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Root Rot and Dieback of Pinus pinea Caused by Phytophthora humicola in Tuscany, Central Italy

High mortality was noticed in a 10-year-old stand of Pinus pinea in the Alberese area (Grosseto, central Italy, elev. 40 m, 42° 39′ 46″ N, 11° 06′ 25″ E) in July 2010. Aerial symptoms of trees included chlorosis, crown thinning, stunted growth, bark lesions at the stem base with resinous exudations, and extensive necroses of the underlying xylem tissue. Woody roots of two uprooted trees exhibited bark necroses and a high proportion of fine roots was destroyed. Soil around necrotic roots was baited using apple fruits (cv. Gala). After 1 week of incubation at 24°C, typical firm fruit rot developed and small tissue samples were transferred to clarified V8 agar (V8A) amended with 5 ml/l PARPNH and incubated at 24°C. After 7 days, stellate to rosaceous, finely lobed cottony colonies arose that were transferred to FPM medium and incubated at 24°C. Within 7 days, spherical oogonia with a smooth surface and predominantly paragynous antheridia formed; sporadic amphyginous antheridia could be observed. Colony squares (1 cm²) were then placed in filtered and sterilized pond water. After 48 h, ovoid, obpyriform, or clavate, nonpapillate, persistent sporangia with internal nested and extended proliferation were formed. Fifty oogonia and 30 sporangia were measured. The diameter of the 50 spherical oogonia varied from 33.6 to 44.9 μm (avg. 39 μm); dimensions of the 30 sporangia were 42.6 to 59.8 × 28.9 to 47.8 μm (avg. 52.95 × 38.98 μm; 1:b ratio 1.37). The isolate was identified as Phytophthora humicola W. H. Ko & Ann on the basis of colony type, size and morphology of oogonia and sporangia, average length/width ratio of sporangia, the homothallic formation of oogonia (4), and ITS rDNA sequence information (GenBank Accession No. JQ757060). A BLAST search of the ITS sequence of P. humicola isolate B33 revealed a 99% identity with the Phytophthora ITS Clade 6 species P. humicola and P. inundata (2). This latter species could be ruled out, however, since it is self-sterile, whereas our isolate B33 was self-fertile (3). A strain of P. humicola was deposited in the CBS-KNAW Fungal Biodiversity Centre, strain no. CBS129249. Pathogenicity tests were conducted on 10 one-year-old twigs of Pinus pinea. A bark portion was removed aseptically and a V8A disc (0.5 cm diam.) of P. humicola mycelium was placed on the wound. Control twigs (3) received sterile V8A discs. Inoculated and control twigs were incubated at 20°C in the dark. Clearly noticeable necrotic lesions (avg. length 2.2 × 0.68 cm) were observed after 15 days on inoculated twigs. Control twigs showed no symptoms. Reisolations on selective V8-PARPNH-agar confirmed P. humicola as the causal agent. P. humicola is mainly associated with woody horticultural crops (1, 3), while the other taxa grouped with this species in Clade 6 are mainly found in forest and riparian ecosystems (1). These aquatic Phytophthora species normally have a saprophytic lifestyle, but under favourable environmental circumstances can act as opportunistic pathogens, attacking susceptible trees and causing scattered mortality in forest stands and natural ecosystems (3). To our knowledge, this is the first report of P. humicola from a pine stand. It is supposed that the pathogen reached the stand through infected plant material or infested soil introduced into the stand.

References: (1) C. M. Brasier et al. Mycol. Res. 107:277, 2003. (2) D. E. L. Cooke et al. Fungal Genet. Biol. 30:17, 2000. (3) T. Jung et al. Persoonia 26:13, 2011. (4) W. H. Ko and P. J. Ann. Mycologia 77:631, 1985.