First Occurrence of Downy Mildew on Boxleaf Veronica Caused by Peronospora grisea in Italy.Plant Dis 2004; 88(4):424PD
Boxleaf veronica (Hebe buxifolia (Benth.) Cockayne & Allan), native to New Zealand, is an annual or perennial shrub widely cultivated in Mediterranean zones. During late spring 2003, after seasonably wet and cool weather, a downy mildew epidemic occurred on potted, overhead irrigated, 1-year-old seedlings at a commercial nursery in eastern Sicily (southern Italy). Infections affected boxleaf veronica at first, but spread to variegated boxleaf (H. buxifolia cv. variegata). Initial symptoms on the upper side of leaves were small, gray brown patches that gradually spread, eventually resulting in necrosis in the center of infected areas. At that point, brown patches were evident on both sides of the leaves. Lesions on the lower leaf surface became covered with a fairly dense, pale gray-to-brown layer of conidia and conidiophores. As the disease progressed, these spots coalesced into large and conspicuous brown lesions. The youngest, most succulent shoots withered and died. The large brown lesions on the leaves were disfiguring and affected 95 to 100% of plants in the nursery. All diseased nursery stock had to be discarded. Oospores were not observed in leaf tissues. The fungus recovered from leaves with abundant gray brown sporulation was identified as Peronospora grisea (Unger) Unger. Microscopic observations revealed conidiophores that branched dichotomously five to seven times with branch ends 6 to 10 × 2 to 3 μm, slightly curved, and tapered to a blunt apex. Hyaline conidia were ellipsoid, brownish when mature, and measured 23 to 27 × 15 to 18 μm (mean = 25.2 × 17.1 μm), falling within the range of those reported for P. grisea (1). Pathogenicity was confirmed by inoculating 10 1-yr-old seedlings (10 cm tall) by gently pressing infected leaves with abundant sporulation onto healthy leaves and maintaining inoculated plants in a humid chamber at 21°C. An equal number of noninoculated plants served as controls. After 9 to 11 days, symptoms similar to those originally observed developed onto inoculated plants, and after 12 to 15 days, grayish mildew grew on leaves. Microscopic examination of the developing mycelium confirmed that leaves were infected with P. grisea. Uninoculated control plants did not develop any symptoms. The disease was also confirmed in this way on variegated boxleaf veronica (H. buxifolia cv. variegata). Downy mildew of Hebe spp. has been recorded in New Zealand, Britain, France, Germany, and more recently, West Sussex (2). Heavy rainfall during the spring of 2003 in eastern Sicily could have favored disease development. To our knowledge, this is the first report of downy mildew of boxleaf veronica and variegated boxleaf veronica caused by P. grisea in Italy. References: (1) S. M. Francis and A. M. Berrie. Peronospora grisea. No. 766 in: Descriptions of pathogenic fungi and bacteria. CMI, Kew, Surrey, UK. 1983. (2) J. M. Whipps and C. A. Linfield. Plant Pathol. 36:216, 1987.