The cultivation of medicinal plants is increasing in some areas of central Italy where the climate is suitable for organic farming and the production of high-quality plant products. During April and May 2003, plants of Althaea officinalis L. at the seedling stage (two-to-four true leaves) maintained in unheated greenhouses before their transplantation to open fields showed an unusual foliar disease. Necrotic leaf spots of variable shape and size were followed by a rapid wilting of leaves that frequently resulted in a blight of the young plants. Small leaf pieces showing symptoms were sampled, surface treated in 0.1% HgCl2 for 30 s, rinsed twice in sterile water, placed on potato dextrose agar (PDA) (pH 5.5) in petri dishes, and incubated for 7 days at 25 ± 2°C. Colletotrichum malvarum (Braun & Casp.) Southworth (1,2) was consistently recovered from affected tissues. The fungus produced dark colonies with whitish aerial mycelium and acervuli containing hyaline, cylindrical conidia (14 to 25 × 3 to 6 μm) on PDA. The pathogenicity of four fungal isolates was tested by inoculating two, true leaves of 10 plants (A. officinalis) with a conidial suspension (5 × 105 conidia ml-1) from a 10-day-old culture. Plants sprayed with water served as controls. All seedlings were placed in a greenhouse at 24± 2°C under natural light conditions and covered with plastic bags for the first 24 h. Each pathogenicity test was repeated one time. After 5 to 7 days, the inoculated seedlings showed small necrotic leaf spots identical to those observed under natural conditions. Affected leaf areas rapidly enlarged and within a few days, the young plants wilted. No symptoms appeared on the noninoculated controls. C. malvarum was consistently reisolated from the symptomatic test seedlings, whereas the fungus was never isolated from control plants. Standard seed health methods (agar plate and blotter) carried out on samples from the same seed lots used for the unheated greenhouse trials were negative for the presence of the pathogen. The occurrence of anthracnose may be attributed to windborne conidia of C. malvarum coming from infected wild malvaceae species and cultivated hosts grown in open fields in the neighborhood of seedling greenhouses. To our knowledge, this is the first report of C. malvarum on A. officinalis in Italy.
References: (1) W. Brandenburger. Page 386 in: Parasitische pilze an gefäbpflanzen in Europa. Fisher Verlag, Stuttgart, Germany, 1985. (2) B. C. Sutton. The genus Glomerella and its anamoroph Colletotrichum. Pages 1--26 in: Colletotrichum, Biology, Pathology and Control. J. A. Bailey and M. J. Jeger eds. CAB International, Wallingford, U.K., 1992.