Broccoli (Brassica oleracea var. italica), cauliflower (B. oleracea var. botrytis), and cabbage (B. oleracea var. capitata) have been grown in central Mexico since 1970, with 21,000 ha cropped in 2001. In contrast, areas grown with oilseed rape (B. napus) are very limited in Mexico (<8,000 ha). Blackleg, a destructive disease of B. napus in most parts of the world, was first observed in Mexico in Zacatecas and Aguascalientes in 1988 on B. oleracea, causing as much as 70% yield loss. A species complex of two closely related Dothideomycete species, Leptosphaeria maculans and L. biglobosa, is associated with this disease of crucifers (1), but leaf symptoms on susceptible plants are different, with L. maculans typically causing >15-mm pale gray lesions with numerous pycnidia, whereas L. biglobosa causes dark and smaller lesions only containing a few pycnidia. Having a similar epidemiology, both species can be present on the same plants at the same time, and symptom confusion can occur as a function of the physiological condition of the plant or expression of plant resistance responses. A total of 209 isolates from symptomatic B. oleracea leaves were collected from three fields in central states of Mexico (58 to 71 isolates per location). All leaves showed similar symptoms, including a 10- to 15-mm tissue collapse with an occasional dark margin. Cotyledons of seven B. napus differentials were inoculated with conidia of all the isolates as described by Dilmaghani et al. (1). Two hundred isolates caused tissue collapse typical of L. maculans. However, nine obtained from white cabbage in a single location in Aguascalientes caused <5-mm dark lesions. When inoculated onto cotyledons of three B. oleracea genotypes commonly grown in Mexico (cvs. Domador, Monaco, and Iron Man), the nine isolates caused a range of symptoms characterized by tissue collapse (maximum 10 to 15 mm), showing the presence of patches of black necrotic spots within the collapse. The occasional presence of a few pycnidia allowed us to reisolate the fungus for molecular identification. ITS1-5.8S-ITS2, (internal transcribed spacers and 5.8S rDNA), actin, and β-tubulin sequences were obtained as described previously (4). Multiple gene genealogies based on these sequence data showed two subclades of L. biglobosa: L. biglobosa ‘occiaustralensis’ (one isolate; ITS [AM410082], actin [AM410084], and β-tubulin [AM410083]) and L. biglobosa ‘canadensis’ (eight isolates; ITS [AJ550868], actin [AY748956], and β-tubulin [AY749004]) (3,4), which were previously described on B. napus in the United States, Canada, and Chile. To our knowledge, this is the first report of L. biglobosa in Mexico. Previously, this species has only been reported once on B. oleracea without discrimination into subclades (2). In the Aguascalientes sampling, 24% of the isolates were L. biglobosa, similar to Canadian locations where this species is still common as compared with L. maculans (1). The large proportion of sampled L. biglobosa ‘canadensis’, highlights the prevalence of this subclade throughout the American continent (1). References: (1) A. Dilmaghani et al. Plant Pathol. 58:1044, 2009. (2) E. Koch et al. Mol. Plant-Microbe Interact. 4:341, 1991. (3) E. Mendes-Pereira et al. Mycol Res. 107:1287, 2003. (4) L. Vincenot et al. Phytopathology 98:321, 2008.
A new avirulence gene of
Leptosphaeria maculans
,
AvrLm14
, identifies a resistance source in American broccoli (
Brassica oleracea
) genotypes