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Title: First report of black swallow-wort as an alternate host of the two-needle pine stem rust pathogen, Cronartium flaccidum (Alb & Schwein.) G. Winter, in France

item BON, MARIE-CLAUDE - European Biological Control Laboratory (EBCL)
item GUERMACHE, FATIHA - European Biological Control Laboratory (EBCL)

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2011
Publication Date: 4/15/2012
Citation: Bon, M., Guermache, F. 2012. First report of black swallow-wort as an alternate host of the two-needle pine stem rust pathogen, Cronartium flaccidum (Alb & Schwein.) G. Winter, in France. Plant Disease. 96 (4),585.

Interpretive Summary: The invasive weed black swallow-wort was introduced prior to 1900 from southern Europe into North America. In its native range it occurs as scattered plants or small patches. Conversely, this vine is a highly invasive weed in natural and managed habitats in the NE U.S. and SE Canada. By forming dense stands, this weed has replaced much of the native vegetation, including milkweeds that are food plants for the protected monarch butterfly. Mechanical and chemical methods are not effective in controlling black swallow-wort, prompting the USDA Agricultural Research Service to initiate a biological control program. In the native European range, few or no pathogens have been recorded on black swallow-wort. During a search for new natural enemies of black swallow-wort in France, we isolated and identified a pine stem rust, CRONARTIUM FLACCIDUM, from this target weed. Black swallow-wort thus represents a potential reservoir in North America for this destructive pine rust, should it be accidentally introduced.

Technical Abstract: Black swallow-wort, Vincetoxicum nigrum (L.) Moench, is a non-indigenous species that is highly invasive over large areas in the U.S., especially in the northeastern U.S. and southeastern Canada where it displaces desirable vegetation in upland natural areas, old fields, and landscape settings. Efforts have been underway to discover biological control agents of black swallow-wort across its native range of southern France, the Iberian Peninsula, Portugal, and northern Italy. Surprisingly little or no disease damage has been reported from black swallow-wort, probably becauseit produces alkaloids with antimicrobial activity. The occurrence of a stem rust, Cronartium flaccidum, was reported in 1918 in Spain and Portugal, on swallow-wort under the species name C. asclepiadum, which now includes both black and white swallow-wort species. In 2011, we found in the vicinity of Aleppo pines in southern France several black swallow-worts with orange yellow pustules of rust on the underside of leaves. Observed morphological features of these pustules were uredinia that eventually formed telia and which were consistent with those of C. flaccidum found on white swallow-wort in Finland and on pines in Italy. Molecular characterization using DNA sequencing of internal transcribed spacer regions confirmed the identification of this rust. This is the first report of occurrence of telia and uredinia of the two-needle pine stem rust on black swallow-wort definitively identified in France.