Sublethal effects of the methyl benzimidazole carbamate "fungicide" thiophanate-methyl applied to prevent Diplodia shoot blight of pines

Abstract

Shoot blight, canker, and collar rot diseases caused by the fungus Diplodia sapinea damage red pine (Pinus resinosa) seedlings in forest tree nurseries in the northcentral USA. Application of fungicides, including the methyl benzimidazole carbamate chemical thiophanate-methyl (TM), have been used to reduce disease incidence. But because Diplodia sapinea is known to persist on or in asysmptomatic seedlings in sprayed beds and to subsequently proliferate and kill outplanted seedlings, we tested the effects of various concentrations of TM on D. sapinea in vitro.Mycelial growth on potato dextrose agar (PDA) was greatly inhibited at concentrations of ≥ 1 μg TM ml-1. However, frequencies of conidial germination on water agar (WA) were not reduced even at 1000 μg TM ml-1 (compared to those on non-amended WA). Although exposure to concentrations ≥ 1 μg TM ml-1 in WA resulted in deformed germ tubes and hyphae, colonies resulting from germination and growth for four hours on WA amended with up to 1000 μg TM ml-1 resumed normal mycelial growth when transferred to non-amended PDA. The observed fungistatic, rather than fungicidal, effects of TM might help explain persistence of D. sapinea on or in asymptomatic nursery seedlings treated with TM or similar chemicals.