Publication Type:Journal Article
Source:Plant Disease, Volume 100, Issue 8, p.1796 - 1796 (2016)
In June 2015, ∼10% of 10,000 8-year-old almond [Prunus dulcis (Miller) D.A. Webb. cv. Ferragnes] trees grafted on the GF677 rootstock in a commercial almond orchard in Besni of Adıyaman Province, showed wilting and lack of vigor, with severely infected trees dying from root and crown rot. The incidence of infected trees on soils with poor water drainage was greater than that on well-drained soils. Crowns of symptomatic plants had a reddish brown discoloration in the inner bark with a sharp contrast between infected and healthy tissue. Most fine roots were completely rotted and the inner bark of infected larger roots showed similar discoloration. Tissue samples taken from the margins of crown and root lesions were placed on carrot agar (CA) amended with 5 mg of pimaricin, 250 mg of ampicillin, 10 mg of rifampicin, 100 mg of pentachloronitrobenzene, and 50 mg of hymexazol (P5ARPH) per liter. Plates were incubated for 5 days at 28°C in the dark and Phytophthora chlamydospora was consistently isolated from the tissues. Isolates produced petaloid colonies and abundant spherical, thin-walled, intercalary but lateral and terminal chlamydospores (30 to 46 μm diameter, 38.5 μm average). Colonized agar discs were submerged in sterilized deionized water at 25°C. After incubation on submerged discs for 24 h, all isolates produced hyaline, nonpapillate, noncaducous sporangia of ovoid to obpyriform shape, with internal and external proliferation, 34.0 to 56.5 μm long, 26.5 to 39.5 μm wide, with a length/breadth ratio of 1.4 to 1.7, formed on simple, unbranched, occasionally sympodial sporangiophores. Globose to subglobose hyphal swellings in branched chains also formed in water. The optimum and maximum temperatures for mycelium growth on CA were 27 and 37°C, respectively. All these characteristics were similar to those described for P. chlamydospora Brasier and Hansen sp. nov. (Hansen et al. 2015), previously known as P. taxon Pgchlamydo. Genomic DNA was extracted from two representative isolates. The ITS region of rDNA was amplified using the ITS6/ITS4 primer pair and sequenced (GenBank KU647271 and KU647272). BLAST searches showed 99 to 100% identity to P. chlamydospora isolates, including the type isolate P236 (99% identity) (AF541900) (Hansen et al. 2015), deposited in GenBank and Phytophthora-ID databases, which confirmed morphological identification. Primary roots of ten 1-year-old P. dulcis cv. Ferragnes seedlings were wounded with a scalpel, and mycelial disks (3 per plant) of P. chlamydospora grown on CA 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. P. chlamydospora caused severe wilt and browning of leaves within 1 and 2 months and extensive root necrosis and rot after 2 months of incubation in a greenhouse. During this period, soil was kept wet by watering plants daily as required. Koch’s postulates were satisfied after reisolating P. chlamydospora from symptomatic roots of inoculated plants. No disease occurred in 10 controls, similarly inoculated with sterile CA disks, and the pathogen was not recovered from these plants. P. chlamydospora has only rarely been recovered from several ornamental and woody species (Blomquist et al. 2012; Brasier et al. 2003; Ginetti et al. 2014), but not from fruit trees so far. To our knowledge, this is the first report of P. chlamydospora infection of almond, which represents a new host for this pathogen. This is also the first record of P. chlamydospora in Turkey. Within a month, all naturally infected almond trees in this orchard were collapsed and destroyed and the number of infected trees increased dramatically, indicating that it might be a significant threat to almond plantations and orchards, particularly on sites where excess soil water occurs periodically.