First Report of Canker and Root Rot of Almond Caused by Phytophthora plurivora in Turkey.

Almond [Prunus dulcis (Miller) D.A. Webb.] is one of the most frequently consumed nut species grown in Turkey. In early March 2015, ~20% of 13,000 one-year-old almond (P. dulcis cv. Ferragnes) seedlings in a nursery in Gaziantep showed sunken and blackened cankers on aboveground parts and root rot. Tissue samples taken from the margins of stem cankers and root lesions were placed on grated apple corn meal agar (Türkölmez et al. 2015) amended with P5ARPH per liter. Plates were incubated for 4 days at 20°C in the dark and a Phytophthora species was consistently isolated from the tissues. Isolates were homothallic with paragynous antheridia, smooth-walled spherical oogonia (24.7 to 29.6-μm diam), and both plerotic (1.3 to 1.4-μm wall diam) and aplerotic golden brown oospores (23.3 to 27.8 μm diam) on V8 juice agar (V8A) and produced semipapillate, noncaducous sporangia (35.5 to 53.2 × 26.3 to 38.4 μm; L:B ratio from 1.3 to 1.4 μm), which were usually obpyriform, ovoid, ellipsoid or irregular, and rarely possessed more than one apex, formed on simple or sympodial sporangiophores in nonsterile soil extracts. On V8 agar, colonies had a slight chrysanthemum growth pattern with limited aerial mycelium at the center of the colonies, whereas on PDA the colonies had a clear chrysanthemum growth pattern and aerial mycelia at the center of the colony. The minimum, optimum, and maximum temperatures for mycelium growth on V8A were 5, 25, and 32°C, respectively. On the basis of the morpho-physiological features, isolates were identified as P. plurivora T. Jung & T.I. Burgess from the ‘P. citricola complex’ (Jung and Burgess 2009). Genomic DNA was extracted from two representative isolates. The internal transcribed spacer (ITS) region of rDNA was amplified using the ITS6/ITS4 primer pair and sequenced (GenBank Accession Nos. KU221328 and KU310937). BLAST searches showed a 100% identity with a type isolate of P. plurivora(FJ665225). The sequence also contained the polymorphic nucleotides that are distinctive of this species (Jung and Burgess 2009). Pathogenicity was tested on shoot segments from ten 1- to 2-year-old P. dulcis cv. Ferragnes (5 to 7 mm diam, 20 cm length) by replacing the 5-mm bark disks of shoots in the middle of each shoot segment with the same-sized mycelial disks of P. plurivora grown on V8 agar. Ten control segments were similarly inoculated with sterile V8 agar disks. The wounds were wrapped with Parafilm, and the shoots were incubated in moist chambers in 100% RH at 23°C. Phytophthora plurivora caused cankers on almond shoots within 5 days after inoculation, and the noninoculated control shoots remained symptomless. To test the pathogenicity on roots, primary roots of 10 P. dulciscv. Ferragnes seedlings (1-year-old) were wounded with a scalpel, and mycelial disks (3 per plant) ofP. plurivora grown on V8 agar were placed under the cortical tissue after removing 10 to 15 cm of soil around the root base. The inoculated and exposed roots were then covered with soil. Ten control seedlings were similarly inoculated with sterile disks. The soil was kept wet by watering plants daily as required. All seedlings were grown in a greenhouse at air temperatures from 24 to 30°C. No disease developed in the controls, but P. plurivora caused wilt and root rot after 2 months. Koch’s postulates were satisfied after reisolating P. plurivora from all inoculated shoots and roots. To our knowledge, this is the first report of P. plurivora causing canker and root rot on almond plants. In late April 2015, all naturally infected almond seedlings in the nursery were collapsed and destroyed by this oomycete, indicating that it might be a significant threat to almond plantations and orchards in the postnursery phase. © 2016 The American Phytopathological Society.