Marsh mallow is cultivated in Switzerland for the production of candies of an internationally known brand. Since 2000, important losses have been caused by a new disease with typical anthracnose symptoms, i.e., dark lesions on leaves and stems including stem girdling and wilting, especially after abundant rainfall in the spring and summer. In older lesions with a clear center, black dots appeared and setae were observed with a dissecting microscope. When incubated with high relative humidity, abundant, pink spore masses were produced. During 2002, spores were isolated on potato dextrose agar (PDA) containing an antibiotic, and the size of the conidiospores and setae produced in acervuli were measured. Artificial inoculation of a range of potential host plants including Althaea officinalis, A. rosea (hollyhock), Lavatera trimestris, Malva alcea, M. crispa, M. moschata, M. silvestris, Fragariae × ananassa (strawberry), and Hypericum perforatum (St. John's-wort) was carried out in a glasshouse. A spore suspension of 106 spores per ml was sprayed on plants at the 4- to 6-leaf stage until runoff. High temperature (30/20°C, day/night) and relative humidity (>95%) was maintained for 2 weeks after which the plants were inspected visually for the occurrence of disease symptoms. Althaea officinalis, A. rosea, and M. alcea showed typical anthracnose symptoms and were considered as host plants after reisolation of the pathogen from lesions. On the basis of the restricted host range and form and size of the conidiospores (length of 10 to 13 μm and width of 3 to 4 μm), and setae (length of 62 to 75 μm), the pathogen was identified as Colletotrichum orbiculare f. sp. from A. officinalis (1). During 2003, a laboratory investigation was conducted to determine whether C. orbiculare f. sp. from A. officinalis is seed transmitted in marsh mallow. The appearance of the disease in marsh mallow fields simultaneously in several regions of Switzerland, where all growers plant the same variety from the same seed source, instigated this work. Seeds from this variety were tested on CGPIM, a semiselective medium for the isolation of Colletotrichum spp. (2). Per seed lot, 400 grains were placed on CGPIM, and C. orbiculare f. sp. from A. officinalis was found in 15% of seeds harvested from a highly diseased plant. Detection in commercial seed was hindered by extensive contamination with Alternaria spp. and Fusarium spp., but nevertheless, C. orbiculare f. sp. from A. officinalis could be found at low frequency (1%). The occurrence of Colletotrichum malvarum on marsh mallow was recently described in Europe (3), but to our knowledge, this is the first observation of seed transmission of C. orbiculare f. sp. from A. officinalis on marsh mallow seeds.
References: (1) J. A. Bailey et al. Phytopathology 86:1076, 1996. (2) J. B. Manandhar et al. Plant Dis. 79:376, 1995. (3) L. Tosi et al. Plant Dis. 88:425, 2004.
