Released September 4, 2009
URBANA, Ill. -- Several problems can strike perennials, said a University of Illinois Extension specialist and pesticide safety educator.
"The key is to understand the various problems and how to deal with them," said James Schuster.
Schuster described some of the common perennial plant problems.
"Achillea sp. (yarrow) is often seriously inflicted with the disease Botrytis. Botrytis (gray mold) appears as brown dead areas and under proper moisture may have a gray fuzzy appearance. It attacks buds, flowers, leaves, and stems. It may eventually kill the perennial," he said. "In addition, Botrytis attacks a wide range of plants. Fungicides and sanitation are helpful."
Powdery mildew is a leaf-surface growing disease. It is whitish in appearance and is often described as a white powder on the leaves.
"Several cool nights and warm, dry days in a row help promote the outbreak of this disease," Schuster explained. "There are many fungi that can cause powdery mildew. Powdery mildew tends to be worse on plants growing in shade."
Fungicides and improving sunlight and air circulation can help reduce infection.
"Alcea sp. (hollyhock) is often attacked by a rust disease," he said. "On hollyhock the rust pustules may vary from yellow to red to gray. Severely infected leaves will die. Depending on the plant, pustule color may vary. Fungicides and sanitation may help reduce infection."
Leaf spot caused by Cercosporella causes gray to black spots (older spots may have tan centers with brown margins). The spots may merge and tear, leaving leaves with a shredded appearance. Again, fungicides and sanitation are helpful.
Aquilegia sp. (columbine) gets several foliar diseases, but one of the most common problems is a serpentine-type leaf miner that leaves light tan wiggly lines through leaves.
"Insecticides may help, but ignoring the problem may be easier on you and better for the environment," Schuster noted. "This insect is not fatal to the plant - mostly it is just an aesthetic problem."
Calendula sp. may get Calendula smut that causes brown round spots or Alternaria leaf spot that causes small reddish-purple spots.
Aster yellows (caused by a phytoplasma rather than fungus or bacterium) causes stunting, yellowish-green foliage and flowers followed by the death of plant. Eradicate the plant as soon as symptoms are noticed. This disease is carried by insects. Powdery mildew may also occur. Fungicides (except for aster yellows) and sanitation are important.
"Chrysanthemum sp. (Shasta daisy) can get Alternaria leaf spot that causes concentric rings of light and dark," he said. "Septoria leaf spot usually causes brown along the margins and tips of leaves. Botrytis may cause flowers to brown and die early as well as cause irregular brown spots on leaves and shoots. Fungicides and sanitation are helpful."
Chrysanthemum sp. (mums) may get Botrytis on buds, flowers, leaves, and stems. Infected areas are brown to dark brown in color (the gray fuzzy appearance requires proper moisture conditions). They start as small spots and rapidly expand to encompass the entire bud or flower. Sometimes mums get rust diseases that can be whitish or brownish in appearance. Fungicides and sanitation are helpful.
"Fusarium wilt is also a possibility. Fusarium may enter through roots or at the soil line (collar rot)," he explained. "Plants wilt, then turn brown and die. Plants may go "piecemeal" or all at one time.
"Rhizoctonia and Pythium can cause root rots and collar rots. Pythium occurs when soils drain poorly or are kept wet by overwatering. Rhizoctonia may occur when soils are wet followed by hot dry conditions."
Echinacea sp. (purple coneflower) may get Pseudomanas or Xanthomonas leaf spot. Both are bacterial diseases. Sanitation is the main control for these leaf spots. Botrytis may also occur. Botrytis causes brown dead areas as well as distorted leaves. There is an eriophyid mite that causes the flower to develop lumps or bumps — remove infested flowers and destroy them.
"If the bumps are on stalks, the plants are infected with aster yellows, and the infected plants need to be removed and destroyed immediately," he said.
Hemerocallis sp. (day lily) may get one of three fungi that cause dead streaks in the leaves. Collectotricum causes dead brown streaks with reddish-brown margins down the middle of the leaf. Aureobasdium causes diamond-shaped spots, reddish in color, which can kill large sections of the leaf when spots join. Colleocephacus causes brown streaks with yellowish margins. Usually these leaf diseases cause mostly an aesthetic problem -- not death of the plant. Fungicides and sanitation are helpful.
Hosta sp. has several insects that eat holes in the foliage," said Schuster. "Large irregular holes during drought weather tend to be caused by grasshoppers. Black vine weevils chew paper punch notches along the leaf edge/margin. Slugs (not an insect) eat small irregular holes. Both the black vine weevil and slugs are night feeders. Use insecticides on the insects. Controlling the growing conditions is more useful for slugs such as keeping plants on the dry side, minimize use of mulches, and water infrequently but deep.
"Anthracnose, sun burning, and drought stress are the other problems. Anthracnose causes brown lesions usually on the leaf edges that may spread to encompass the entire leaf. Sun burning often bleaches (light tan to whitish) the leaves. Drought stress usually causes dead leaf margins that may become infected with saprophytic fungi. Anthracnose tends to occur more often on hostas in wet locations, he added.
Iris sp. may get rust. Rust often causes rectangular brown leaf lesions. There are two other leaf diseases Didymelcina and Mycosphaerella.
"But the more serious problem is the iris borer that allows a bacterial rot inside," he said. The borer enters the leaf when they are about four to six inches high. The insect works its way down the leaf into the tuber. The bacterium follows and causes decay.
"Use a preventative insecticide at the correct time and sanitation (dig up and remove the borers, scrape out decay, dip in a 10% chlorine bleach solution, dry for several hours in sunlight). Replant so that top of rhizome is visible. After watering, cut leaves to four to six inches in height."
Peonies are attacked by Botrytis and anthracnose as well as several other diseases. Both diseases cause buds, flowers, leaves, and stems to die. Both cause similar death symptoms. Grow in full sun and good air circulation. Fungicides and sanitation are helpful.
Phlox have several foliar diseases, but the most common is powdery mildew. Buy resistant varieties. Improve sunlight and air circulation. Fungicides and sanitation are helpful.
Rosa sp. (rose) gets many diseases and insects. Viruses, bacteria, and fungi are the pathogens. Aphids, rose midge, and mites are some of the insects or insect-like problems. Virus diseases include ring spot and mosaic viruses. Eradicate infected plants.
"There are no chemical controls," he said. "Crown gall is the main bacterium problem. Crown gall causes growths to occur on roots and stems. Eventually the plants die. Eradicate infected plants. Many plants can get crown gall so it is best to grow monocot plants after removing and destroying infected plants.
"Or if one wants to grow plants that are susceptible to crown gall, it is best to wait at least five years before replanting."
Fungal problems include canker problems (Coniothyrium, Phomopsis, and several other canker-causing fungi). Cankers are dead areas on the stem that can enlarge and eventually kill the stem or even the entire plant. Avoid stressing plants. Prune, water, and fertilize to improve vigor.
"Fungicides are not very effective on canker diseases," said Schuster.
Powdery mildew causes a whitish appearance on leaves. Mildew needs three consecutive cool humid nights and warm dry days.
Black spot causes black to brownish spots with wide yellow halos. It is the most common leaf disease on roses in most growing areas. Mycohaerealla leaf spot causes tan to gray spots with purple to red halos.
"Tulips tend to get a virus (mosaic-type) disease," he said. "Botrytis infects the leaves and is called "fire." Tulips are also prone to bulb rot. Eradicate virus-infected plants. Discard rotting or moldy bulbs. Fungicides and sanitation are helpful for Botrytis"
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http://www.aces.uiuc.edu/news/stories/news4885.html
Source: James Schuster, (217) 244-9646
Writer: Gary Beaumont, (217) 333-9440, beaumont@uiuc.edu
