A Semi-Selective Medium to Evaluate Over-Summering Survival of Neopestalotiopsis sp. in Florida Strawberry Fields

Abstract

Devastating disease outbreaks with leaf spot symptoms and fruit rot caused by a fungus identified as a cryptic species of Neopestalotiopsis have generated concern in the Florida strawberry industry. Some Neopestalotiopsis species are saprobes in soil and plant debris, but the ability of the new Neopestalotiopsis sp. to survive in strawberry debris was unknown. Therefore, the objectives of this study were to develop a semiselective medium for isolation of Neopestalotiopsis spp. and to evaluate Neopestalotiopsis sp. survival in soil and strawberry plant debris over the summer in Florida. The Neopestalotiopsis semiselective medium (NSM) inhibited growth of most fungal species pathogenic to strawberry in Florida, except Neopestalotiopsis rosae, Neopestalotiopsis sp., and Phomopsis obscurans, in addition to Penicillium spp., which are nonpathogenic. However, Neopestalotiopsis species, P. obscurans, and Penicillium spp. could be distinguished in the medium through morphological characteristics. Soil samples arbitrarily collected from six commercial fields toward the end of the season (May) and before (July) and after (September) preplant soil fumigation in the following season were processed with NSM, and Neopestalotiopsis spp. populations were calculated as CFU per gram of soil. CFU ranged from 48.3 to 2,410.8 at the end of the season and from 1.7 to 630.8 before soil fumigation, but Neopestalotiopsis spp. were not recovered after soil treatment. However, 1.7 to 25 CFU were obtained from the nontreated areas in the row middles. Neopestalotiopsis sp. survival was also evaluated on diseased strawberry plants (leaves and crowns) exposed to the environment for 17 months. On leaves, Neopestalotiopsis spp. were recovered for 6 months before leaf decomposition, whereas the number of CFU on crowns declined monthly, but colonies were still recovered during the final evaluation months. A high-resolution melting analysis confirmed most of the colonies from soil and crowns were the new Neopestalotiopsis sp. Our results show Neopestalotiopsis sp. can survive in soil and strawberry debris under Florida summer conditions and may serve as a source of inoculum for the subsequent season.