Pale swallowwort (PSW) and black swallowwort (BSW) are two viney milkweeds native to Europe that have increasingly become problematic and noxious weeds in eastern North America. An indigenous fungal isolate, Sclerotium rolfsii VrNY, was discovered causing significant mortality in a dense stand of PSW in a park in upstate New York. Although this fungus is a known pathogen with a broad host range, we evaluated the host potential of S. rolfsii VrNY on a limited range of related and nonrelated U.S. species as a critical first step to assess its suitability as a mycoherbicide for PSW and BSW. In addition, PSW and BSW produce the specific stereoisomer (−)-antofine, a compound with antimicrobial and phytotoxic activity that could inhibit the pathogen. Tests revealed this compound had no effect on S. rolfsii VrNY. This isolate caused significant mortality on all broadleaf plants tested (Asclepias syriaca, Asclepias curassavica, Apocynum cannabinum, Monarda fistulosa, Rudbeckia hirta, PSW, BSW) with the exception of Glycine max, and had no effect on the monocots Schizachyrium scoparium and Zea mays. Although these laboratory studies indicate that most broadleaf vegetation may be susceptible to the pathogen, S. rolfsii might have potential as a mycoherbicide in natural eco-niche environments where invasive PSW and BSW have already become the predominant vegetation. Further laboratory testing of S. rolfsii and limited field testing at the initial discovery site are needed in order to prevent premature rejection of this isolate as a potential management tool against these highly invasive weeds.
Nomenclature: Black (or Louise's) swallowwort, Vincetoxicum nigrum (L.) Moench, syn. Cynanchum louiseae Kartesz & Gandhi, bloodflower milkweed, Asclepias curassavica L., common milkweed, Asclepias syriaca L, hairy coneflower, Rudbeckia hirta L., hemp dogbane, Apocynum cannabinum L., little bluestem, Schizachyrium scoparium (Michx.) Nash, pale (or European) swallowwort, Vincetoxicum rossicum (Kleopow) Barb., syn. Cynanchum rossicum (Kleopow) Borhidi, wild bergamot, Monarda fistulosa L., corn, Zea mays L., soybean, Glycine max (L.) Merr.
Management Implications: Pale swallowwort (PSW) and black swallowwort (BSW) are two viney milkweeds native to Europe that have become invasive in eastern North America. Long term biological control efforts are the best hope for mitigating the spread of these weeds, but mycoherbicides might also offer a complementary approach for integrated pest management. An indigenous fungal isolate, Sclerotium rolfsii VrNY, was discovered casuing significant mortality in a dense stand of PSW in a park in upstate NY. This fungus is known to be a pathogen of a wide range of plants, but the possibility exists for a pathogen strain to occur that has a narrower host range. Even if not, the fungus might still have potential as a mycoherbicide in limited natural environments where PSW and BSW have become the predominant vegetation. In a laboratory study, we tested the ability of S. rolfsii VrNY to infect related and nonrelated (habitat associated) native U.S. species as well as soybean and corn, known hosts in the southern U.S. The isolate caused significant mortality on all broadleaf plants tested (common milkweed, bloodflower milkweed, hemp dogbane, wild bergamot, and hairy coneflower) except soybean, and had no effect on the monocots little bluestem and corn. Further laboratory testing of S. rolfsii and limited field testing at the initial discovery site are needed in order to prevent premature rejection of this isolate as a potential management tool against these highly invasive weeds.
