The impact of pesticides on the pathogen Batrachochytrium dendrobatidis independent of potential hosts.

Amphibians around the world are experiencing the greatest organismal decline in recent history. Xenobiotics, such as pesticides, and pathogenic biotic perturbations, including the fungus Batrachochytrium dendrobatidis (Bd), have played major roles in amphibian decreases. We conducted laboratory culture studies to determine the effects of three pesticides {carbaryl, glyphosate, and thiophanate-methyl [TM; Topsin-M(R) (Cerexagri-Nisso LLC)]} on Bd zoospore production and zoosporangia growth. We applied Bd to pesticides mixed in an agar culture to simulate pathogen introduction to a system with pre-existing pesticides (Bd addition). Alternatively, pesticides were applied to pre-established Bd to simulate pesticide introduction after Bd establishment (pesticide addition). We then measured Bd zoosporangia and zoospore production. All pesticides significantly inhibited zoospore production; however, glyphosate and TM were more effective at doing so than carbaryl. In addition, only carbaryl and glyphosate inhibited zoosporangia production. Our data suggest that carbaryl and glyphosate are equally effective at inhibiting both zoosporangia and zoospore production; however, TM is selectively toxic to zoospores but not zoosporangia. One possible explanation for this observation could be that TM is toxic to zoospores but not the protective zoosporangia. In the case of pesticides applied to established Bd cultures, all pesticides caused significant mortality in both zoosporangia and zoospores, and no differences were found among pesticides. We conclude that examining pesticide and pathogen interactions independent of hosts provides mechanistic understanding of such interactions before and after host infection or contamination.