First Report of Early Blight Caused by Alternaria linariae on Potato in Algeria

Potato (Solanum tuberosum L.) occupies a leading place in Algerian agriculture, where it is grown over an area of approximatively 160,000 ha and can be planted and harvested in virtually any month of the year. However, this production remains threatened by early blight disease caused by Alternaria species including A. solani, A. grandis, and A. protenta (Ayad et al. 2017; Bessadat et al. 2017). High incidences of early blight (up to 80%) have previously been observed in northwestern parts of Algeria on potato as well as tomato (Bessadat et al. 2017). Potato disease surveys were carried out between 2012 and 2015 with sampling performed in 12 potato-growing regions of Algeria. One hundred sixty-four leaf samples exhibiting typical early blight symptoms (dark elongated or circular lesions with concentric rings surrounded by a yellow halo) were collected. Small pieces (3 to 4 mm²) were cut off from the lesion edges, surface disinfested, and plated on potato dextrose agar medium at 22°C. From the isolates obtained, 82 exhibited morphological characteristics associated with species of Alternaria belonging to the section Porri. Twenty-two pure cultures were induced to sporulate by plating on V8 medium and incubating for 2 weeks under alternating 12-h darkness and 12-h near-ultraviolet light. Although most of the isolates had typical morphological and sporulation characteristics of A. solani and A. grandis, five isolates, originating from north Algeria, produced conidia whose body and beak length (up to 108 and 225 µm, respectively) were in the range of those described for A. linariae (Gannibal et al. 2014). For confirmation of the identity of these isolates at the species level, a partial region of the calmodulin (cal) gene was amplified using a published primer set (Gannibal et al. 2014) and sequenced. Maximum-likelihood cluster analysis of the resulting nucleotide sequences (GenBank accession nos. MH243795, MH243769, MH243793, MH243789, and MH243794) and additional reference sequences of species within the section Porri confirmed that the five isolates (DA01, DA02, and DA03 from the Tipaza province; and DA06 and DA07 from the Alger province) could be assigned to A. linariae with 100% sequence similarity to a reference strain (CBS 109161; GenBank accession no. JQ646254) (Woudenberg et al. 2014). To confirm pathogenicity of the five A. linariae isolates, 3-week-old leaves of susceptible varieties of tomato (Marmande and St Pierre) and potato (Spunta and Sarpomira), cultivated in a pot under greenhouse conditions at 28°C with 16 h/day of light, were inoculated by depositing 20-µl drops of a 10⁴ conidia/ml suspension. Irrespective of the tested plant species and variety, all inoculated leaves showed extending lesions that may reach up to 60% of the leaf area at 21 days postinoculation. No symptom was observed on control plants treated with distilled water. Together with A. solani, A. linariae is considered the main causal agent of tomato early blight (Gannibal et al. 2014), but it has never been described on potato. To our knowledge, this is the first report for the occurrence of A. linariae on potato. The fact that potato and tomato fields often coexist in close proximity in northwestern Algeria with farmers even using tomato in rotation with potato may favor the development of A. linariae on the latter plant species.