Effectiveness of calcium salts, hydrogen peroxide, azoxystrobin, and antagonistic bacteria to control post-harvest rot on tobacco caused by Rhizopus oryzae

Abstract

Rhizopus rot, caused by the fungus Rhizopus oryzae (Mucorales, Zygomycota) is the most economically important post-harvest disease of flue-cured tobacco and is also known to cause diseases on vegetables and fruits. In case of high infection rates, losses in can reach up to 100% during curing of tobacco leaves. The primary sites of entry of this pathogen are wounds on leaves which are inflicted during harvest and handling. Several compounds were first tested in vitro to determine their antifungal capacities, then applied to racks filled with freshly harvested leaves to prevent colonization by R. oryzae during the subsequent curing process. Calcium propionate, hydrogen peroxide, and azoxystrobin reduced mycelial growth, spore production, and germination of R. oryzae in in vitro tests, whereas calcium chloride mainly influenced the production of fungal biomass. As calcium propionate was phytotoxic when applied to leaves, only calcium chloride, hydrogen peroxide, and azoxystrobin were selected for rack treatments. In addition, fluorescent pseudomonads were tested against natural infections of leaves by the fungus. Hydrogen peroxide had no effect on post-harvest decay. However, disease severity in flue-cured tobacco was significantly reduced by calcium chloride and fluorescent pseudomonads used alone or in combination, and nearly repressed by azoxystrobin. Therefore, an application of antagonistic bacteria alone or in combination with calcium chloride is a valuable alternative in pest management of flue-cured tobacco where residual fungicides have to be avoided.

Keywords:

• Flue-cured tobacco
• Pseudomonas fluorescens
• rack treatments

Acknowledgemets

The German Federal Association of Tobacco Planters is greatfully acknowledged for financial support. Thanks are also extended to P. Groß, D. Kolb and their families for providing everything needed. The author wish to thank H. Buchenauer for critically reading the manuscript, R. Belz for her help in statistical analysis, and A. Münzenmaier for language-related corrections.