A New Phytophthora Root Disease of Alder in Italy.

Abstract

A Phytophthora was found associated with wilt and mortality of Italian alder (Alnus cordata) seedlings in a nursery in northern Tuscany, Central Italy. This disease is one of the major constraints to alder survival in northern Europe (3). Symptoms included sparse yellowish brown foliage with abnormally small leaves, dark stained necrosis of the bark at the collar level, and reduction of the root system due to the death of the tap root and lateral roots. These symptoms resemble those on chestnut, walnut, and oak plantlets colonized by P. cambivora. The diseased seedlings were 1 to 2 years old and were grown on both beds and soil. Tissue isolation from infected parts of the plants yielded a P. cambivora-like species in culture. Pure cultures were appressed and felty in appearance with sparse or no aerial mycelium when grown on V8 agar, carrot agar, or potato dextrose agar. Sporangia were ovoid, non-papillate, and averaged 51.8 μm in length and 36.6 μm in width (range: 45 to 60 μm and 30 to 40 μm). The isolates were homothallic with smooth-walled oogonia with a diameter ranging from 20 to 45 μm and with two-celled, amphigynous antheridia. Colonies showed rapid development at 25°C (mean radial growth = 6 mm/day), but at 30°C growth was strongly inhibited, unlike P. cambivora isolates previously collected in the same area, which were more thermophilic. The morpho-physiological features of the alder isolates resembled those of the Swedish variant of the alder Phytophthora (2). Identification was confirmed by C.M. Brasier (Forestry Authority Research Station, UK). Two-year-old alder seedlings (1.3 cm diameter, height 70 cm) grown in pots were wound-inoculated in the trunk. Six weeks after inoculation, symptoms were produced identical to those described above, as well as necrosis of the bark tissue varying in length from 0.5 to 3 cm. Isolations from diseased bark tissue yielded fungal colonies resembling those described above. The pathogen, described for the first time in England in 1995 (1), is now present throughout northern Europe. This is the first report for the Mediterranean region, which suggests that the fungus will cause disease in the drier and warmer climates of southern Europe.