First report of wilt of Amygdalus communis caused by Verticillium dahliae in China

Verticillium wilt caused by the fungus Verticillium dahliae is found world-wide and attacks a wide range of plants. In the summer of 2001, a wilt disease of Amygdalus communis (sweet almond) cultivated in the Xinjiang municipality in China was first observed. The characteristic symptoms of typical wilt included wilt of leaves and twigs, and brownish discoloration of vascular tissues. Ultimately, the branches and entire trees wilted and died. To identify the causal agent, both traditional and PCR-based methods were attempted.

In 2004 and 2005, a Verticillium sp. was isolated from the xylem of diseased branches on potato dextrose agar (PDA. The fungus produced dark colonies on PDA, produced rotiform conidiophores with 2-4 layers, one-celled colourless olivary conidia, and small black microsclerotia. It was identified as V. dahliae based on morphological characteristics according to the description of Smith (1965). Ribosomal DNA (regions ITS1, 5·8S rDNA and ITS2) was amplified and sequenced (GenBank Accession No. EU109532). Sequence analysis revealed that the fungus isolated from A. communis is identical to a Greek strain of V. dahlia (GenBank Accession No.AF104926).

Each of six seedlings of A. communis, approx. 20 cm high in sterile soil, was inoculated by injecting 20µL single-conidial suspension containing 1×10⁶ conidia/mL into the base of the stem. The inoculated seedlings were incubated at 25°C. Water-soaked lesions appeared on the leaves of all of the inoculated seedlings at the fourth day and then dark brown lesions appeared around the injected sites and spread rapidly upwards. The inoculated seedlings wilted and died after seven days. The pathogen was reisolated from the inoculated stems. Control seedlings, inoculated with an equal volume of sterile water, remained healthy.

The fungus was previously recorded in Xinjiang municipality and other provinces in China infecting many Rosaceae and Malvaceae plants (Tai, 1979), but this is the first report of V. dahliae associated with wilt of A. communis in China.