Landscape - Recognizing Diseases on Perennials and Annuals in the Landscape

The following are some tips on recognizing diseases on perennials and annuals in the landscape.

Infectious plant diseases in the landscape are caused by pathogenic microbes such as fungi, bacteria, phytoplasmas, and viruses. Symptoms such as wilt, leaf spot, root rot, canker, and blight resulting from microbial infections represent the plant's reaction to disease. When evidence of the pathogen such as fungal spores or mycelium, bacterial ooze, or fungal fruiting bodies can be seen, they are regarded as signs of disease. Foliar diseases of landscape plants are most noticeable and they can often be identified by symptoms seen by the unaided eye and signs visible with a hand lens.

Rust diseases.

Rust diseases often produce raised pustules on the leaf surface which produce spores which are distinctively brown, reddish brown, orange or yellow in color. Infected leaves often produce a yellow spot with pustules that are found on the undersides of infected leaves. Rust diseases are favored during wet weather with moderate temperatures. Heavily infected leaves become yellow, then turn brown and die. In Delaware landscapes, rust can commonly be found on aster, daisy, dragonroot, geranium, hollyhock, jack-in-the-pulpit, rudbeckia, snapdragon, and sunflower.

Leaf spot diseases.

Leaf spots caused by fungi or bacteria are common on annuals and perennials. Symptoms vary depending on the host and the pathogen, but common leaf spot forms include angular brown or gray spots, brown spots with yellow halos, irregular blotches, tan or gray spots with reddish margins, reddish streaks and target-shaped circular spots. Some of the spots may contain tiny black pimple-like fungal fruiting structures called pycnidia. These pycnidia, as seen with a hand lens, are visible signs of disease. Spots may coalesce and blight affected leaves and heavily spotted leaves usually shrivel up and die or drop from the plant. Fungal leaf spot diseases are favored by wet, rainy seasons or frequent overhead irrigation. Examples of leaf spot of annuals and perennials include:

Alternaria fungus leaf spot on impatiens, marigold, rudbeckia, sunflower, and zinnia.
Fungal leaf streak, sometimes confused with rust, occurs on daylily.
Reddish leaf petiole streaks on peony are symptoms of Cladosporium infections.
Rose black spot is the most common defoliating disease of roses.
Iris is subject to a fungal and a bacterial leaf spot, both of which produce a gray spot with yellow halo.
Bacterial and fungal leaf spots also affect English ivy.
Begonia, chrysanthemum, and zinnia are subject to bacterial leaf spot.
Geranium bacterial blight begins as a leaf spot.
Foliar nematodes, common on shade-grown plants, cause angular spots on brunnera and heuchera, irregular blotches on anemone and salvia, and wedge-shaped dead patches on hosta. Signs of foliar nematodes may be glimpsed in leaf water droplets with a good hand lens.

Powdery mildew.

Signs of the powdery mildew fungus can be seen first as small patches and spreading to cover leaves, petioles, flowers, and stems of infected plants. These signs consist of a whitish or grayish mat of fungal mycelium, conidiospores, and conidiophores which bear the spores. Infected leaves may be twisted, curled, and distorted while covered with fungal signs. In some cases, fungal signs are sparse and leaves develop reddish splotches or take on a yellowish cast. In the landscape, powdery mildew especially affects begonia, chrysanthemum, columbine, monarda, phlox, rhododendron, rose, sedum, and zinnia. Powdery mildew does not require periods of rain or dew to thrive; warm weather and high humidity favor the disease.

Downy mildew.

Yellow patches are often observed on the upper surface of leaves infected with downy mildew. On the leaf underside, the fungus produces a whitish or grayish fuzzy fungal growth consisting of sporangial stalks and sporangia, best seen in early morning while the leaves are still moist. On rose, symptoms include dark, angular spots which produce fungal signs on the leaf undersides. Infected leaves shrivel up and die. Downy mildew can be found on alyssum, ornamental tobacco, pansy, rose, salvia, and snapdragon.

Botrytis blight.

When Botrytis blight (gray mold) is active, flowers are often attacked and blighted flowers may have dead, tan spots or blotches, or turn completely brown. Botrytis also causes tan to brown leaf spots and shoot blights, especially during cloudy, cool, moist weather. When the disease is active, signs of disease appear on dead tissues as gray or tan moldy growth of the causal fungus. Under moist conditions, almost all annuals and perennials can be affected by gray mold.

Virus diseases.

Plant virus symptoms appear commonly on the foliage, but plants are typically systemically infected. Rose mosaic virus symptoms can be seen as patterns of yellow and light-green lines, splotches, or speckles on infected leaves. Yellow or brown ring spots can be one symptom of impatiens necrotic spot virus (INSV) on New Guinea impatiens. INSV and its close relative, tomato spotted wilt virus, can also cause malformed strap-shaped leaves and stunting. During recent years, unusual virus diseases of landscape perennials such as anemone, hosta, and peony have appeared in the U.K. Plant Disease Diagnostic Laboratory with symptoms of ring spots, chlorotic spots, and mottled leaves.

Adapted from "RECOGNIZING FOLIAR DISEASES OF LANDSCAPE ANNUALS AND PERENNIALS" By John Hartman in the February 6, 2006 edition of the Kentucky Pest News form the College of Agriculture, University of Kentucky.