Patogenicidade de Pratylenchus brachyurus e severidade da murcha bacteriana quando associada a este nematóide em fumo

Abstract

The pathogenicity of Pratylenchus brachyurus on tobacco (Nicotiana tabacum) cultivar K326 was evaluated in experiments testing different initial population densities of the nematode (Pi = 0; 1,000; 3,000; 9,000 and 27,000 eggs and larvae per plant), in eight replications. The isolate was monoxenically multiplied on carrot discs. At the end of the experiment (70 days after inoculation), the above ground part of each plant was weighed and measured and, the respective root system was processed. The reproduction rate of Pratylenchus brachyurus was estimated by extracting the nematodes of the roots and of the soil. Necroses on the roots of the tobacco plants were observed. Pratylenchus brachyurus caused well-delimited root lesions on tobacco root. None of the analyzed plants died; however plant growth was reduced. Also, it was observed a decrease of the fresh mass of roots and of the dry top weight, mainly under the highest population density (27,000 nematodes per plant). The reproduction rate was low. Besides the interaction between Pratylenchus brachyurus and Ralstonia solanacearum in two tobacco varieties, Virgínia K326 and Burley By21 was evaluated. Test plants were inoculated with the nematode in the initial population density of 3,000 individuals (eggs and larvae) by plant and, 15 days later, the plants were inoculated with a sample of soil infested by the bacterium, in estimated population density of 106 CFU.g-1 of soil. Besides the mixed inoculation with Pratylenchus brachyurus and Ralstonia solanacearum, tobacco plants were single inoculated. The plants were monitored daily. Dates of the beginning of symptom expression were registered, as well as symptom evolution. When the plants were inoculated with Pratylenchus brachyurus and Ralstonia solanacearum, symptom of wilt that is caused by the bacterium Ralstonia solanacearum began earlier than in single inoculated plants, and they also evolved quicker, inclusive in the variety Virgínia K326 that was considered more tolerant. In this case, the presence of Pratylenchus brachyurus, in the root system of these plants, probably favored the infection by Ralstonia solanacearum, even before the symptoms caused by the nematodes could be detected. This study was done in greenhouse and under laboratory conditions.