First Report of Alternaria alternata Causing Leaf Spot on Aloe vera in Louisiana
Authors: da Silva W. L. , Singh R. 
Published: June 2012

Aloe vera (L.) Burm. f. is a perennial succulent plant that is grown worldwide mainly for medicinal and cosmetic uses. In the USA, it is mainly cultivated in some southern states to produce aloe gel for the cosmetic industry (3), and in Louisiana it is also sold commercially as an ornamental. During the summer of 2011, several A. vera plants infected with leaf spots were observed on the campus of Louisiana State University, Baton Rouge. Large, necrotic, sunken, circular to oval, dark brown spots were present on both surfaces of the leaves. Infected leaf tissue pieces were surface disinfested with 1% NaOCl solution for 1 min and plated on potato dextrose agar (PDA). Plates were incubated at 28°C in the dark for 4 days. A dark olivaceous fungus with profuse golden brown, branched, and septate hyphae was consistently isolated from the infected tissue on PDA. The fungus produced conidia with longitudinal and transverse septa, and was morphologically identified as an Alternaria sp. (4). Conidia were produced in long chains, pale to light brown, obpyriform, with a beak (6.0 μm long), one to seven transverse and up to three longitudinal septa, and measured 10 to 45 μm long × 7 to 18 μm wide. Conidiophores were straight, septate, light to olive golden brown with conidial scar, and measured 35 to 100 μm long × 2 to 5 μm wide. Genomic DNA from a single-spored isolate was extracted and the internal transcribed spacer (ITS1-5.8s-ITS2) regions were amplified and sequenced using primers ITS1 and ITS4. BLASTn analysis of a 486-bp sequence (GenBank Accession No. JQ409455) resulted in 100% homology with A. alternata strain DHMJ16 (GenBank Accession No. JN986768) from China and several other Alternaria spp. The fungus was identified as A. alternata based on mycelial and conidia characters after being grown under standard, previously described conditions (4). Pathogenicity tests were carried out by inoculating six potted aloe plants with 0.5-cm diameter discs taken from a 6-day-old culture grown on PDA. Four discs were placed on the upper surface of each of the bottom leaves of every plant. Inoculated plants were individually covered with a plastic bag and maintained in a greenhouse for 1 week at 25 ± 2°C. Six control plants received only agar plugs. Seven days after inoculation, necrotic leaf spots were observed on the inoculated plants and A. alternata was reisolated from these spots. No leaf spots were observed on control plants. To the best of our knowledge, this is the first report of leaf spot caused by A. alternata on A. vera in Louisiana. Several outbreaks of the disease have been reported in Pakistan and India as damaging aloe gel production in those countries (1,2). An outbreak of this disease in Louisiana could represent a serious issue for the state's A. vera ornamental commerce. References: (1) R. Bajwa et al. Can. J. Plant Pathol. 32:490, 2010. (2) A. Kamalakannan et al. Australas. Plant Dis. Notes 3:110, 2008. (3) T. Reynolds. Aloes: The Genus Aloe. CRC Press, Boca Raton, FL, 2004. (4) E. G. Simmons. Alternaria: An Identification Manual: Fully Illustrated and with Catalogue Raisonné 1796-2007. CBS Fungal Biodiversity Centre, Utrecht, The Netherlands, 2007.