Agriculture: Almond Pest Management Guidelines

Bacterial Spot

  • Xanthomonas arboricola pv. pruni
  • Symptoms and Signs

    Fruit infections begin as small water-soaked, circular spots. Infections may enlarge slightly and become necrotic and continue to develop into the mesocarp, endocarp, and endosperm. Infection sites on the fruit often profusely exude amber-colored gum. Multiple lesions may develop on a fruit and cause the fruit to drop. Persistent lesions containing the pathogen are found on mummified fruit.

    Leaf symptoms are less common than fruit symptoms. On leaves, small water-soaked circular lesions [0.02–0.08 inch (0.5–2 mm diameter)] develop mainly along the midrib and toward the tip and margin of the leaf. Lesions become chlorotic and then turn brown and necrotic. Some of them abscise, creating small holes with the appearance of shot holes. Lesions may coalesce to create larger, irregular lesions. Under severe disease severity, some defoliation of trees may occur.

    Comments on the Disease

    Bacterial spot is caused by Xanthomonas arboricola pv. pruni. While the disease is a major problem on stone fruit crops (especially peach) in the eastern United States, it is a relatively new disease of almond and other Prunus spp. in California (first reported in 2013). The disease is found in the Sacramento and northern San Joaquin Valleys.

    Fruit mummies are common bacterial spot overwintering sites in almond, but twig cankers are considered additional possible sites to harbor the pathogen during tree dormancy. In early spring as temperatures warm, the pathogen begins to multiply. However, under California conditions, infection of young green shoots in the spring, and subsequent development of small twig cankers, rarely occurs.

    The bacteria are spread from mummified fruit by dripping dew and by water splashing or wind-driven rain to the newly emerging leaves and developing fruit. The pathogen infects through natural openings or wounds. High-moisture conditions and warm (greater than 68ºF) temperatures are very favorable for infection. Severe fruit infections are more common with frequent periods of rainfall or irrigation during fruit development.

    Fritz is one of the most susceptible almond cultivars. Others, such as Nonpareil, Butte, Carmel, and Price are also affected although with generally lower disease severity. The disease is also occasionally found on other stone fruit crops at low incidence.

    Management

    Delayed dormant and in-season treatments with copper products and mancozeb to protect immature, developing fruit are highly effective. Low rates or reducing rates of copper with each application are effective strategies to prevent copper phytotoxicity to almond leaves. There are no reports of copper resistance in populations of this pathogen in California; however, resistance has been reported elsewhere. Rotate or mix copper applications with other modes of action when possible.

    Cultural Control

    • Use strategies to improve air movement to reduce relative humidity in the orchard.
    • Design irrigation systems to reduce or prevent wetness of the tree canopy.
    • Follow good sanitation practices, including
    • dormant fruit mummy removal from trees and mowing to grind up infected fruit.
    • cleaning harvesting equipment carefully to prevent the movement of infected fruit between orchards.
    Common name Amount per acre REI‡ PHI‡
    (Example trade name) (hours) (days)
    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.
    DORMANT
    A. COPPER#
    (various products) Label rates See label See label
    MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
    . . . OR . . .
    COPPER#
    (various products) Label rates See label See label
    . . . PLUS . . .
    NARROW RANGE OIL#
    (various products) Label rates See label See label
    MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1) + Contact
    COMMENTS: Not all copper and oil products are organically acceptable; check labels. Apply 1 to 2 lb metallic copper equivalent (MCE)/acre.
    B. MANCOZEB
    (Manzate Pro Stick) 4–6.4 lb 24 145
    MODE-OF-ACTION GROUP NUMBER1:
    COMMENTS: Apply with copper (1–2 lb metallic copper equivalent (MCE)/acre) and 3 to 4.8 lb ai/acre mancozeb. Do not apply more than 19.2 lb of product (14.4 lb ai)/acre per season.
    BLOOM AND PETAL FALL
    A. COPPER
    (various products) Label rates See label See label
    MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
    COMMENTS: Not all copper and oil products are organically acceptable; check with certifier. Apply with reducing rates of copper (1, 0.5, 0.25 lb metallic copper equivalent (MCE)/acre) from full bloom through 5 weeks after petal fall if environmental conditions favor disease.
    B. MANCOZEB
    (Manzate Pro Stick) 4–6.4 lb 24 145
    MODE-OF-ACTION GROUP NUMBER1: Multi-site contact (M3)
    COMMENTS: Can be applied alone at 4 to 6.4 lb product/acre (3–4.8 lb ai/acre) or with reducing rates of copper (1, 0.5, 0.25 lb MCE/A) with each application from full bloom through 5 weeks after petal fall if environmental conditions favor disease. Do not apply more than 19.2 lb of product (14.4 lb ai)/acre per season. Full bloom and two petal fall applications (a minimum of 7 days apart) prior to rain events provide excellent control.
    C. BACILLUS SUBTILIS strain QST 713
    (Serenade Opti, Serenade Optimum) 14–20 oz 4 0
    MODE OF ACTION: Biopesticide
    COMMENTS: The product is best used under low disease pressure and can be mixed and applied with copper. When using copper, use reducing rates (e.g., 1, 0.5, 0.25 lb metallic copper equivalent (MCE)/acre with each application) from full bloom through 5 weeks after petal fall if environmental conditions favor disease.
    D. STREPTOMYCES LYDICUS strain WYEC 108
    (Actinovate AG) 3–12 oz 1 0
    MODE OF ACTION: Biopesticide
    COMMENTS: The product is best used under low disease pressure Applications should be made from full bloom through 5 weeks after petal fall if environmental conditions favor disease.
    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. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
    * Permit required from county agricultural commissioner for purchase or use.
    # Acceptable for use on organically grown produce.
    1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. 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 a fungicide with a different mode-of-action group number.
    Text Updated: 08/17
    Treatment Table Updated: 08/17