First Report of Leptosphaeria biglobosa as a Stem Canker Pathogen of Brassicas in New Zealand

Abstract

Phoma black leg or stem canker, caused by Leptosphaeria maculans or L. biglobosa, is an important disease of brassicas, causing significant crop losses in areas such as Europe, Australia, and North America (1). Samples collected in 2011 from canola and forage brassica (swede, kale, and turnip) crops in the main New Zealand growing regions (Southland, Central Otago, Canterbury, Hawkes Bay, and Manawatu) to identify the causal agent(s) of the characteristic stem cankers, found many isolates of L. maculans, which has been reported previously in New Zealand (2), and three isolates identified by colony characteristics as L. biglobosa. Of the latter, two isolates were from canola (Brassica napus) stem cankers from Darfield and Lincoln, Canterbury, and one was from a kale (B. oleracea) stem canker from Lincoln. An isolate (ICMP10665) of similar morphology, from the International Collection of Microorganisms from Plants (ICMP), obtained from a basal rot lesion on a cauliflower (B. oleracea var. botrytis) plant in Levin, New Zealand in 1979, was also evaluated. The initial, incorrect identification of the latter isolate as L. maculans predates the reclassification of L. maculans group B isolates as a new species, L. biglobosa (1). These four isolates produced fluffy white mycelium and a yellow pigment on potato dextrose agar (PDA) after 5 days' growth, and abundant black-brown, globose pycnidia containing cylindrical hyaline conidia after 7 days. In contrast, L. maculans isolates had slower growth and no pigment production (4). Amplification of genomic DNA using species-specific primers LmacR, LmacF, and LbigF (1) generated a PCR product of 444 bp that is typical of L. biglobosa isolates. Sequencing of the PCR product from each of the four isolates showed they were 100% identical to a sequence of L. biglobosa ‘brassicae’ in GenBank (JF740198). To confirm the species identity of the isolates, the rDNA, actin, and β-tubulin gene regions were amplified (1,3). Sequences for the rDNA (568 bp), actin (941 bp), and β-tubulin (410 bp) gene regions were 99% identical to sequences of the same regions of isolates in GenBank for L. biglobosa ‘brassicae’ (AY48997, AY748949.1, and AY748997.1, respectively). The four L. biglobosa isolates were tested for pathogenicity on a canola cultivar commonly grown in New Zealand (Flash). Cotyledons of 10-day-old seedlings (n = 12 seedlings/isolate or control treatment) grown in a potting mix in pots were pricked with a sewing needle, and each wound inoculated with 10 μl of the appropriate conidial suspension (106 conidia/ml) or 10 μl sterilized distilled water for the control treatment. Leaf lesions that developed on the inoculated cotyledons were characteristic of those caused by L. biglobosa, i.e., small and dark with a distinct margin. No pycnidia were produced on the lesions. No lesions developed on the cotyledons of the non-inoculated control plants. The causal agents were confirmed as L. biglobosa by the colony morphology of isolates that grew from surface-sterilized, inoculated leaf lesions plated on PDA amended with 100 μg/ml ampicillin. The fungus was not isolated from control leaf tissue. To our knowledge, this is the first report of L. biglobosa as a pathogen of canola and kale in New Zealand. This finding shows that both causal agents of black leg are present in New Zealand's brassica cropping areas. References: (1) S. Y. Liu et al. Plant Pathol. 55:401, 2006. (2) H. C. Smith and B. C. Sutton. Trans. Brit. Mycol. Soc. 47:159, 1964. (3) L. Vincenot et al. Phytopathology 98:321, 2008. (4) R. H. Williams and B. D. L. Fitt. Plant Pathol. 48:161, 1999.