Southern Blight on Groundcovers

Southern blight on stem

Black Spot on Roses

Black spot is the most important fungal disease of roses worldwide. The initial symptoms start as feathery edged, black spots on lower leaves. As theses spots enlarge the leaves turn yellow and drop off. The disease will continue up the stems until the entire plant may become defoliated. Stem lesions are less obvious, but start as dark irregular blotches that eventually become blistered. Stem lesions are the most important source of fungal spores for initiation of the infection cycle next season.

The disease is caused by the fungal pathogen Diplocarpon rosae (imperfect stage: Marssonina rosae). Leaves are most susceptible when young and must usually be moist overnight before infection can occur. The disease can be spread by rain, dew, irrigation, people, insects and transport of infected plants. The fungus cannot live in the soil or last on pruning tools for longer than a month. Black spot spores can survive in fallen leaves and stem lesions over the winter, and will remain active year round on the plant in mild climates.

Management: Sanitation is critical for black spot management. Removal of fallen leaves and pruning infected canes will dramatically slow initial spring infections. Good air circulation will reduce the incidence of black spot by promotion of faster drying of leaf surfaces. Restrict irrigation during cloudy humid weather. Rose cultivars resistant to black spot are increasingly more available, but resistance can be regionally variable. Most people will need to use labeled fungicide sprays every 7-14 days as the first leaves emerge in the spring through the fall for adequate control of this disease.

Bacterial Leaf Scorch

Bacterial leaf scorch is a relatively new disease of shade trees and affects a large number of plants including elm, oak (see above photo), sycamore, maple, mulberry and hickory in the landscape. Symptoms typically appear in mid to late summer on lower branches as irregular marginal browning on interior leaves. Symptoms progress along the branch towards the tip. Symptoms will occur every year and progress through the crown. Scorched areas may have a yellow halo around them depending on the tree species. Reduced growth and dieback are also common in severely infected plants. These symptoms are sometimes mistaken for drought, environmental stress, or other diseases.

Bacterial scorch symptoms differ from drought scorch symptoms, in that they appear first on the lower branches and on the older interior leaves. Drought scorch symptoms will be more uniform and will first appear near the upper branches and on the younger leaves near the tips of the branches.

Management: There are not treatments for bacterial leaf scorch. However, infected trees may continue to persist in the landscape if symptomatic branches and dead wood is pruned out promptly. Antibiotic trunk injections have shown promise but they only relieve symptoms and don’t provide a cure.

Powdery Mildew on Shade Trees

Powdery mildew on dogwood

Powdery mildew is the common name for the disease and symptoms caused by a closely related group of fungi. These fungi grow on the upper and lower leaf surfaces, young stems, and shoot tips of plants. As they grow, they produce microscopic chains of spores that give infected areas their characteristic white powdery appearance.

The fungi parasitize the tissues of the plant causing a decline in its vigor. They also block light needed for photosynthesis. Infection is rarely lethal, but does cause leaf yellowing and browning, leaf distortion, early fall coloration, premature leaf drop, and blemished or aborted flowers and slower-than -normal growth.

The optimum conditions for powdery mildew development are warm days followed by cool, humid nights. Dry daytime weather allows spores to spread to other plants on air currents. On a cool evening they absorb enough moisture from the air to germinate and cause infection. The entire powdery mildew life cycle can take place in less than a week under ideal conditions, and many overlapping infection cycles can occur within a single growing season.

Management strategies: Control begins with selection of plants resistant to powdery mildew. Place susceptible plants where there is adequate sunlight and good air circulation to reduce humidity levels. Allow proper plant spacing for the same reasons. Pruning for better air circulation also may help. Registered fungicides may be needed if disease is severe. Check the label registration on horticultural oil products for powdery mildew control listings.

When selecting new dogwood varieties, choose powdery mildew resistant cultivars.  Among the cultivars of C. florida, ‘Cherokee Brave’, ‘Kay’s Appalachian Mist’, ‘Jean’s Appalachian Snow’ and ‘Karen’s Appalachian Blush’ have shown significant resistance to powdery mildew.  Most of the C. kousa and C. kousa x C. florida hybrids are also resistant.  Resistant Kousa dogwoods include ‘Milky Way’ and ‘Steeple’.  The resistant hybrids include ‘Celestial’, ‘Stardust’, and ‘Stellar Pink’.

Powdery Mildew on Herbaceous Plants

Powdery mildews grow as a white powdery coating over the surfaces of leaves, buds, flowers, twigs and stems. Another characteristic of powdery mildews is that they produce their sexual spores within a round dark colored structure called a cleistothecium. These structures observed under a hand lens appear as very small dark spheres with attached appendages. These fungi can over winter as spores on fallen leaves or in buds. Spores are carried to the leaves by air currents. The first symptoms of infection usually occur as a superficial white coating of mycelium on older leaves. Disease spread occurs as spores are released from the surface layer of mycelium. Perennials commonly troubled by powdery mildews include Aster, Centaurea, Coreopsis, Lathyrus, Monarda, Phlox, and Rudbeckia.

Management: Most foliar diseases can be lessened with proper watering. Water plants as early in the day as practical to allow foliage to dry before nightfall. Alleviate poor air circulation or crowded conditions with proper plant spacing. Inspect new growth and older foliage regularly for signs of infection to catch infections early. Sometimes simply removing the infected spotted leaves or plants will solve the problem. Selection of resistant varieties will also help to eliminate the application of costly controls.

Brown patch, or Rhizoctonia sheath and leaf blight, is a common summertime disease of cool-season turfgrasses in Maryland, usually caused by the fungus Rhizoctonia solani. Another species, R. zeae, is more common on perennial rye grasses on golf courses. Even though zoysiagrass is susceptible it's not commonly infected in Maryland.

Grasses Affected:
The disease affects most turfgrasses, especially young stands of tall fescue and perennial ryegrass.

• Primary symptoms:
  • Circular or irregular brown patches, 6 inches to 3 feet in diameter.
  • On higher cut home lawns, athletic fields, and parks, with tall fescue, bluegrass and perennial rye grasses, the brown appearance is primarily due to leaf tip die back and a thinning of the turf. Usually this disease attacks leaves and sheaths, but won't kill the crowns or roots, so that grasses can recover in the fall.
  • On tall fescue, individual leaves will have elongated tan lesions across the blade width bordered above and below by dark brown bands. Other grasses will have similar symptoms but with smaller lesions accompanied by tip die back.
  • When the fungus is active and moisture is present during the early morning hours a cobwebby growth can sometimes be seen growing on the grass blades.
• Season: Brown patch appears June to September but is generally most severe July through August.
• Environmental Conditions:
  • Minimum air temperatures of 61 degrees F coupled with rainfall or humidity above 95% for 8 hours are conducive for disease development.
  • Severe brown patch usually develops when nighttime thunderstorms or evening watering occurs during July and August when air temperatures are above 68 degrees F coupled with day temperatures of 85 degrees or higher with high humidity.
  • Late spring and summer applications of quick release water soluble nitrogen fertilizers, especially urea or ammonium based products, will also increase disease severity.

Management:· The general recommendation for homeowners is to tolerate a certain amount of brown patch damage to their lawns in the summer because with proper maintenance most of the recommended turfgrass cultivars will exhibit some recovery during the fall. Under severe conditions overseeding will be needed to thicken the affected turf.

Most of the required fertilizer for turf in Maryland (75-100%) should be applied in the fall between September and mid November. Slow release nitrogen formulations such as IBDU, methylene urea, sulfur coated urea, and natural organics can reduce the brown patch severity next summer.

If irrigation is necessary apply it early in the day to promote drying before nightfall. Frequent mowing at the proper height to avoid a high canopy will also speed drying of the foliage.

Perennial rye grass is a poor choice for home lawns in Maryland because of brown patch susceptibility and other serious diseases.

While fungicides are available to turf care professionals for control of brown patch most homeowners don't have the proper application equipment and several of the most effective products, which are costly, are not for sale to homeowners.

Daylily leaf streak, caused by the fungus Aureobasidium microstictum is very common disease on daylilies.  The first symptoms are visible in early spring as dark green spots that later turn into brown spots along the leaf midrib.  These spots quickly run together under wet weather conditions and cause brown streaks typically with yellow margins that extend the entire leaf length.  Infected leaves eventually will turn yellow or brown and die.

Management:  There are no registered fungicides for this disease.  There is a wide range of disease resistance between daylily cultivars to this disease.  This disease is often visible at the time of purchase and so purchase only healthy plants.  Also, select among the more resistant named cultivars for landscape plantings.  Check with the various daylily societies and state extension web sites for local recommendations.  Prompt removal of symptomatic leaves will help slow disease spread during the growing season.  Since the fungus can survive and overwinter in dead leaves a good fall clean up and removal of dead foliage will help slow disease spread next season. 

Eastern Filbert Blight is a fungal canker disease caused by Anisogramma anomala on several filbert species.  The most common ornamental filbert cultivar grown in landscapes is a European Filbert, Corylus avellana ‘Contorta’, also called Harry Lauder’s Walking Stick.  Filberts are also sometimes called hazelnuts.  Early symptoms include wilted or dead leaves hanging from infected branches.  Distinctive rows of parallel elongate black lesions are often visible along infected twigs and branches.  These perennial lesions elongate each year and will eventually girdle and kill the infected branch.  The disease cycle may take 2-3 years after infection to cause dieback.  There appear to be eastern and western strains of the pathogen.

Management:  The cultivar Harry Lauder’s Walking Stick appears to be fairly susceptible to infection in the mid Atlantic region.  In the landscape diseased branches should be pruned out by cutting well below visible lesions.  Commercial filbert production in Oregon relies on annual pruning and fungicide sprays.  In addition, commercial orchards are evaluating the use of resistant nut cultivars. 

Although shot holes in leaves are not definitively diagnostic the most common shot hole leaf disease of flowering cherry in the landscape is caused by the fungus Blumeriella jaapii.  This fungus overwinters on fallen leaves and causes new leaf infections as they have nearly finished expanding in the spring.  Wet spring weather encourages disease severity and warm rains will spread the fungal spores throughout the tree.  Leaf spots appear initially as purple spots that eventually turn reddish brown. New spots continue to appear through late summer.  Severe spotting will cause browning and the foliage will turn yellow and defoliate.  The characteristic shot hole symptom occurs when a brown border forms around the infected spot and the center falls out. 

Another shot hole symptom on flowering cherries is caused by the bacterium Xanthomonas arboricola pv. pruni.  Spring leaf infections often initiate from twig infection through leaf scars from the previous season.  Angular brown spots, often concentrated near leaf margins and tips, appear during warm spring rains.  Severe infections will cause leaf yellowing and defoliation.  The shot hole symptom on leaves occurs when the centers of the spots fall out.

Management:  On ornamental flowering cherries frequent scheduled sprays for fungal and bacterial diseases are not recommended. Although unsightly in wet years these diseases usually do not have a major impact on the overall health of infected trees.