First Report of Black Spot Caused by Alternaria alternata on Grafted Cactus

Abstract

Chemical Biotechnology Research Center, KRICT, Daejeon 305-605, Korea(Received on June 25, 2009; Accepted on November 25, 2009)A stem spot disease was observed on Gymnocalyciummihanovichii (Korean name: Bimoran), a scion of graft-cactus, in major growing areas of Goyang and Eum-seong, Korea during 2008 and 2009. Typical symptomswere initial blackish brown lesions produced mainly onareoles and scars of injured stem tissues, graduallybecoming large black spots. A causal organism isolateCD2-7A isolated from the infected stems was identifiedas Alternaria alternata based on its morphologicalcharacteristics and confirmed by the DNA sequencinganalysis of ITS, glyceraldehydes-3-phosphate dehydro-genase (gpd), and allergen Alt a1 (Alt a 1) genes.Artificial inoculation of the fungal isolate CD2-7Aproduced the same rot symptoms on the cactus stems,from which the same fungus was isolated and identified.This is the first report of the black spot caused by A.alternata in the grafted cactus.Keywords : Alternaria alternata, graft-cactus, Gymnocaly-cium mihanovichii, pathogenicityMost cactuses are stem succulents, having fleshy stems. Agrafted cactus, which is composed of stock and scioncactuses, is a major commercial product in Korea and oneof the most important exporting ornamental plants due to itshigh quality in the world market. The most widely culti-vated stock cactus is a three-angled cactus (Hylocereustrigonus), while two scion cactuses commonly used inKorea are Gymnocalycium mihanovichii and Chamaecereussilvestrii, of which the Korean common names are Bimoranand Sanchui, respectively. G. mihanovichii is more popular-ly cultivated in Korea than C. silvestrii.One of the limiting factors for cultivating grafted cactusesin greenhouses is due to stem rot diseases especially causedby fungi. They include stem rots caused by Fusariumoxysporum (Chang et al., 1998), Bipolaris cactivora (Hyunet al., 1998; Kim et al., 2004), and Glomerella cingulata(Kim et al., 2000). In our surveys of cactus diseases during2008 and 2009, however, we often observed some stemdiseases of G. mihanovichii with no fungi belonging to theabove reported genera. Therefore, we report the occurrence of a fungal stemdisease of G. mihanovichii and the identification andcharacterization of the fungal pathogen.Disease occurrence and symptoms. Occurrence of a stemdisease was observed frequently on G. Mihanovichii inmajor growing areas of Goyang and Eumseong, Koreaduring 2008 and 2009. Symptoms were characterized byblackish brown lesions produced mainly on areoles andscars of injured stem tissues (Fig. 1A, B). Sometimes largeportions of the cactus stems were diseased with large blackspots at advanced infection stages. Fungal isolation and pathogenicity. Pathogen isolationwas conducted using infected G. mihanovichii stems. Smallstem tissues were cut from the edge of the diseased areawith flame-sterilized razor blade and surface-sterilized with75% ethanol for 30 sec and 1% sodium hypochlorite for 60sec, and rinsed with sterile water. The stem tissues wereplaced on potato-dextrose agar (PDA) supplemented with0.1% lactic acid, incubating at 25