During the springs of 2003 and 2004, a serious outbreak of root and stem disease occurred on Aloe barbadensis L. grown in commercial fields in Yuanjiang, Yunnan Province, Southwest China. The disease began with brown root rots that advanced to collars and stems, followed by internal decay of the lower stem. Diseased tissue samples collected from plants in different fields were surface disinfested and plated on potato dextrose agar (PDA). A Fusarium species was consistently isolated, and pure cultures were established by initiating new cultures with single spores. Grayish white and fluffy mycelium with a deep violet-blue pigment developed on PDA agar, with a growth rate of 3.4 cm in 3 days, and a dark, blue-green mass of confluent sporodochia or pionnotes covering the central part of the culture. Microconidia in false heads on long phialides were abundant when initially isolated but were infrequent in later cultures. Microconidia were generally single celled, oval to kidney shaped, and 8 to 16 × 2.5 to 4 μm. Macroconidia were abundant, cylindrical, slightly curved with blunt and rounded apical cells, and foot-shaped or notched basal cells, and mostly three to four septate, 27 to 50 × 3.6 to 5.7 μm. Chlamydospores were sparse, single or in short chains, thick walled, and 7 to 10 μm in diameter. This description corresponds to Fusarium solani (Mart.) Appel & Wollenw. emend. Snyd. & Hans as described by Nelson et al. (1). Inoculations with F. solani were made by dipping healthy aloe roots into a 300-ml suspension of 3 × 105 conidia/ml. Control plants were dipped in sterile water. All plants were potted in soil and kept in a greenhouse (2). After 30 days, all inoculated plants developed root rots followed by collar and stem rots that were similar to those observed in the field. Cultures of F. solani were reisolated from the diseased plants and identification was confirmed by conidial characteristics. No symptoms were observed on the control plants. Tests were repeated with similar results. In China, the Yunnan Province produces one-third of all aloe grown and this disease has a major impact on production. Aloe is grown in certified organic fields and chemical pesticides are not used. To our knowledge, this is the first report of F. solani causing root and stem rot of A. barbadensis.
References: (1) P. E. Nelson et al. Fusarium Species: An Illustrated Manual for Identification. Pennsylvania State University Press, University Park, PA, 1983. (2) X. Ruan et al. Pages 211--215 in: Phytopathology Laboratory of Yunnan Province. Vol. 2. Y. Y. Shengfu, ed. Yunnan Science and Technology Press, Kunming, Yunnan, China, 1998.
