Anthracnose Leaf Diseases of Shade Trees

anthracnose on sycamore

anthracnose on oak

Anthracnose on Herbaceous Plants

Anthracnose symptoms typically appear as dark colored spots or sunken lesions that can quickly run together to form irregular dark lesions that cause rapid blighting of leaves or stems. This blighting can result in severe plant losses if not diagnosed in the early stages of infection. Under wet conditions these diseases can have multiple infection cycles during the growing season. Anthracnose diseases are caused by various fungi that produce their spores within a fungal fruiting body called an acervulus. The acervuli are often visible as small dark dots within the necrotic leaf or stem lesions and the spores are often released in slimy mass.

Perennials commonly infected by anthracnose diseases include Acorus, Bellamcanda, Bergenia, Calamagrostis, Convallaria, Dianthus, Epimediium, Heuchera, Hosta, Liatris, Liriope, Luzula, Malva, Miscanthus, Panicum, Pennisetum Phlox, Polygonatum, Rudbeckia, Saponaria, Sedum, and Tiarella.

Management strategies: If possible water early in the day, allowing leaves to dry before evening. Keeping plant foliage dry overnight helps to prevent fungal diseases, because infection occurs when leaves are wet for prolonged periods of time. Pick and discard infected leaves and stems, if practical. Cut infected plant to ground level in the fall and discard in the trash instead of composting. Thorough fall clean up will help to reduce the amount of infection during the next growing season.

Rusts of Ornamental Trees

Cedar-apple rust, quince rust and hawthorn rust are caused by fungi in the genus Gymnosporangium. These rust fungi require two types of plants, a pomaceous plant and a cedar in which to complete their life cycle. Juniperus virgiana, the Eastern Red cedar, is the most common rust-susceptible cedar in Maryland. Conspicuous symptoms are produced on both apples and cedars. Fungal spores produced on one type of host plant are carried by wind to infect the other.

These rusts damage many popular ornamental plants. Colorful leaf spots, twig and fruit malformations are produced on pomaceous plants. Peculiar round leaf galls, twig galls, cankers and twig dieback are produced on cedar hosts.

Highly susceptible, heavily infected pomaceous plants may be defoliated by mid-summer. Leaf spots are conspicuous and detract from ornamental value. The loss of leaf area due to spotting and defoliation reduces the vigor of trees, decreases yields, and makes the plants more susceptible to winter injury and other diseases.

Most infected fruit drops in June, while the remainder may be misshapen, cracked, and subject to secondary fruit rots. Quince rust galls on twigs may result in branch dieback and distorted growth. In moist spring weather, gelatinous masses of reddish-orange fungal spores are produced on infected twigs.

On cedar hosts, galled leaves and twigs may die back resulting in distorted growth. Perennial rust cankers may cause witches' brooms and large swollen cankers.

Management strategies:

1. Separate alternate hosts. Do not plant susceptible apples near susceptible junipers: a distance of 1 to 2 miles greatly reduces infection. (This is obviously not practical in areas where Eastern Red cedar is common.)

2. Hand pick and destroy cedar galls by April 1, before the spore-producing tendrils are formed. Cedar galls are most conspicuous and easy to see in wet weather when the orange spore tendrils are extruded.

3. Select resistant varieties of pomaceous plants.

Virus Diseases of Ornamental Plants

Virus symptoms often include color changes in the leaves such as a mosaic or mottled pattern of light and dark green. Other color changes could appear as ring patterns of light green or necrotic areas. Additional symptoms include stunted or malformed new growth, unusual flower colors, and dieback of terminal growth. These disease symptoms in many plants can be confused with nutritional deficiencies or environmental effects such as air pollution.

Most plant viruses cause systemic infections and there is no cure for infected plants. Plant viruses are commonly spread from plant to plant by insects that feed on sap such as aphids, thrips and white flies. Other common modes of virus dissemination include infected seeds, sap, divisions, grafted plant varieties, bulbs and pollen. Viruses can also be spread by horticultural activities such as vegetative propagation and pruning. Many viruses can survive in weeds adjacent to the horticultural plants that become infected in our gardens.

Management strategies: Management of virus diseases often involves removal of the infected plant to prevent spread to adjacent healthy plants. Insect management is critical since viruses can also move through the garden in insects feeding on infected plants before feeding on near by healthy plants. Weed removal is also very important since they can be the primary source of infected plants where insects breed or overwinter before moving onto healthy plants in the garden. Never knowingly plant virus infected varieties into your garden. Purchase certified virus clean stock and be alert to remove plants that exhibit disease symptoms.

Rose Rosette

This disease abbreviated (RRD), is believed to be caused by a virus, and infects multiflora as well as ornamental roses. The initial symptoms are reddening of the stems and stunted growth. Sometimes there is a proliferation or clustering of stems with excessive soft pliable thorns. The flowers may also abort. Infected roses may die within two years.

The disease is transmitted by an eriophyid mite and by grafting. Mites can be carried by wind currents from infected roses to healthy plants downwind. Multiflora rose serves as the main source for infection in ornamental plantings.

Management Strategies: Early detection is critical to prevent further disease spread within a planting. Infected roses should be uprooted and removed promptly. Remaining roses should be closely monitored for symptoms. If possible multiflora roses should be removed from the vicinity. Ornamental plantings should be planted upwind of multiflora roses if possible and spaced so that their foliage will not be touch. There are no pesticides for this disease and the control of eriophyid mites is very difficult.