Risk of bacterial blast of almond is high when freezing and wet conditions occur during bloom

Florent Trouillas, Associate Professor of Cooperative Extension, Department of Plant Pathology, University of California, Davis

Tawanda Maguvu, Postdoctoral Researcher, Department of Plant Pathology, University of California, Davis

Jim Adaskaveg, Professor of Plant Pathology, Department of Microbiology and Plant Pathology, University of California, Riverside

2/28/2024: Updated to include the Kasugamycin Section 18 approval for 2024

Bacterial or blossom blast of almond is caused by the bacterial pathogen Pseudomonas syringae pv. syringae and can be devastating in orchards when freezing temperatures combined with high humidity occur during bloom. The disease can affect all Prunus species including almond, apricot, peach, European plum and French prune, and sweet cherry. Severe outbreaks of bacterial blast were reported in 2023 in almond orchards throughout the Sacramento and San Joaquin valleys of California. As bud break and the almond bloom are approaching in the central valley of California, concern is rising among farmers about a possible recurrence of bacterial blast. Here, we summarize conditions that lead to high risk of bacterial blast in almond orchards and share knowledge about the disease biology and best management strategies.

Symptoms

Bacterial blast and bacterial canker of almond are two phases of a disease that can affect most parts of the almond tree, including flowers, leaves, as well as trunk and branches. Bacterial blast causes a sudden withering of blossoms that turn brown while remaining attached to the tree (Fig. 1). Infected shoot tips may become necrotic and exude gum; infected buds may die and fail to push or grow in the spring. Symptoms on young leaves include necrotic lesions with a surrounding chlorotic halo (Fig. 2). Bacterial blast infections can progress into spurs, twigs and branches, killing the next growing season fruiting wood and leading to the canker phase of the disease. Bacterial canker can produce amber-colored gum balls with underlying necrotic tissues (the canker) in the bark of trunks and branches. Islands of necrotic tissues (flecks) develop at the canker margin and eventually coalesce to form a canker of dead phloem tissues. Large cankers and extensive infections of trunk may cause tree death. However, the rootstock portion of the tree generally survives, producing several suckers around the base of the tree, as cankers generally do not extend below the soil line. Although both phases of the disease can be devastating to almond, blossom blast is generally more harmful to an orchard as the disease can significantly reduce the number of flowers and the overall fruit set, thus leading to a significant reduction in yield.

Fig. 1. Blossom blast resulting from an infection caused by Pseudomonas syringae pv. syringae. Blossoms wither, turn brown and remain attached to the twig. Leaves may develop reddish or necrotic spots with chlorotic halos. Shoot infections may result in death of the entire spur and lead to twig cankers.

Biology of the causal organism

In California, bacterial blast and canker is caused by the ubiquitous bacterial pathogen P. syringae pv. syringae. This bacterium occurs commonly on plant tissues in most almond orchards.  P. syringae lives as an epiphyte on aerial plant parts of almond trees and on weeds. During the epiphytic phase, the bacteria cause no harm to the almond tree. The pathogen only kills blossoms when freezing and wet conditions occur during bloom. Under these favorable conditions, the pathogen multiplies rapidly on plant tissues and is disseminated to infection sites by water splashing during rain. P. syringae pv. syringae strains produce an ice nucleation protein that can trigger ice crystal formation at temperatures where water would normally remain liquid. This results in freezing injuries and wounds on flowers that allows bacterial entry, leading to the blossom blast phase of the disease. P. syringae pv. syringae also produces cell-wall-degrading enzymes and various toxins in infected plant tissues that function as virulence factors and further exacerbate disease symptoms. Trees with poor vigor and stressed trees, including trees in sandy soils with high populations of ring nematodes and trees with low nitrogen nutritional status are more prone to damage. Populations of P. syringae have been shown to decline quickly on plant surfaces following hot summer days, however, the bacteria can survive in cankers and dormant buds, thereby serving as inoculum for the following growing season.

Fig. 2. Bacterial blast causing necrotic lesion with a surrounding chlorotic halo on young, developing leaves.

Management of bacterial blast

Almond growers with susceptible cultivars (e.g., Independence, Aldrich and Monterey) or with orchards that have a history of bacterial blast are more at risk for the disease, particularly if conditions during bloom are cold and wet. In that case, growers should be prepared to respond to the possibility of blast. Cultural practices that reduce tree stress combined with the use of a bactericidal antibiotic can decrease the risks and severity of bacterial blast in almond. Protecting trees from frost damage by using sprinkler irrigation or machines to produce good airflows in orchards can lessen bacterial blast. However, if temperatures drop too low, no treatment will be effective.

Recent work by Dr. Jim Adaskaveg (UC Riverside) indicated that the use of the antibiotic kasugamycin (Kasumin®, UPL, USA) prior to a forecasted frost event during bloom can significantly reduce blossom blast. Kasugamycin is an aminoglycoside antibiotic (bactericide) produced by the soil bacterium Streptomyces kasugaensis and initially isolated in Japan. Kasugamycin has a preventive, systemic localized activity and a unique mode of action that allows it to inhibit the proliferation of other bacteria by interfering with their ability to make proteins. Currently, kasugamycin is not labeled for almond under a full registration (Section 3) but an emergency exemption registration under Section 18 was recently approved for Kasumin® in 2024. Kasugamycin provides a protective layer against the blast bacterium but does not have any beneficial impact once the disease has established. Kasugamycin can be applied as a foliar treatment during bloom and up to 7 days prior up to a forecasted frost event with wet conditions. If the risk of bacterial blast is high in an orchard, two applications may be warranted. A strong disease pressure is considered when multiple frost events at 7- to 10-day interval and wet conditions are forecasted during bloom. Currently there are no reports of P. syringae populations resistant to kasugamycin in California, making this antibiotic the most effective compound to control bacterial blast. For best efficacy, kasugamycin must be applied within 7 days prior to a frost event and at times during the day with low light intensity to limit photodegradation.

Copper is usually less effective for preventing bacterial blast in most almond orchards as copper resistance in almond populations of P. syringae is widespread in California. Moreover, copper applications can be phytotoxic to almond blossoms and green tissues such as young leaves causing injury resulting in spotting, marginal necrosis, and leaf drop. 

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