Bacterial Spot

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.

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.

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.

• 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.