Mefenoxam sensitive isolates of Phytophthora infestans can quickly acquire and lose resistance to this fungicide

Phytophthora infestans is an oomycete that causes late blight disease in multiple solanaceous crops, including potato and tomato. This makes it a worldwide concern for farmers, given the level of crop loss and its explosive epidemic potential. Although fungicides have traditionally been used for managing this disease, populations of P. infestans resistant to fungicides have been documented. Furthermore, it has been shown that isolates considered originally sensitive to mefenoxam acquire resistance to it after a single exposure in vitro to low concentrations of this widely used fungicide. Previous studies have measured such resistance in terms of mycelial growth. However, there is still much unknown regarding how this phenomenon is triggered. Here, we explored the dynamics of how this resistance is gained and lost, both in terms of mycelial growth and sporangial production. Isolates that acquired resistance to mefenoxam: i) did not sporulate significantly more than they did before, ii) were not affected by repeated exposures to a high concentration of mefenoxam, iii) lost the acquired resistance after a single passage through medium without mefenoxam, and iv) re-acquired the resistance after a single re-exposure to a low concentration of mefenoxam. Such dynamic behavior aligns well with the pleiotropic drug resistance mechanism that has been proposed as the molecular process behind the acquired resistance phenomenon, as well as with an epigenetic mode of regulation. This study narrows our knowledge gap on this phenomenon and reinforces the importance of deciphering whether it could be an issue under field conditions.