Medlar decline caused by Phytophthora cactorum in Hungary

In August 2006, a 10-year-old medlar tree (Mespilus germanica) growing on a quince (Cydonia oblonga) rootstock in a home garden in Budapest exhibited signs of severe decline. The foliage was withered and the fruit remained immature. The following spring, the tree failed to leaf out and the collar of the quince showed conspicuous bark necrosis.

Isolations from symptomatic bark tissue on carrot agar (CA) yielded three pure cultures with similar petaloid colonies and sparse aerial mycelia. The hyphal growth was optimal at 26-27°C while completely inhibited above 32-33°C. Terminal sporangia on sympodial sporangiophores formed abundantly on CA plates flooded with sterile distilled water for 3-4 days at 20°C. Caducous sporangia with short (<4 µm) pedicels were papillate and ovoid to spherical, 27-41 µm (35.4 µm ± 0.40) × 21-34 µm (29.1 µm ± 0.4), with a length and breadth ratio of 1.22:1. The isolates were homothallic forming smooth-walled oogonia, 22-29 µm (25.2 µm ± 0.34) in diameter with paragynous, monoclinous antheridia at 26-27°C on CA plates. The oospores were plerotic. Based on these features, we identified our isolates as Phytophthora cactorum, a pathogen of more than 200 plant species (Erwin & Ribeiro, 1996), but not known naturally to infect medlar or quince.

The ITS1/ITS2 including the 5.8S subunit of rDNA of the three isolates were sequenced (GenBank accession numbers EU109567, EU109568 and EU109566) and showed 100% identity with known sequences (EU106589 and AY848931) of P. cactorum..

Since medlar is commonly grafted onto a quince rootstock, Koch's postulates were completed on both quince, which has been reported to be susceptible to P. cactorum by artificial inoculation (Smith, 1937), and medlar, grafted on a quince rootstock. Stems of potted 1-year-old quince and 2-year-old medlar (three of each) were wound-inoculated 8-10 cm above the soil line and the grafting site, respectively, with 5 mm mycelial discs from 3-day-old CA cultures. Bark discolorations, dark brown on quince and greyish on the graft, were apparent within 3 weeks of incubation at 18 to 25°C, for all of the inoculated plants. Within 2 months, discoloured areas gradually spread (Fig. 1) and the infection reached the vascular bundles (Fig. 2).

To our knowledge, this is the first report of P. cactorum causing decline of medlar, most probably via infection of the quince rootstock. Although previously known from Hungary, new technology has been used to provide the first definitive description of this pathogen in the country.