Resistance of citrus green mold Penicillium digitatum Sacc. to benzimidazole fungicides
Wild, Brian Lindsay.
Ph.D. Thesis, University of California, Riverside, U.S.A., 1980. 120 pp.
1980
บทคัดย่อ
Green mold, incited by Penicillium digitatum, is the principal cause of postharvest decay of citrus fruits produced in arid subtropical areas. The benzimidazole fungicides, benomyl, carbendazim, thiabendazole, and thiophanate methyl effectively controlled this disease for several years after their introduction in the late 1960's. However, the effectiveness of these compounds has been reduced recently by the appearance of resistance strains of P. digitatum throughout the citrus growing areas of the world. This dissertation is a study of the ecology and epidemiology of these resistant strains.
Variant strains of P. digitatum were classified into five categories according to their resistance to carbendazim and thiabendazole, as measured by fungus growth and sporulation in the presence of these inhibitors in vitro. The capability of P. digitatum strains in each category to infect fruit treated with high concentrations of benomyl and thiabendazole was determined. Every isolate of P. digitatum tested that was resistant to one of these fungicides was resistant also to the other (cross-resistant).
Carbendazim-resistant isolates representative of categories I, II and III were as pathogenic as the sensitive strain (category O) when single strains were inoculated alone into oranges. However, when the inoculum was a mixture of sensitive (wild) strains and resistant (variant) strains, the latter decreased in frequency in subsequent spore generations. Suppression of resistant strains by sensitive strains appeared to be a general phenomenon and was not influenced by the proportion of spores of each strain in the initial inoculum mixture. However, some sensitive strains isolated from lemon packinghouses were only capable of suppressing the resistant strain in lemons but not in oranges.
Despite the suppression of the resistant strains by the sensitive strains in citrus fruit in the laboratory, it was demonstrated that a high level of resistant strains could be maintained in the spore population of a citrus packinghouse through the mechanism of double-resistance to the benzimidazole fungicides and either sodium o-phenylphenate (SOPP) or sec-butylamine (SBA). These doubly-resistant isolates were characterized according to their in vitro tolerance to pairs of chemically-unrelated fungicides, and were found to grow as well on medium amended with either of the fungicides as did the single resistant strains.
Competition experiments in untreated oranges showed that the frequency of the doubly-resistant isolates decreased when co-inoculated with a sensitive strain. The suppressive action of the sensitive strain was overcome by treatment of inoculated (doubly-resistant + sensitive strains) fruit with a mixture of benomyl and either SOPP or SBA. Residues of benomyl on the fruit provided selection pressure for the proliferation of the benzimidazole resistant strains irrespective of the second fungicide in the treatment mixture. No isolates were found that were triply-resistant to the benzimidazole fungicides, SOPP and SBA, or doubly-resistant to SOPP and SBA.
An investigation of the infectivity of inoculum mixtures (sensitive + resistant strains) in benomyl-treated fruit demonstrated that the sensitive strain increased the infectivity of the resistant strain, despite the inability of the sensitive strain alone to incite disease in benomyl-treated fruit. Microscopic examination of the inoculation site on benomyl-treated orange revealed that conidia of the sensitive strain germinated and formed distorted non-infective germ tubes. Morphologically-similar germlings produced in the presence of carbendazim in liquid culture elaborated pectin-degrading enzymes that reduced the inoculum threshold for carbendazim-resistant strains.