Phosphite is more effective against Phytophthora Crown Rot and Leather Rot caused by Phytophthora cactorum than P. nicotianae

Abstract

Phytophthora crown rot (PhCR) and leather rot (LR) caused by Phytophthora spp. are major threats to strawberry production worldwide. In the United States, these diseases are mainly caused by Phytophthora cactorum; however, P. nicotianae has also been recently reported causing PhCR. Growers have relied on three different chemical products, i.e. mefenoxam and phosphites for PhCR and LR, and azoxystrobin for LR. Since resistance to mefenoxam and azoxystrobin has been reported, this study aimed to assess the in vitro sensitivity of Phytophthora spp. isolates from strawberry to phosphites and investigate its efficacy on in vivo assays. In vitro sensitivity of P. cactorum (n=128) and P. nicotianae (n=24) isolates collected from 1997 to 2018 was assessed at 10, 50, 100, 150, and 300 µg/ml of phosphite. Regardless of the Phytophthora species and isolation organ, most of the isolates (75% for P. cactorum and 54.2% for P. nicotianae) had EC50 values ranging from 50 to 100 µg/ml. In vivo tests with strawberry fruit and plants revealed that commercial formulations of phosphite applied at the highest field rate controlled P. cactorum isolates but failed to control PhCR and LR caused by some isolates of P. nicotianae. In this study, EC50 results from in vitro assay did not truly translate the efficacy of phosphites on controlling LR and PhCR caused by P. cactorum and P. nicotianae. Our findings support the hypothesis that the product acts in a dual way: direct on the pathogen and stimulating the plant immune system. Moreover, this has important implications for disease management, highlighting the importance of a correct diagnosis before phosphite recommendations, since its efficacy varies within Phytophthora spp.