Xanthomonas fragariae is a leaf spot disease found in strawberry.
Life cycle and appearance of Angular leaf spot
Xanthomonas fragariae overwinters in plants and dead leaves, that become the primary source of infection in the spring. The bacteria are also introduced in a crop in new transplants. This bacterium is very resistant to desiccation and can survive well in dry leaves or leaves in the soil, but not independantly in the soil. Under humid conditions, bacteria ooze from the lesions on the leaves and form a secondary source of infection. They are dispersed by rain, irrigation water or handling of the plants. Infection of the plants happens both passively and actively. Infection and disease development are favoured by temperatures around 20 °C during the day and cold nights, in combination with high humidity or presence of water. These conditions occur mostly in the spring if it rains or when sprinkler irrigation is used. Healthy, well growing plants are more susceptible than diseased or stressed plants.
Xanthomonas fragariae initially causes small, water-soaked lesions on the underside of the leaf. They enlarge to angular spots, often limited by the small veins. The distinctive feature is the way the lesions look when using different light sources: when reflected light is used, the lesions look dark green, but when transmitted light is used, they are translucent. When conditions are humid, a viscous bacterial exudate forms on the underside of the leaf. This exudate changes to a white film when it dries. Lesions may merge as they grow and later also become visible on the upper side of the leaf as irregular spots which are first reddish brown and then become necrotic. The foliar symptoms are the most important, but also all types of vascular tissue of the crown can be infected, creating a look similar to that of a Phytophthora fragariae infection.