leaf scorch caused by drying conditions. To help distinguish between Guignardia blotch disease and simple physiological leaf scorch on Aesculus, use a hand lens to check for tiny black pimple-like fruiting bodies of the Guignardia fungus present within the blotched areas.
In 1999, a number of Ohio State University horticulturists and plant pathologists will be doing a survey of various buckeye and horsechestnut taxa to check for relative incidence of leaf blotch disease and leaf scorch problems.
At the end of each weekly BYGL, the authors try to include a meaningful and/or humorous quote for the edification of BYGL readers. Now, to conlude this disease summary, here are a few favorite BYGLosophies from 1998.