Root and Basal Stem Rot of Rose Caused by Phytophthora citrophthora in Italy.

Approximately 800 ha of cut flower roses are cultivated for commercial production in Italy. During autumn of 2004 in an experimental greenhouse in western Sicily (southern Italy), 60% of 2-year-old plants of rose cv. Red France on Rosa indica cv. Major rootstock grown in soil showed leaf chlorosis and wilt. A dark brown lesion lined by a water-soaked area was noticeable at the stem base near the soil surface. Root rot was found consistently associated with aboveground symptoms and plants collapsed within 4 months after the appearance of the first symptoms. The same symptoms were observed sporadically on rose plants of the same cultivar during the last 6 years in commercial nurseries in western Sicily. In all cases, a Phytophthora species has been consistently isolated from rotted roots and stems on Phytophthora-selective media. Pure cultures were obtained by single-hypha transfers. The species was identified as Phytophthora citrophthora on the basis of morphological characters and electrophoretic analysis of mycelial proteins on polyacrylamide gel (1). On potato dextrose agar, isolates produced petaloid colonies with optimum growth temperature at 25°C. On V8 agar, mono- and occasionally bipapillate, ovoid to limoniform sporangia, measuring 44 to 55 × 27 to 28 μm, with a mean length/breadth ratio of 1.4:1 were produced. All isolates were heterothallic but did not produce gametangia in dual cultures with P. nicotianae isolates of A1 and A2 mating type. Electrophoretic patterns of total mycelial proteins and four isozyme (acid and alkaline phosphatases, esterase, and malato dehydrogenase) of the isolates from rose were identical to those of reference isolates of P. citrophthora, but clearly distinct from isolates of other heterothallic species with papillate sporangia, including P. capsici, P. nicotianae, P. palmivora, and P. tropicalis. All isolates from rose showed the same electrophoretic profiles. Blast search of rDNA-ITS sequence from PCR-amplified ITS4/ITS6 primers (2) of a representative isolate from rose (IMI 392044) showed 98% homology with a reference isolate of P. citrophthora (GenBank No. EU0000631), thus confirming the identification. Pathogenicity of isolate IMI 392044 was tested on 10 12-month-old plants of rose cv. Red France grafted on R. indica cv. Major transplanted in pots containing a mixture of sphagnum peat moss and sandy loam soil (1:1 vol/vol) infested with 80 g of inoculum per liter of mixture. Inoculum was produced by growing the isolate on wheat kernels. Plants transplanted in pots containing noninfested soil served as controls. Plants were kept in a greenhouse at 22 ± 3°C and watered to soil saturation once a week. Inoculated plants developed symptoms of leaf chlorosis and root and crown rot within 15 to 30 days and wilted within 40 to 80 days after inoculation. Control plants remained healthy. P. citrophthora was consistently reisolated from inoculated plants. Root and basal stem rot of rose may be caused by several Phytophthora spp. and has been reported in various countries of Asia, Europe, and North America (3,4). However, to our knowledge, this is the first report in Italy. The occurrence of this disease may be attributed to excessive irrigation practices. References: (1) S. O. Cacciola et al. EPPO Bull. 20:47, 1990. (2) D. E. L. Cooke et al. Fungal Genet. Biol. 30:17, 2000. (3) Y. Nagai et al. Phytopathology 68:684, 1978. (4) B. W. Schwingle et al. Plant Dis. 91:97, 2007.