Submitted to: Journal of Plant Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/29/1996
Publication Date: N/A
Interpretive Summary: Production of high-value crops including ornamental plants for cut flowers and foliage requires intensive use of pesticides to assure desirable floral and leaf appearance that consumers demand. Recently the appearance of abnormalities in leaf shape and color of leatherleaf fern (used as green foliage in cut flower arrangements) occurred so frequently in many production fields that much of the harvestable leaves were unmarketable. One chemical most commonly used by all affected ferneries was a fungicide (Benlate DF) which was applied very often during fern growth to control diseases that affect fern leaf appearance. We found that plants continuously treated with the fungicide developed optimum conditions in the soil around roots for growth of certain microorganisms that produced chemicals toxic to plant growth. Thus, continuous use of the same fungicide year after year on the same field was causing an apparent buildup pin the soil of plant-detrimental microorganisms unrelated to the disease fungi the fungicide was to control. This information is being used to change approaches for managing diseases of fern and to renovate fields previously receiving continuous applications of the fungicide.
Technical Abstract: Foliar damage symptoms to leatherleaf fern (Rumohra adiantiformis) in Florida during widespread use of the fungicide Benlate DF could not be attributed to new pathogens developing on the crop. The objective of this study was to assess the involvement of rhizosphere bacteria in the damage symptoms using bioassays to detect phytotoxic activity. Rhizosphere bacteria were cultured from rhizospheres of leatherleaf ferns sampled from ferneries where Benlate DF was routinely applied and from a check not receiving the fungicide. Using a lettuce seedling bioassay, the proportion of rhizosphere bacteria that was plant growth-inhibitory ranged from 7.5% for isolates from a check fernery to 70% for those from a fernery previously treated with Benlate DF. Rhizosphere bacteria originating from Benlate DF-treated leatherleaf ferns caused the most severe damage on seedlings with some isolates, reducing root growth 70% compared to control seedlings. Other symptoms induced by these bacteria, which were mainly fluorescent and nonfuorescent pseudomonads, included necroses and inhibition of root hair development. Results suggested that Benlate DF affected the composition of bacteria in leatherleaf fern rhizospheres by promoting a bacterial component with phytotoxic properties toward plant growth.