Agriculture: Floriculture and Ornamental Nurseries Pest Management Guidelines

Bacterial Leaf Spots, Blights, Cankers, and Rots

  • Bacterial blight of chrysanthemum: Dickeya (=Erwinia) chrysanthemi
  • Bacterial blight of geranium: Xanthomonas axonopodis pv. pelargonii (formerly X. campestris pv. pelargonii)
  • Bacterial leaf spot of begonia: Xanthomonas axonopodis pv. begoniae (formerly X. campestris pv. begoniae)
  • Bacterial leaf spot of poinsettia: Curtobacterium flaccumfaciens pv. poinsettiae
  • Bacterial wilt of carnation: Burkholderia (=Pseudomonas) caryophylli
  • Black leaf spot of delphinium: Pseudomonas syringae pv. delphinii
  • Symptoms and Signs

    Foliar diseases caused by plant pathogenic bacteria result in a wide range of symptoms that vary greatly depending on the particular pathogen and the host plant. Leaf spots (also called leaf lesions) are discrete diseased sections of leaves that initially appear water-soaked, but later turn yellow, brown, or black. Leaf spots are usually angular in shape and bordered by the veins in the leaf. Leaf petioles can also develop such spots. Merging of numerous leaf spots results in the infection of large portions of the foliage; such symptoms are called blights. Rots occur when the bacteria infect fleshy stems, crown, bulbs, corms, and other parts of plants and cause a soft, watery decay. Infections on twigs and branches can cause cankers, which are sunken, discolored, and cracked areas in the woody tissue. In some cases, bacteria inside the diseased tissue will ooze to the surface of the plant and are visible as cream to yellow colored exudates, collecting outside the plant.

    Comments on the Disease

    There are many different kinds of bacterial diseases of flower and nursery crops. In many cases these bacteria are host specific and will only infect one plant species. In other cases, for example with soft rot bacteria, the pathogen is able to infect a larger number of plant hosts. All bacteria in this section can be spread through infected cuttings and other propagative material, and some of these bacteria may be carried in seed and irrigation water. Bacteria are dependent on splashing water for their dispersal and for creating a suitable environment for infection and disease development.

    Management

    Use pathogen- and disease-free seed, cuttings, transplants, and other propagative materials.

    • Implement sanitation measures when dealing with containers, flats, benches, pruning tools, and other items that come in contact with plants.
    • Avoid using overhead sprinkler irrigation.
    • Rogue out (remove) diseased cuttings, transplants, and plants.
    • Sanitize hands or use disposable gloves when handling diseased plants.
    • Prevent injuring bulbs, corms, and other fleshy parts of plants to avoid soft rot problems.

    Copper-based fungicides may provide some control but can be phytotoxic to some ornamentals.

    Common name Amount to use REI‡
    (Example trade name) (hours)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
     
    A. FIXED COPPER
    (Kocide 2000)# Label rates 48
    MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M 01)
    COMMENTS: A protectant fungicide. Growth of some plants may be reduced by this material; follow label directions carefully to reduce the risk of phytotoxicity. Check label for registered use ornamental species list in California. Not all copper compounds are approved for use in organic production; be sure to check individual products.
    1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action group number.
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing.
    # Acceptable for use on organically grown ornamentals. Check with your certifier.
    Text Updated: 11/20
    Treatment Table Updated: 11/20