Symptoms and Signs
Infection first appears as minute, water-soaked spots on the lower surface of leaves and calyxes. The lesions enlarge to form translucent, angular spots that are delineated by small veins and under conditions of high humidity will exude a viscous ooze of bacteria and bacterial exudates, which appears as a whitish and scaly film after drying. As the disease progresses, lesions coalesce and reddish-brown spots, which later become necrotic, appear on the upper surface of the leaves. A chlorotic halo usually surrounds the infected area.
Comments on the Disease
This bacterium is not free-living in soil. It can, however, overwinter in soil on previously infected plant material. It is host-specific and highly resistant to degradation (i.e., it can persist on host plant debris in the soil for long periods of time). Lesions on the leaf surface serve as a source for secondary inoculum; cells are dispersed by splashing rain or overhead irrigation. The disease is favored by cool, moist days with cold nights near freezing.
Xanthomonas fragariae can cause vascular collapse, although this is uncommon in California. This symptom initially appears as a water-soaked area at the base of newly emerged leaves. Shortly after, the whole plant suddenly dies, much like plants infected with crown rot. Xanthomonas fragariae is also associated with strawberry blossom blight in California.
Angular leaf spot generally has a minor impact on fruit yields in California. However, it is a concern at strawberry nurseries, which may be subject to quarantine regulations for angular leaf spot on nursery stock for export.
Angular leaf spot is kept to a minimum by using certified planting materials. Chemical controls are typically ineffective against this pathogen. Copper-containing compounds are registered but have caused phytotoxicity with repeated applications. Rotate crops to avoid infesting fields and avoid overhead irrigation when possible.
Organically Acceptable Methods
Use certified plants, use drip irrigation instead of overhead irrigation, and rotate crops in organically grown strawberries.