Abstract
Clavibacter michiganensis subsp. nebraskensis (Cmn), the causal organism of Goss’s wilt and leaf blight of maize, can be detected in the phyllosphere of its host prior to disease development. We compared the morphology and pathogenicity of 37 putative isolates of Cmn recovered from asymptomatic and symptomatic maize leaves. Thirty-three of the isolates produced mucoid orange colonies, irrespective of the source of isolation and all but four of these isolates were pathogenic on maize. The remaining 4 isolates recovered from asymptomatic leaves had large fluidal yellow colonies, and were non-pathogenic on maize. Isolates varied in their aggressiveness on a susceptible hybrid of maize but no significant differences in aggressiveness were detected between epiphytic isolates and those recovered from diseased maize tissues. The genomics of Cmn is poorly understood; therefore as a first step to determining what genes may play a role in virulence, we compared 33 putative virulence gene sequences from 6 pathogenic and a non-pathogenic isolate recovered from the phyllosphere. Sequence polymorphisms were detected in 5 genes, cellulase A, two endoglucanases, xylanase B and a pectate lyase but there was no relationship with pathogenicity. Further research is needed to determine what genes play a role in virulence of Cmn. Our data show however, that the virulence factors in Cmn likely differ from those reported for the closely related subspecies michiganensis and sepedonicus.
Highlights
Goss's wilt and leaf blight of maize, caused by Clavibacter michiganensis subspecies nebraskensis (Cmn), is economically important in the United States (US) [1]
The goals of this study were (i) to compare the morphology and pathogenicity of isolates of Cmn recovered from the phyllosphere of apparently healthy maize leaves with isolates recovered from maize leaves with characteristic Goss’s leaf blight symptoms, and (ii) to assess sequence heterogeneity between a non-pathogenic isolate of Cmn and pathogenic isolates in 33 genes that encode for putative virulence factors
The 37 putative strains of Cmn were classified into six groups based on colony morphology after 5 days’ growth on Nutrient Broth Yeast Extract (NBY) medium (Fig 1) and pathogenicity on maize (Table 1)
Summary
Goss's wilt and leaf blight of maize, caused by Clavibacter michiganensis subspecies nebraskensis (Cmn), is economically important in the United States (US) [1]. After decades of reduced incidence, the disease recently reemerged throughout the Midwest [1, 4,5,6] Texas, [7, 8], Louisiana [9, 10], and in Canada [3, 8, 11]. Cmn can infect maize at any crop developmental stage causing either leaf blight symptoms and/or wilting of the plant. Wilt occurs when the bacteria in the vasculature becomes systemic and is characterized by an orange to brownish discoloration of the internal vascular tissue of the stalk and stunting of the maize plant [5]. The wilting is more common on young seedlings and when seed-transmission of the pathogen occurs
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