Bacterial spot of stone fruits

Xanthomonas arboricola is a devastating bacterial pathogen responsible for various diseases in fruit and nut trees. This bacterium colonizes the intercellular spaces of plant tissues, leading to the development of necrotic lesions. It is highly specialized, with different pathovars targeting specific plant hosts, such as walnut (juglandis) or stone fruits (pruni).

The primary hosts include species from the Prunus genus, such as peach, nectarine, apricot, and cherry, as well as the walnut tree. The pathogen is widespread globally and poses a significant threat to nursery operations and fruit production, as it can cause complete defoliation and severe fruit damage, rendering the crop unsaleable.

Symptoms are typically observed as small, angular, water-soaked spots on leaves that turn brown or black over time. Fruit infections start as tiny spots that evolve into deep, unsightly scabs or cracks, often accompanied by bacterial ooze. On branches, the infection causes cankers, which serve as long-term reservoirs for the bacteria within the tree's vascular system.

Environmental conditions such as high humidity, frequent rainfall, and moderate temperatures are ideal for the development and spread of Xanthomonas arboricola. The bacteria move through water splash, wind, and contaminated tools. Once established in an orchard, the pathogen can persist in dormant buds, fallen leaves, and diseased bark, making early detection critical.

Preventative management is the most effective way to combat this bacterial disease, as chemical control options are limited. Key agricultural strategies include:

• obtaining certified pathogen-free nursery stock;
• pruning during dry weather and disinfecting tools with effective bactericides;
• applying copper-based protective sprays during the dormant season;
• maintaining tree vigor to improve natural resistance;
• removing and destroying infected plant debris to limit the primary inoculum source.