First Report of Foliar Infection of Cabbage by Phytophthora drechsleri in Argentina.

Abstract

Cabbage (Brassica oleracea var. capitata L.) is a popular crop grown along the northeast horticultural belt of Buenos Aires Province, Argentina. In the summer of 2010, fields in this region remained flooded for long periods due to frequent and intense precipitation (560 mm from January to March). Commercial cabbage crops in the cities of Luján and General Rodríguez developed patches of diseased plants that were stunted and wilted. Affected plants had necrotic areas in the crowns and roots. Symptoms expanded to the upper stems, leaving vascular tissues exposed. During April 2010, samples from 2 fields were brought to the laboratory where the stems were washed thoroughly and disinfected with a 1% bleach solution for 2 minutes. Small pieces (5 mm in diameter) were removed from the lesion edge, plated on V8 agar (V8A) plates, and incubated at 24°C in the dark for 5 days. Four isolates were transferred to V8A using hyphal tips. Morphological studies were performed on the V8A cultures as well as plates flooded with tap water. Sporangia were obpyriform, nonpapillate, persistent, and variable in size, averaging 44 × 28 μm. Each isolate belonged to the A1 mating type when paired with P. capsici tester isolates, CBS 370.72 and CBS 111.334 (Fungal Biodiversity Centre, CBS, Utrecht, the Netherlands). The isolates produced amphigynous antheridia, and chlamydospores were present but scarce. Maximum temperatures for growth (37°C) were also performed. Edited sequences of the internal transcribed spacer (ITS) region of the rDNA (GenBank Accession Nos. JQ653300, JQ653301, JQ653302, and JQ653303) were compared with Phytophthora sequences available in GenBank using the BLASTN search utility (1) and aligned to the data set of Cooke et al. (2). Sequences of the four isolates (strains 2: R-cai-cuello-col-3, 3: R-cai-cuello-col-18, 4: R-AN-col-1A and 5: R-AN-col-1B) matched 100% with GenBank sequences of P. drechsleri (100% coverage, 100% identity and no gaps). Based on these results, the four Argentinian cabbage isolates were identified as P. drechsleri (3). Pathogenicity tests were completed using three detached heads of mature cabbage plants (B. oleracea var. capitata) for each isolate. A 5-mm colonized mycelial plug of the appropriate isolate was placed on the main vein of the outermost leaves. For the control treatments, three heads were inoculated with non-colonized V8A plugs. The inoculated and control heads of cabbage were incubated in plastic boxes wrapped in black nylon bags at 24°C for 4 days. Broccoli (B. oleracea var. italica) and cauliflower (B. oleracea var. botrytis) were also tested following the same procedure. All heads of cabbage, broccoli, and cauliflower developed necrotic lesions 2 to 4 cm in diameter and a dark grey color. Control heads of each plant remained green. P. dreschleri was consistently reisolated as described above from the inoculated heads, but not from the control heads. To our knowledge, this is the first report of cabbage as a host for P. dreschleri in Argentina. Frezzi (4) reported this species as a pathogen of Chrysanthemum cinerariefolium, Celosia plumosa, Schinus molle, and Solanum lycopersicum in Argentina in 1950.