Epidemiology and Management of Bacterial Spot of Almond Caused by Xanthomonas arboricola pv. pruni, a New Disease in California

Abstract

Bacterial spot caused by Xanthomonas arboricola pv. pruni was first detected on almond in California in 2013, and it is reported herein as a new disease in California based on fulfilling Koch’s postulates and identification of the pathogen using species-specific PCR primers. Infected mummified fruit from the previous growing season and their peduncles were identified as primary overwintering sites of the bacterium on the tree. Twig cankers were not observed, and the pathogen was not recovered from dormant buds. Isolation from flowers and emerging leaves was only successful when they were collected within 20 cm of an infected, mummified fruit on the tree. Inoculation of flowers and immature fruit as well as immature and mature leaves resulted in disease development, indicating a long period of host susceptibility in the spring, but disease incidence was highest in fruit inoculations. In split-plot trials over 3 years, dormant applications in December or January with copper or copper-mancozeb significantly reduced the disease compared with untreated controls in seasons with high rainfall, but they had no effect in seasons with low rainfall. In-season applications of copper-mancozeb at petal fall or at full bloom and petal fall were also effective in reducing the disease. Phytotoxicity was observed after repeated applications of copper bactericides, especially in low-rainfall seasons. Dormant and in-season treatments of copper-mancozeb mixtures integrated with removal of mummified fruit are currently the best management strategies for bacterial spot of almond in California.

Characterization of Xanthomonas arboricola pv. pruni from Prunus spp. orchards in Western Balkans

“…The diseased area drops out, giving a shot‐hole appearance, and affected leaves turn yellow and fall prematurely. Bacterial spots on fruit take the form of water‐soaked or dark brown sunken lesions (Haack et al, 2020; Iličić & Popović, 2021). Severe defoliation and spots on the fruit reduce its commercial value, while prematurely defoliated trees become more susceptible to low winter temperatures.…”

Proteome Analysis of Walnut Bacterial Blight Disease

“…Copper-based biocides are effective in many pathosystems involving Xanthomonas spp., although some Xaj isolates are especially resistant [ 23 , 24 ]. Some pathosystems such as the bacterial blight of rice, bacterial spot, and black rot diseases have been studied in molecular detail, revealing specific plant-microbe protein interactions [ 25 , 26 , 27 , 28 , 29 ].…”