Fungistatic Action of Diphenyl on Citrus Fruit Pathogens

Abstract

1. The effect of diphenyl vapor on the growth of ten citrus fruit pathogens was studied. Growth of a few organisms was totally inhibited, some were moderately or only slightly checked, and one was aPparently stimulated. Likewise, differences in response were observed when spores placed in water drops and on agar plates were exposed to diphenyl vapor. In the case of all the fungi so exposed, the developing hyphae were larger in diameter than normal and secondary branching was greatly increased. With some species of fungi there was a tendency to produce short, malformed or giant cells. In the presence of diphenyl the spores frequently became enlarged and distorted and often burst before germ tubes were developed to any appreciable extent. Death of cells did not occur except when they actually burst. Upon removal of diphenyl from cultures the organisms resumed growth and spore production, showing that the action of this chemical is fungistatic rather than fungicidal. 2. A new method of using diphenyl as a fungistatic agent in the control of citrus fruit decay was demonstrated. Unwrapped oranges placed in cuplike depressions in special paper-pulp trays impregnated with diphenyl developed little or no decay when packed in tight fiberboard boxes, whereas the controls developed considerable rot. 3. The control of rot in citrus fruit during transit, storage, and marketing through the use of diphenyl-treated wraps or trays is due to the inhibition of germination of contaminating surface spores, the retardation of growth of exposed hyphae already present on the fruit, and the prevention of new spore formation that often is the source of infections during handling. 4. Diphenyl vapors should be effective in checking development of the following diseases of citrus fruit: blue mold rot (Penicillium italicum), Botrytis rot (B. cinerea), Diplodia stem-end rot (D. natalensis), green mold rot (Penicillium digitatum), and Phomopsis stem-end rot (P. citri). The failure of diphenyl to inhibit to any marked extent the mycelial growth and spore germination of the causal organisms of Alternaria rot (A. citri), anthracnose (Colletotrichum gloeosporioides), brown rot (Phytopthora citrophthora), cottony rot (Sclerotinia sclerotiorium), and Trichoderma rot (T. viride) suggests that these diseases are not likely to be controlled satisfactorily by this chemical.