Peony Insect Pests
Foliar nematodes are microscopic organisms that feed within plant tissue. They require a layer of moisture in order to move about on the outside plant surfaces. The species that infects peony is Aphelenchoides fragariae, commonly known as the Strawberry Leaf Nematode. First symptoms appear as bud blast, where infect buds fail to open. If you cut open the suspected bud the inside will be rotted while the outside still appears healthy. From the buds the foliar nematodes move to the leaves when there is adequate surface moisture. They enter through stomata (breathing pores) and feed between the veins. Feeding damage thus appears angular with discoloured wedge-shaped sections first appearing water soaked then yellowing and turning brown or black. In order to cross a vein, they must exit the leaf and travel on a layer of moisture to the other side of the vein and reenter the leaf through stomata. They are most problematic later in the season as their populations rise.
To manage leaf nematodes, remove infected plant tissue and black bag. Clean up plant debris in the fall, as foliar nematodes can live up to 3 years in plant litter. Keep foliage dry so they cannot spread. The tubers when dormant can be soaked in hot water to kill the nematodes see the American Peony Societies post on Foliar nematodes in peonies for instructions. Aphelenchoides fragariae, feed on a wide variety of ornamental plants, weeds and crops. Examine other nearby plants such as Hosta, for infection.
There are several types of microscopic root nematodes that are parasitic. Some of them are stationery and feed inside the root from the same spot their entire lives, such as root knot nematodes (Meloidogyne hapla). Other types feed inside the roots, but are able to move around, such as root lesion nematodes (Pratylenchus penetrans). Still other types live and feed on the outside of the root such as cyst nematodes like dagger nematode (Xyphinema spp.) and pin nematode (Paratylenchus) among others. Nematode feeding damage to peony roots varies slightly depending on the species involved and more than one species can be present at the same time. In addition to feeding damage, root nematodes also cause a variety of plant diseases, such as Tobacco rattle virus. To determine the exact species at hand a lab analysis is required.
Root knot nematodes are immobile and feed in one spot their entire lives. Their feeding causing the surrounding tissues to swell up over them, forming small galls on the roots, that are visible to the naked eye. These galls interfere with the normal flow of water and nutrients to the plant and may cause above ground symptoms such as the plant failing to thrive, stunting, wilting, thin stems, leaf discolouration and bud blast. Below ground if you dig up a section of root you will see heavily branched feeder roots with small galls on them. In Peony the most common genus appears to be Meloidogyne spp. but Rotylenchus and Ditylenchus can also infect peony.
Root lesion nematodes on the other hand do not cause galls to form on the roots rather their feeding damage appears as dark necrotic lesions on the root surfaces. They feed within the root cortex but unlike the root knot nematodes these ones move around. As they move about, the lesions grow larger and can coalesce even to the point of completely girdling the root. The damage done to the roots can cause the same type of above ground symptoms as root knot nematodes. The root lesion nematode species usually involved with peony is Pratylenchus sp. especially Pratylenchus penetrans in temperate climates.
Managing root nematode infections is very difficult and usually involves digging up infected plants and soil and black bagging them. Soil can remain contaminated for 2-3 years and since these nematodes can infect a wide variety of ornamental plants grasses are usually replanted in their spot. French marigolds can also be planted instead of grasses. The chemical released by their roots has been proven to be affective at reducing soil nematodes by preventing their eggs from hatching. Repeated tilling of the soil is also helpful in reducing soil nematode populations as they die when exposed to the sun. Improving the diversity of soil microorganisms by adding organic material regularly to the soil can help to establish certain fungi populations that parasitize root nematode eggs.
The most common outdoor thrip species is the western flower thrip (Frankliniella occidentalis). They feed on a wide variety of ornamental garden plants including peony. The insects are very tiny (1-2mm) and difficult to see with the naked eye but in mass they can cause significant damage. Damage occurs in spring when the peony is putting on their new growth. The thrips feed by sucking the plant juices out of the leaves, leaving behind tiny dead spots giving the leaves a dull or silvery appearance. Heavily infested leaves may have a scorched appearance. Flower buds and petals can also be affected. Buds distort and fail to open, and petals appear discoloured and distorted. In addition to feeding damage western flower thrips also vector the diseases Impatiens necrotic spot virus and Tomato spotted wilt virus, two serious viral diseases for which there is no cure.
Managing thrips involves monitoring for their presence in spring by watching for signs of their damage. Peony stems can be shaken over a light coloured fabric so they are easier to spot and identify. Thrips have very thin, elongated bodies and range in colour from yellow to red or dark brown. Adults have 4 wings but are poor fliers. If present, they can be sprayed with insecticidal soap. Attracting parasitic wasps to your garden will help to control their populations naturally.
Armored scale such as oyster shell scale (Lepidosaphes ulmi) can occasionally infest peony plants. These brownish/grey, tiny insects measuring about 3mm (1/8 “) in length suck the plant juices out of the plant causing mottling, yellowing and premature leaf drop. The adults remain stationary, feeding in one spot for the remainder of their life. They cover themselves in a hard scale coating for protection. When populations are high plants may become stunted.
Manage scale populations by scrubbing the scale off using a cotton swab dipped in rubbing alcohol. Spraying the plants in spring with horticultural oil works quite well at suffocating them.
Mealybug occasionally feed on peony. These small oval insects measure between 2-6mm in length and half as wide, depending on species. The adult females, like scale insects, are a stationary feeder. Males are small gnat like flies that do not feed. The females have segmented bodies, waxy filaments extending from their sides and are covered in white mealy secretions. They can be found on the underside of leaves and on stems usually in groups. They feed on the plant juices and a byproduct of their feeding is the excretion of a sticky honey dew that coats the leaves and attracts ants and wasps. If populations are high enough leaves will begin turning yellow and dropping from the plant.
Mealybugs can be washed off with a strong jet of water or wiped off using a cotton swab dipped in rubbing alcohol. Remove badly infested stems, Attract natural predators to your garden such as mealybug destroyers, lacewings, syrphid flies, ladybugs, and parasitic wasps. Insecticidal soap or horticultural oil can be sprayed at the crawler stage but that may also affect your natural predator populations. Reduce nitrogen fertilizer that produces the young succulent growth that they are attracted to and control ant populations who may be transporting the mealybugs to your peony.
Bulb mites and very tiny, measuring about 0.55 to 0.75 mm long and are whitish with brown legs. They feed on damaged and rotting areas of the tubers. Affected areas of the tubers will be soft and a reddish brown colour. Above ground, plants may be stunted or collapse.
Manage bulb mite populations by only planting disease and damage free tubers. Do not dig to close to your peony plants where you could damage the tubers. Remove the tubers and carefully clean them, remove damaged and diseased portions then dip them in a sulfur fungicide solution and let them dry.
Ants are more of a nuisance to gardeners who want to cut peony blooms to bring into the house. Outdoors the ants do no harm to the plant and are attracted to the sugary sap given off by peony flower buds. This sap is a food source for them. No control is warranted but flowers can be washed off or shaken off before bringing into the house.
Peony Leaf Blotch (Graphiopsis chlorocephala)
Peony Leaf Blotch (also called red spot or peony measles)
Peony leaf blotch is caused by the fungal pathogen Graphiopsis chlorocephala (formerly Cladosporium paeoniae). The disease goes by several names depending on the symptoms it is exhibiting. In late spring, the overwintering fungal spores may get splashed up on the upper surface of young leaves and cause small reddish or purplish spots to form on the leaves. As the season progresses the spots enlarge and may coalesce, forming dark purple blotches that are shiny and irregularly shaped. Stems and petioles can also be affected developing red streaks on them. The tissue with the spots may eventually begin to die turning a tan colour with some dark markings. Leaves may distort a bit and if the disease is severe entire leaves can die but remain on the plant. The dark, leathery, irregular leaf spots that form with Botrytis blight look similar but there is an accompanying fuzzy grey mould.
Managing leaf blotch involves removing diseased plant material and black bagging it. Cut herbaceous peony stems back to the ground in the fall. Avoid overhead watering and ensure there is good air circulation so the leaves will dry as quickly as possible.
Powdery Mildew (Erysiphe polygoni)
Powdery mildew on peony is caused by the fungal pathogen Erysiphe polygoni. This is an extremely common disease that causes white powdery growth to form on the leaves, stems and sometimes flowers of peony. The fungus spreads quickly on the wind, particularly in spring and fall when the night temperatures are cooler and humid, and the day temperatures are warmer but less humid. Contrary to popular belief, rain and wet leaves are not the cause of powdery mildew. Actually, water can actually inhibit its growth, as can high temperatures. In severe infections the entire plant can become covered in a thick grey growth that can distort the plants and cause them to yellow and drop foliage.
Managing powdery mildew involves good sanitation, and good fall clean up, including cutting herbaceous peony back to the ground. Ensure good air flow around peony plants and keep plants well-watered, watering in the morning to reduce humidity at night. At the first sign of powdery mildew spray peony with a baking soda and water spray or other sulfur-based fungicide earlier in the morning, before the heat of the day.
Phytophthora Blight (Phytophora cactorum)
Phytophthora Blight of peony is caused by the fungal pathogen Phytophora cactorum. This soil borne fungus thrives in cool wet soils causing young shoots to blacken and rot at soil level. Roots may also rot. Higher up on the plant leaves may develop black leathery areas as well as flowers and buds.
Managing Phytophthora Blight begins with ensuring the site has free draining soil. Remove infected plant material. If roots are also infected remove the entire plant and surrounding soil. If the disease is a problem, begin applying a protective copper-based fungicide early in the growing season. Symptoms of Botrytis blight look similar to Phytophthora Blight, but it has the presence of a fuzzy grey mould.
Botrytis Blight aka Grey Mold or Peony Wilt (Botrytis paeonia)
Botrytis Blight of peony is caused by the host specific fungal pathogen Botrytis paeoniae. 3 other non-host specific Botrytis species which can also infect peony (especially later in the season) are Botrytis cinerea, Botrytis pseudocinerea and Botrytis euroamericana (Agriculture and Horticulture Development Board, (n.d.)). Botrytis paeonia infects newly emerging peony early in spring, during cool rainy weather. Infection usually takes place near the soil line, forming a water soak lesion and a thick layer of grey mould that rots affected tissue causing the stems to quickly wilt and turn brown. Flowers and buds are also very susceptible to infection, bud blast is common, and open flower petals will first develop brown spots that will quickly progress to entire flowers becoming covered in grey mould, collapsing and blackening. Leaves may also be infected developing large irregularly shaped leaf spots, that often starts at the leaf tip and spreads inwards.
Managing Botrytis Blight involves removing infected plant tissue or entire plants and soil when most of the plant is infected. Good sanitation is important to prevent spreading the disease. Ensure soil is free draining and that there is good air circulation around plants. Where this disease is a problem begin applying a protective copper-based fungicide early in the growing season. Avoid overhead watering.
Southern blight aka Crown Rot or White Mold) (Sclerotinia sclerotiorum and Sclerotium rolfsii)
Southern blight aka Crown Rot or White Mold is usually a fatal fungal disease caused by the fungal pathogens Sclerotinia sclerotiorum and Sclerotium rolfsii. This is primarily a disease of tropical and sub-tropical regions, but it occasionally affects cooler regions like northern U.S states and Southern Canada, especially in contaminated plant shipments. This disease affects a wide range of plants. The fungi overwinter and survive in the soil and organic matter for years. In early summer during periods of moisture and high humidity. Peony infected with Southern blight appear much like the 2 previous diseases with water-soaked stem lesions occurring near the soil line that cause the leaves and stems to wilt discolour and collapse. White cottony growth forms on the soil and stem bases which later turns reddish.
Managing Southern blight usually involves the removal of the plant and infected soil. Ensure there is good drainage, avoid overhead watering, and reduce humidity by proper spacing of plant and controlling nearby weeds.
Verticillium Wilt of peony is caused by the fungal pathogens Verticillium alboatru, and Verticillium dahliae. These fungi attack a wide variety of plants causing peony stems to wilt and die. Unlike the former 3 diseases the crown is not damaged.
Managing verticillium wilt involves the removal of infected plants and surrounding soil.
Other Root Rot, Stem Rot, and Crown Rot Diseases
Fusarium, Rhizoctonia, Armillaria and Pythium fungi can also infect and be fatal to peony. Peony become yellowed, stunted, wilt, and die.
Remove infected plants and surrounding soil.
Crown Gall (Agrobacterium tumefaciens)
Crown Gall caused by the soil bacterium Agrobacterium tumefaciens, can infect a wide range of plants. On peony the bacteria enter the plant usually through wounds on stems and roots near the soil line. Once inside the plant the bacteria genetically modifies the DNA of the plant causing uncontrolled cell division, resulting in the smooth, whitish, tumor like growths called galls. The galls slow down the transmission of water and nutrients to the plant, which can cause stunted growth, yellowing of leaves or tip die back. As the galls enlarge and age they become hard and bumpy and turn brown. The bacteria survive in the soil, organic matter and water for long periods of time. For more information visit Crown Gall.
To manage this disease, ensure good sanitation practices, remove diseased plant material and black bag, replace soil or solarize and avoid injuries to plants.
Bacterial Blight (Xanthomonas hortorum)
Bacterial Blight of peony, caused by the bacterium Xanthomonas hortorum, causes purple spotting on leaves and stems. The spots may have reddish or yellow haloes. Spots enlarge and coalesce causing leaves to blight.
To manage bacterial blight of peony, avoid overhead watering, ensure good sanitation practices, ensure there is good air circulation around plants, cut herbaceous peony back in the fall and remove diseased plant material. Apply copper-based fungicides or Bacillus-based biocontrol’s to help suppress the bacteria.
Virus diseases are very difficult to diagnose without lab analysis. The leaves may exhibit ring spots or appear mottled. Plants may appear healthy otherwise or they may be stunted, the leaves may be curling, the stems may be spindly and/or they may not be blooming. Many virus diseases are vectored by insects such as nematodes, mites, thrips and others, or they can be transmitted by tools and hands.
Managing virus diseases often involves removing the infected plant, as there is no cure.
Although there are several virus and virus-like diseases that can infect peony I am only going to briefly touch on a few.
Tobacco Rattle Virus
Tobacco rattle virus, (formerly known as Peony ringspot virus or Peony mosaic virus) is vectored by Trichodorus and Paratrichodorus root nematodes. Infected plants develop yellowish or light green mottling, mosaic patterns or circular patterns on their leaves.
It is considered a mild disorder for which there is no cure.
Tomato Spotted Wilt Virus
Tomato spotted wilt virus, is vectored by thrips. Infected plants develop yellowish to light green mosaic or wavy line patterns on their leaves. This disease has a very large host range.
Management of this disease involves managing thrip populations.
Little is known about this disease and the specific pathogen or combination of pathogens involved. Roots develop a series of small gall-like growths within them. The tissue within these growths is yellowish in colour. Above ground symptoms may involve dwarfing, spindly shoots, failure to flower.
There is no known cure, practice good sanitation practices to help avoid infection.
Other Conditions Affecting Peony
Is a common condition of peony whereby the flower buds form but eventually die rather than open. There are several causes of bud blast such as diseases like Botrytis blight, tubers being planted to deeply, drought, not enough sun, and plants being too immature to bloom.
Peony is fairly draught tolerant once established but they still require at least 1″ of precipitation per week. Symptoms of insufficient water may be wilting, buds may droop, leaves will turn brown (usually beginning at the margins) brown areas will become brittle and leaves may curl.
Peony dislike wet feet and require good drainage. Over watering your peony can cause root rot. Other symptoms may appear as wilting, and brown mushy leaves. Water established peony deeply (about 1″ of water) weekly. Younger plants may require more.
Not Enough Sun
Peony requires 6 to 8 hours of sun per day to flower well. They can grow in shadier sites, but they likely will not flower.
Improper Planting Depth
Herbaceous peony is planted about 2″ deep in colder climates, like my own, and less for warmer climates. Tree peonies that have been grafted however need to be planted deeper to protect the graft union. Plant tree peony at about a depth of 4-6″.
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