Induction of systemic resistance in tobacco against metalaxyl-tolerant strains of Peronospora tabacina and the natural occurrence of the phenomenon in Mexico

Field experiments were conducted in San Andres Tuxtla, Veracruz, Mexico, during 1986 and 1989 to test the effectiveness of induced systemic resistance (immunization) against metalaxyl-tolerant strains of Peronospora tabacina, causal agent of tobacco blue mold. Plants of Nicotiana tabacum cultivar Jaltepec were stem-injected with sporangiospores of metalaxyl-tolerant strains of P. tabacina. Half of the plants were treated with metalaxyl. The number and size of blue mold lesions that developed because of infections caused by ambient inoculum were significantly fewer and smaller on plants injected with P. tabacina as compared with noninjected stem controls, regardless of metalaxyl applications. Some vigorously growing, naturally infected plants with necrotic stem lesions but markedly reduced foliar lesions of blue mold were observed in commercial fields surrounding the experimental plots. Stem necrosis was consistently associated with these with these plants. Stem necrosis was not detected in plants infected heavily with foliar lesions of blue mold. Several attempts to isolate the causal agent of necrosis associated with acquired resistance to blue mold failed. Only live sporangiospores of P. tabacina produced necrosis typical of that found in the naturally protected plants. Natural immunization apparently occurs at scattered locations on the gulf coast of Mexico and is associated with vigorous growth. Furthermore, the endemic nature of the disease, presence of sexual stages of the fungus, long-term moisture in the field, and high constant disease pressure created conditions very favorable for the development of tolerance to any protective agent, such as metalaxyl, with a single site of action; however, development of tolerance to immunization has not yet been observed.