White Blister Rust Caused by Albugo candida on Oilseed Rape in Korea

Abstract

Oilseed rape, Brassica napus ssp. oleifera (DC.) Metzger, is grownworldwide for the production of vegetable oil for human consump-tion, animal feed, and biodiesel. In Korea, this crop has also beenused as an important vegetable by harvesting the leaves nilat e winteror early spring or by picking the main flowering shoots just beforethe first flowers open. Since May 2004, typical symptoms of whiteblister rust disease on oilseed rape have been continuously found inseveral localities of Korea. Leavesof infected plants had whitishs orimostly on the lower surfaces and reddish violet blotches on thecorresponding upper leaf surfaces (Fig. 1A-C). Representativesamples were deposited in the herbarium (KUS-F20204, F22717,F24893). Microscopic examination of the sori from fresh materials wasperformed under a DIC-light microscope (Zeiss AX10). Thegrouped sporangiophores were hyaline, clavate or cylindric, andmeasured 20−40×12−15 µm (Fig. 1D & E). The sporangia werearranged in basipetal chains, hyaline, globose to subglobose, withuniform wall thickness and measured 15 −22×12−19 µm (Fig. 1F).The primary sporangia were similar to the secondary sporangia,although the former exhibit a slightly thicker wall than the latter. Noresting organs were observed. Up to now, the white blister rustpathogen on oilseed rape has been considered Albugo candida or A.cruciferarum (cf. Farr and Rossman, 2010). Albugo cruciferarum isregarded as a synonym of A candida . (cf. Choiet al ., 2007). Based onthe morphological characteristics and the specific host plant, thecausal agent of this disease was identified as A. candida (Pers.)Kuntze (Choi et al., 2007).The amplification and sequencing of cox2 mtDNA and ITSrDNA were performed according to a method of Choi et al. (2006),and the resulting sequences were deposited in GenBank (Acc. No.DQ409817 and DQ409816, respectively). In cox2 mtDNA-basedphylogenetic analysis with Albugo s.str. sequences from GenBank(Fig. 2), the present causal agent formed a well-supported group withA. candida from Brassica juncea (AY927046) as well as variousbrassicaceous plants am, ong which no sequence difference was found.The sequence of ITS region was also identical to those of A. candidafrom B. oleracea (AF241767) and B. juncea (AY929828). Therefore,the sequence analysis verified the pathogen to be A. candida .In Korea, it has been reported that A. candida attacks B. junceaand B. campestris subsp. pekinensis (Korean Society of PlantPathology, 2009), but to our knowledge this is the first record of A.candida on oilseed rape. As the white blister rust caused by A.candida is one of the most devastating diseases of oilseed rape inmany countries where the crop is cultivated, the occurrence of thisdisease poses a significant threat for the commercial large-scalecultivation of oilseed rape in Korea.