**5.1** *Xanthomonas fragariae* **(***Xf***) Kennedy and King, 1962, Bacterial angular leaf spot**

Angular leaf spot is a potentially threatening disease of strawberry. The causative agent was identified as *Xanthomonas fragariae* and was firstly reported in the USA [72]. *Xanthomonas fragariae* is regulated as a quarantine organism in most EU countries [73]. The disease starts with small water-soaked blotches on young leaves. These symptoms form angular spots. These spots are usually bordered by small veins. Observing the lesions appear dark green under light, but using transmitted light, they become translucent. In high humidity and high-temperature conditions (over 20°C), a sticky bacterial ooze forms on the leaves. Disease lesions may coalesce as they grow and then appear as irregular stains on the upper side of the leaf. Reddish-brown lesions then become necrotic. Vascular tissue in the trunk may also be infected [74]. Bacteria cause latent infections by moving systemically via the vascular system of the plant [75]. Infected plants become less productive. The disease can result in up to 10% crop loss in strawberry yield. Plants may even die in severe infections [76]. Plants that are infected systemically produce the first infected leaves, and they served as the primary inoculum source in newly planted fields [77]. Disease symptoms may

### *Pests, Diseases, Nematodes, and Weeds Management on Strawberries DOI: http://dx.doi.org/10.5772/intechopen.103925*

be confused with fungal diseases such as *Mycosphaerella fragariae* and a new pathovar of *X. arboricola* pv. *fragariae* [78]. *Xanthomonas fragariae* overwinter in plant debris by serving as the source of infection. The bacterium is resistant to desiccation and can easily survive on dry leaves in the soil but not independently in the soil. It creates a secondary infection in moist conditions. Infection of plants occurs both passively and actively. It is spread through rain, irrigation water. Daytime temperatures of around 20°C and cold nights, combined with high humidity or the presence of water, provide a favorable environment for infection and disease to develop [79]. Bacteria may survive for up to 2 weeks on metal and wood materials. That is why agricultural machines may carry the pathogen during an important period of time if not suitably disinfected. Machinery contamination with the bacterial ooze may cause the spread of *X. fragariae* infections [80]. *X. fragariae* is gram-negative bacterium. It is rod-shaped (0.4 × 1.3 μm size), non-spore-forming, and non-capsulated bacterium. Most cells of the bacterium are non-motile, but some of them have a single polar flagellum. Colonies are circular, entire, convex in shape, and glistening, translucent to pale-yellow on beef-extract-peptone agar [81]. Direct isolation of the bacterium on artificial nutrient media is difficult because of very slow growth. Wilbrink's medium with nitrate (Wilbrink-N) is recommended for the most suitable growth medium for isolation of the bacteria [82, 83]. Rapid screening tests based on serological (e.g., indirect immunofluorescence, (ELISA), and molecular methods are used in diagnosis. For confirmation of diagnosis, positive results in serological and molecular tests should be obtained. Several polymerase chain reaction (PCR) detection tests have been improved targeting diverse loci of the bacterial genome [84, 85]. To approve the incidence of *X. fragariae* in symptomatic plant material and latent *X. fragariae* infections and several of these tests have also been used [84–90]. Planting using certified disease-free propagation material is recommended for preventing disease occurrence [91]. Using immune strawberry cultivars such as *F. moschata* instead of susceptible ones (*Potentilla fruticose, P. Glandulosa, F. vesca,* and *F. virginiana*) is recommended [92, 93]. Eliminating infected leaves is important to reduce inoculum sources of bacteria. Copper compounds can be used for the chemical control of *X. fragariae*. Because of resistance established by the bacterium, these compounds must be applied at higher concentrations [84, 91]. Streptomycin and oxytetracycline antibiotics have shown efficacy, but these treatments are not largely registered because of high cost and resistance problems. Induction of systemic resistance using analogs of salicylic acid is also another meaning of control, but still, new developed efficient methods are needed to control angular leaf spot disease of strawberry [84, 91].
