In 2008, we isolated a strain of an Amphobotrys sp., CotAr-12, from a cotton plant showing Verticillium wilt symptoms in Hubei Province, China. In October 2010 and 2011, surveys for Amphobotrys sp. on cotton were conducted in 14 counties in Hubei. No signs of an Amphobotrys sp. was observed on cotton. However, a gray mold disease was found on the weed, Acalypha australis in 40 of 51 cotton fields with an average disease incidence of 18.6%. The disease started on the inflorescences at the top of stems or branches and then spread downward to the main stems, branches, and leaves. Abundant sporulation on the necrotic tissues and formation of sclerotia underneath the epidermis or in the stem pith were evident. A total of 128 isolates, including CopAr-5, were obtained from A. australis (1 to 10 isolates per field). All isolates from A. australis and CotAr-12 appeared similar in colony morphology; ragged colony margins, erect conidiophores with long stipes and dichotomous branches at the top, globose and subhyaline conidia with an average diameter of 5.3 to 8.5 μm, and black, oval sclerotia of 0.6 to 26.2 × 0.5 to 19.0 mm. These characteristics matched the description for Amphobotrys ricini (Buchw.) Hennebert (teleomorph Botryotinia ricini [Godfrey] Whetzel) (2). Strains CopAr-5 and CotAr-12 were selected for molecular identification. DNA was extracted from mycelia and used for cloning of three nuclear genes (e.g., G3PDH, HSP60, and RPB2) using the procedures described by Staats et al. (4). The resulting DNA sequences were used for phylogenetic analysis with the corresponding sequences for Botrytis spp. (4). Results showed that strains CopAr-5 (GenBank Accession Nos. JN681883, JN681881, and JN6981879), CotAr-12 (GenBank Accession Nos. JN681882, JN681880, and JN6981878), and Botrytis (teleomorph Botrytinia) ricini (GenBank Accession Nos. GQ860998, GQ860996, and GQ860997) formed a separate clade that was distinct from Botrytis spp., supporting the distinction of Amphobotrys from Botrytis and suggesting that the two strains were Amphobotrys ricini. Pathogenicity was determined by placing mycelia of strains CopAr-5 and CotAr-12 on 10 detached leaves of A. australis, castor bean, and cotton. Control leaves of these plants were inoculated with potato dextrose agar alone. After incubation at 20°C under moist conditions (>90% relative humidity) for 60 h, the control leaves remained healthy, while the leaves of A. australis and castor bean inoculated with both strains formed extensively expanded lesions of 20 and 27 mm in diameter on average, respectively. Both strains also caused disease on cotton leaves with discontinuous and localized lesions of 18 mm in average diameter. A fungus was reisolated from the leaf lesions on these plant species and were identical to Amphobotrys ricini in colony morphology and conidial characteristics. We conclude that Amphobotrys ricini is a major pathogen on A. australis in central China. It is an important pathogen of castor bean (1) and has been reported to infect several euphorbiaceous plants, including A. hispida (3). To our knowledge, this is the first report of Amphobotrys ricini on A. australis.
References: (1) G. H. Godfrey. J. Agric. Res. 23:679, 1923, (2) G. L. Hennebert. Persoonia 7:183, 1973. (3) B. V. Lima et al. Australas. Plant Dis. Notes 3:5, 2008. (4) M. Staats et al. Mol. Biol. Evol. 22:333, 2005.