Plant Health Problems

Diseases caused by Fungi:

Gray mold blight, Botrytis paeoniae.
This fungus attacks young stems, leaves, and buds, causing a brownish-black rot covered with gray mold. Rot may extend several inches below the bud. Newly emerging shoots develop a blackened, blighted appearance. Half open flowers turn brown in part or entirely. The fungus usually fruits readily on infected parts. Small black sclerotia may be formed on the bases of the stalks and other affected parts that fall to the ground.

Prompt removal of infected plant parts lowers the possibility of new infections. Good sanitation includes cutting and burning all old stalks in the fall and removal of mulch early in spring. Sometimes removal of the top 2 inches of soil, replacing with new soil, is helpful in stubborn cases. Control can also be achieved with the use of fungicides applied when new growth is emerging or as soon as symptoms are visible. Among the compounds registered for use in Connecticut are mancozeb and thiophanate-methyl. Consult the label for dosage rates and safety precautions.

Stem rot, Sclerotinia sclerotiorum.
Symptoms are similar to those of Botrytis blight but sclerotia are large and black and are found in the center of diseased stems.

Control is the same as for gray mold blight.

Leaf blotch, Cladosporium paeoniae.
After blooming, purple blotches may appear on the leaves which in wet weather are covered with an olive-grey mold.

Destruction of old plant parts in the fall is usually sufficient for control.

Leaf spots, Septoria paeoniae, Pestalozzia sp., Alternaria sp., Phyllosticta sp.
Symptoms include spots of varied color and size which develop on the leaves.

Control can be achieved by removal of plant debris in the fall. Chemical control is usually not necessary.

Diseases caused by Nematodes:

Root-knot nematodes, Meloidogyne spp.
Plants are small and weak and produce no flowers. Roots are short and stubby, and fine roots show galls about one-eighth inch in diameter. When sliced, these reveal brown areas containing many nematodes, small round worms that are also called eelworms.

Hot-water treatment at 115 F for one-half hour of roots and crown effectively kills nematodes. Planting heat-treated roots in a new location prevents reinfestation. Since many other species of plants may become infested with these nematodes, care should be taken to plant only nematode-free planting material.

Diseases caused by Physiological/Environmental Factors:

Bud blast, cause unknown.
Buds remain small and turn black but no gray mold appears on them. Cause is unknown but it has been suggested that it may be potash deficiency or late spring frosts.

Insect Problems:

Ants.
The presence of ants on peony buds does not indicate injury to the plant by the insects. They are feeding on the sweet secretion produced by the buds.

Rose chafer, Macrodactylus subspinosus.
The beetles feed upon and soil the white flowers. When abundant, this insect injures roses and many other shrubs and trees when adult beetles skeletonize the leaves. Rose chafers breed most abundantly in sandy waste land but often occur in lawns; they appear each year about June 10 to 12, and feed for about a month, though some of the beetles are present for 6 weeks. The females each lay from 24 to 36 eggs singly in the ground a few inches beneath the surface. They soon hatch and the young grubs feed upon the roots of grass and other plants, becoming full grown by late autumn when they go into the ground several inches to overwinter. The next April or May, they come near the surface and transform to pupae in earthen cells, and from 2 to 4 weeks later the beetles emerge. The adult beetle is yellowish grayish brown or clay colored and is a little over 1/3" (10-12 mm) long, with long sprawling legs. The larva is a white grub about 3/4" long when full grown. Feeding on the foliage can be prevented by using sprays of acephate, azadirachtin, carbaryl or malathion, which are among the products registered for use against this pest in Connecticut. Imidacloprid, applied early in the season as a soil drench, will provide season-long systemic control. Consult the labels for dosage rates and safety precautions. With the exception of imidacloprid, it is difficult to protect the blossoms. Treatment of lawn areas should reduce the numbers of beetles. See Lawns, oriental beetle.