Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant Pathology and Crop Physiology

First Advisor

John P. Jones


Pythium dimorphum Hendrix & Campbell and other Pythium species were determined to be pathogenic fungi that cause the death of longleaf pine seedlings during cold storage. Pathogenicity of these Pythium isolates as determined by infecting newly-germinated slash pine seedlings in vitro was confirmed in field evaluations. An in vitro dual culture screening system was effectively used for selecting Trichoderma isolates capable of killing P. dimorphum. Selected Trichoderma isolates and Gliocladium virens produced volatile and/or non-volatile antibiotics on solid media and in liquid cultures, but antibiotic production could be affected by medium type, pH, and culture age, depending on species or isolate. Some antibiotic-lacking Trichoderma isolates were able to parasitize and kill hyphae of P. dimorphum, but did not kill whole colonies. Antibiotics produced by the Trichoderma spp., rather than parasitism, were responsible for killing and/or inhibition of Pythium in vitro. After being delivered to the root systems of longleaf pine seedlings, both Pythium and Trichoderma could develop populations rapidly during cold storage. Seedling mortality was only correlated with high Pythium populations, where as seedling survival percentage was not always associated with high populations of Trichoderma. All cold-tolerant Trichoderma isolates were effective in biocontrol of Pythium in field experiments where Trichoderma wheat bran inoculum (400 mls) in clay slurry was applied to roots 1 week before the Pythium, regardless of their ability to kill or inhibit Pythium in vitro. These results suggested that the mechanisms involved in killing or inhibition of P. dimorphum in vitro were not useful for predicting biocontrol efficacy in the field. Survival data from 3 years of field experiments provides strong evidence that time and method of application were critical to obtain a successful biological control of P. dimorphum in longleaf pine seedling roots during cold storage.