Common Ornamental Trees Disease

Ornamental trees or plants are plants that are grown for decorative purposes in gardens and landscape design projects, as houseplants, for cut flowers and specimen display. The cultivation of these, called floriculture, forms a major branch of horticulture.

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  • Downy mildew diseases
  • Powdery mildew diseases
  • Rust diseases
  • Grey mold
  • White mold
  • Root rots

Downy Mildew of Grape

  • Caused by Plasmopora viticola

Downy Mildew – A (little) History

  • M.A. Millardet (a French botanist) first used Bordeaux mixture (copper sulfate and lime) to control downy mildew in the vineyards of France
  • He noticed that a copper sulphate-lime powder mixture that was sprinkled on grapevines along highways to prevent stealing of the grapes also controlled downy mildew
  • This observation led to the discovery and development of Bordeaux mixture

Downy Mildew – Symptoms

  • Early symptoms Initially, leaf spots are pale yellow on upper surface

Late symptoms

As lesions age, they become a mosaic of yellow, red, and brown angular spots on the upper leaf surface.

Off-color, diseased fruit shrivel and become covered with white or cream-colored, downy fungal growth.

Downy Mildew – Management

  • Promote well-drained soils, Reduce overwintering infective structures (oospores in fallen leaves, hyphae in buds), Prune out infected shoots
  • Avoid overhead irrigation or practices that maintain wet foliage for extended periods of time
  • Fungicides:
  • In the same group of ‘water mold’ organisms as the late blight pathogen – so chlorothalonil and copper-based products are effective – apply before disease becomes established
  • Applications generally begin before budbreak or at ½-inch shoot length or before bloom (depending on the fungicide)

Downy Mildew of Cucurbits

  • Caused by Pseudoperonospora cubensis

Powdery Mildew Diseases

  • Caused by a related group of host-specific fungi

Powdery Mildew Diseases

  • Each powdery mildew fungus has a narrow host range
  • Main hosts in North Dakota: lilacs, peas, roses, and cucurbits
  • Note: lilac powdery mildew doesn’t infect rose, and so on
  • Brown to black, nearly spherical survival structures of the fungus (cleistothecia)

Favorable conditions and symptoms:

  • Cool, dry periods after prolonged leaf wetness
  • High relative humidity
  • Free moisture not required for infection (unlike most other plant pathogens)
  • White or grayish powdery growth on leaves, stems, or fruit
  • Extreme symptoms = dwarfing, distortion, chlorosis, premature leaf drop, blemishes on fruit

Powdery Mildews – Management

  • Increase air circulation – proper plant spacing, pruning
  • Plant areas with at least 6 hours full sun
  • Plant in well-drained soil
  • Avoid water splashing (sprinklers) – water from the bottom, use soaker hoses
  • Host resistance is reportedly available
  • Purchase plants that look healthy (disease-free)
  • At normal pruning time, remove and destroy diseased terminals of woody plants (lilacs, roses)
  • Rake up and destroy fallen leaves

Fungicides are available

  • Chlorothalonil – for garden vegetables like cucurbits
  • Sulfur-based compounds such as Safer® Brand Garden Fungicide – labeled for some ornamentals and edibles
  • Tebuconazole such as Bayer Advanced Disease Control for Roses, Flowers, and Shrubs – DO NOT USE ON EDIBLE CROPS

Rust Diseases

  • Caused by a related group of host-specific fungi

Rusts on ornamentals

  • May require 2 hosts
  • Very specialized pathogen
  • Typical symptom
  • Orange pustules: powdery masses of yellow, orange, purple, black, or brown spores
  • Usually on leaf underside.
  • Swellings and galls
  • Favored by temperate conditions that also favor host


Common Rust Diseases

  • Aster
  • Achillea
  • Chrysanthemum
  • Sunflowers
  • Liatris
  • Snapdragons
  • Poplar
  • Ash
  • Dry Beans
  • Apple/Crabapple
  • Lilies
  • Irises
  • Rosaceae
  • Grasses
  • Others

Some Rust Pathogens Require Two Different Hosts

  • Cedar apple rust
  • Main host: apple and crabapple
  • Alternate host: juniper and cedar

Rusts that require 2 hosts

  • Cedar apple rust
  • Cedar/junipers and apple/crabapples
  • Wheat stem rust
  • Barberry and wheat
  • Ash rust
  • Cordgrass and ash
  • Oat crown rust
  • Buckthorn and oat


Rusts that require only 1 host

  • Some of the rose rusts
  • Dry bean rust
  • Pea rust
  • Sunflower rust

Managing Rust Diseases on Ornamental trees

  • Protectant fungicides: 1 to several applications before bloom or bud break
  • Usually only if a chronic problem
  • Eradication of alternate host
  • Not always practical
  • Separate susceptible junipers and rosaceous hosts in nurseries, landscapes
  • Prune out rust galls
  • Prune out infections
  • Use genetic resistance
  • Junipers
  • Other plant species
  • Avoid overhead irrigation

Grey Mold

  • Caused by Botrytis

Symptoms of Grey Mold

  • Leaf and petal spots – flecks to completely blighted tissue
  • Fuzzy, gray lesions
  • Possible zonate pattern
  • May result in brown, mushy tissue
  • Favored by cooler temperatures, moisture, and weakened plant tissue

Grey Mold – Management

  • Prevent introduction
  • Manage leaf wetness, maintain or create adequate air circulation
  • Sanitation
  • Prune out and destroy diseased, decaying, or dead plant material
  • Remove and destroy plant tops in fall
  • Apply fungicides in spring

White Mold on Ornamental trees

  • Caused by Sclerotinia sclerotiorum

White Mold on Various Hosts

  • Wide host range
  • Favored by moist conditions

White Mold – Management

  • Increase aeration between plants (decrease RH)
  • Avoid overhead irrigation
  • Irrigate in morning rather than evening to allow foliage to dry out
  • Prevent buildup of sclerotia, which can survive in soil for many years
  • Remove and destroy infected plants (don’t compost)
  • Deep plowing
  • only sclerotia in top 2 – 3 inches of soil germinate
  • Fungicide soil drenches, if perpetual problem
  • Protectant foliar-applied fungicide

Root Rots

  • Caused by various soil borne pathogens

Caused by soil-borne pathogens that attack roots

  • Roots may be discolored internally or externally
  • Symptoms may include wilting, stunting, yellowing
  • Symptoms vary, depending on the pathogen
  • Some root rot diseases cause a light brown discoloration on roots
  • Roots may be mushy or not
  • Others cause distinct, dark reddish-brown lesions on roots

Root Rot Management on Ornamental trees

  • Select and plant healthy material
  • Plant in well drained soils
  • Don’t over water
  • “Hill up” infected plants to promote adventitious root production above rotted area

Virus Diseases of some Ornamental trees

  • Movement from plant to plant can occur in different ways, depending on the virus
  • Some are mechanically transmitted – human touch, pruning activity, shovels, and so on
  • Tobacco Mosaic Virus (TMV) – wide host range, carried on cured tobacco and transferred easily to human hands through smoking, chewing, etc
  • Hosta Virus X (HVX) – primarily hosta, transmitted through wounds via contaminated pruning tools
  • Some are transmitted by insects, other arthropods, fungi, or nematodes
  • Impatiens Necrotic Spot Virus (INSV) – wide host range, vectored by thrips
  • Cucumber Mosaic Virus (CMV) – wide host range, vectored mechanically and by aphids

Managing Virus Diseases of Ornamental trees

  • Not curable
  • Prevention is key
  • Purchase healthy-looking plants
  • Remove and destroy infected plants
  • Manage potential insect/arthropod vectors
  • Sanitize tool

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