Agriculture: Floriculture and Ornamental Nurseries Pest Management Guidelines

Rust

  • Rust: Phragmidium spp., Puccinia spp., and others
  • Symptoms and Signs

    Rust pustules appear as powdery masses of yellow, orange, purple, black, or brown spores on leaves and sometimes on stems. Depending on the particular plant host, pustules can be found on either side of leaves. Severe rust can result in dried, dead foliage. The most significant impact of rust disease is the reduction of quality and marketability of the commodity.

    Comments on the Disease

    The rust fungi are obligate parasites in the order Uredinales. Some have complicated life cycles that include up to five different spore stages and two distinctly different hosts. Other rusts produce less than five or sometimes only one type of spore and infect only one kind of plant. Stem rust (Puccinia graminis) of wheat alternates between barberry (Berberis spp.) and wheat (Triticum vulgare) and all five spore stages are produced. Rose rust produces four spore stages but only roses (Rosa spp.) are infected.

    Urediniospores ("repeating" spores) are produced in pustules that appear yellow, orange, or brown as a result of masses of spores. These spores can re-infect the same host that produces them, making them the damaging phase of most rust diseases. Urediniospores are windborne and infect the plant through stomata. Water is required for short periods (6–8 hours or less) for germination and infection. Heavy dew is often sufficient. Once infection has occurred, water is no longer needed for continued development, and the infection and spore production will continue for the life of the leaf.

    Some rusts, including rose (Phragmidium tuberculatum) may survive the winter on leaves that do not fall off the plant. Rose rust also survives as teliospores (dark, thick–walled, overwintering spores) that form in fall. These spores survive in a dormant stage on fallen leaves.

    Management

    Rust diseases are favored by moderate temperatures that favor the growth of the host. Rust spores can be killed by high temperatures. Some rust infections, such as geranium rust (Puccinia pelargonii-zonalis), can be eradicated by hot water treatment of cuttings, although some damage to the host can occur.

    Because water is necessary for infection, overhead irrigation should be avoided when rust is a problem. Eliminate alternate hosts.

    Chrysanthemum white rust, caused by Puccinia horiana, and Gladiolus rust, caused by Uromyces transversalis, are under an eradication program in California. Gladiolus rust also causes disease on other members of the iris family such as Crocosmia, Tritonia, and Watsonia. See specific guidelines under the DISEASE CONTROL OUTLINE FOR CHRYSANTHEMUM and DISEASE CONTROL OUTLINE FOR GLADIOLUS.

    There are several fungicides that can be used to protect plants from infection. Mancozeb as a protectant and myclobutanil as an eradicant are generally effective against all rusts while triadimefon is effective against only specific rusts.

    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. TEBUCONAZOLE
    (Torque) 4–10 fl oz/100 gal water 12
    MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
    COMMENTS: A systemic fungicide applied as a foliar spray; both a protectant and eradicant.
     
    B. MANCOZEB
    (Dithane 75 DF) 1–1.5 lb/100 gal water 24
    MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M 03)
    COMMENTS: Provides protection only; must be applied before infection. Protects against leaf spots, Botrytis, rusts, and blight. Thorough coverage is important for control.
     
    C. MYCLOBUTANIL
    (Rally 40WSP) 3 oz/50 gal water 24
    MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
    COMMENTS: A systemic fungicide applied as a foliar spray; both a protectant and eradicant of rusts or powdery mildew on carnations, crape myrtle, gerbera, roses, and snapdragons.
     
    D. BOSCALID/PYRACLOSTROBIN
      (Pageant) Label rates 12
      MODE OF ACTION GROUP NAME (NUMBER1): Carboximide (7) and quinone outside inhibitor (11)
     
    E. FLUOXASTROBIN
      (Disarm 480 SC) 1–4 fl oz/100 gal 12
      MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
     
    F. TRIADIMEFON
    (Bayleton Flo) Label rates 12
    MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
    COMMENTS: A long-lasting systemic fungicide that provides for general control of some powdery mildews, some rusts, and leaf blight and spots in greenhouses and commercial nurseries. Because this material is closely related to growth retardant materials, it can have a toxic effect on certain plants, such as greenhouse roses.
     
    G. AZOXYSTROBIN
    (Heritage) 1–4 oz/100 gal water 4
    MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
    COMMENTS: Apply as a broadcast or banded spray targeted at the foliage or crown of the plant. A locally systemic fungicide that is effective against rusts.
     
    H. REYNOUTRIA SACHALINENSIS#
      (Regalia CG) Label rates 4
      MODE OF ACTION GROUP NAME (NUMBER1): Anthraquinone elicitor (P 05)
     
    I. WETTABLE SULFUR# 1–3 lb/100 gal water 24
    MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M 02)
    COMMENTS: Use a wetting agent. Not as effective as other materials. Apply this material with caution when temperatures exceed 85°F.
     
    J. CHLOROTHALONIL
    (Chlorothalonil 720 SFT) 1.375 pt/100 gal water 12
    MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M 05)
    COMMENTS: Provides protection only; must be applied before infection.
     
    K. PROPICONAZOLE
    (Banner Maxx II) 2–16 fl oz/100 gal 12
    MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
    COMMENTS: Controls white rust of chrysanthemums but may cause phytotoxic symptoms on some cultivars.
     
    L. NEEM OIL
    (Triact 70)# Label rates 4
    MODE OF ACTION GROUP NAME (NUMBER1): (NC)
    COMMENTS: Registered for landscape and nursery ornamentals, neem has some protectant properties against rust, but is not effective for rust on rose. Apply on a 14-day schedule.
    * Permit required from county agricultural commissioner for purchase or use.
    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.
    # Acceptable for use on organically grown ornamentals.
    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.
    Text Updated: 11/20
    Treatment Table Updated: 11/20