Effects of Benlate(R) on leatherleaf fern growth, root morphology and rhizosphere bacteria

Reduced frond production as well as foliar disorders and root damage to leatherleaf fern (Rumohra adiantiformis [Forst.] Ching) coincided with the widespread use of Benlate fungicide in grower operations. These symptoms could not be explained by environmental conditions or the presence of new pathogens. The objective of this study was to examine the effects of Benlate at labeled rates on frond production, root morphology, and soil microbial characteristics of featherleaf ferns. A separate experiment was conducted by adding Benlate at labeled drench rate to previously untreated soil to evaluate the promotion of soil bacteria. Benlate was applied to leatherleaf fern plants in the greenhouse at labeled foliar and drench rates for 17 months. Fronds were harvested during the last nine months. Roots of the ferns were examined using electron microscopy at termination of the experiment to determine physical and microbial characteristics-of the roots. Rhizosphere bacteria were evaluated to determine counts and types. Lettuce seedling bioassays were conducted to evaluate bacterial isolates for possible deleterious effects on plants. Over the nine-month harvest period, 181 fronds were harvested per pot from the control treatments, 162 for the Benlate foliar treatment, and 113 for the drench treatment. The Benlate drench rate resulted in significant reduction of frond production. However, the Benlate foliar rate was not significantly different from the control. Benlate application resulted in a reduction of root hairs and disruption of the root surfaces, especially at the drench rate. Total bacterial counts were not significantly different among Benlate treatments; however, the biodiversity of these populations was reduced with 46, 36, and 22 individual isolates identified for control, foliar, and drench rate treatments, respectively. Using a lettuce bioassay, compared to seedlings grown on sterile medium, the average seedling growth in the presence of isolates from the control treatment was 83%, 73% from the foliar rate treatment, and 60% from the drench rate treatment. The predominant deleterious isolates include Pseudomonas fluorescens, Ps. putida, and Ps. cepacia. In a laboratory experiment, these bacteria were shown to be promoted within the soil by Benlate application. These results indicate that Benlate has the potential to reduce yields of leatherleaf ferns. This reduction may be caused by direct action of the chemicals within the Benlate formulations or by the promotion of deleterious bacteria within the growing media.