Efeito de bactérias na promoção de crescimento e no biocontrole da fusariose em mudas de abacaxi micropropagadas

Abstract

Pineapple micropropagation is important due to the quality and quantity of the plantlets produced. After an acclimation period of 5 to 10 months, plantlets are read for planting in field where they will be exposed to several adverse factors including pests and diseases. The fusariosis caused by Fusarium subglutinans may cause losses as high as 80 and 20%, respectively in fruits and plantlets. Plant growth-promoting rhizobacteria have been used to increase development of micropropagated plantlets and for disease biological control. This study aimed to select a bacterization method and bacterial isolates that could efficiently promote growth of micropropagated pineapple plantlets cv. Pérola, increasing biomass and reducing the acclimation period; to test mixtures of isolates efficient in the growth-promotion and to determine their mechanisms of action; and to study the effect of bacteria on the fusariosis in micropropagated plantlets and the mechanisms involved in this protection. Overall the best methods were root dipping + soil drenching and root dipping the second being chosen due to its practicability. The most efficient bacterial strains were C210 (B. cereus), ENF16 (B. pumilus), RAB9 (Bacillus sp.) e ENF10 (B. thuringiensis subvar. kurstakii). Increases as high as 163.6%, 107.7% and 87.0% were obtained by RAB9 applied by root dipping, respectively for shoot dry weight, root dry weigh and leaf area thirty days after bacterization. All strains showed compatibility and combination of ENF10+RAB9, ENF16+C210 and C210+RAB9 induced respectively 100%, 88.1% and 80.1%, for root dry weight. Productions of IAA, HCN or phosphate solubilization were not detected for any of the bacterial isolates under the experimental conditions here utilized.Only nittrogen amounts in bacterized plantlets significantly differ (P=0,05) from the controls. In the study of the influence of bacterial isolates on the pineapple fusariosis by using root dipping the isolates ENF12 (not identified), C21 (Bacillus sp.), C11 (Bacillus sp.), ENF14 (Enterobacter cloacae), C210 and IN937A (Pseudomonas chlororaphis) significantly differ (P=0,05) from the control in relation to disease grade averages. ENF12, C21 and benomyl showed disease severity reduction of 85.2; 79.0 and 88.0% respectively, in relation to control without treatment. In the experiment using bacteria to spray detached leaves, disease index was significantly reduced (P=0,05) by EN5 (Alcaligenes piechaudii) which promoted disease severity reduction of 90.2%. In the experiment spraying bacteria onto attached leaves, only ENF14 significantly (P=0,05) reduced the disease severity in 55.6; 39.4 and 68.4 % respectively in relation to foliar symptoms, number of infected leaves and internal symptoms. None of the bacterial isolates produced celulase, pectinase or HCN. However C210, R14 (B. subtilis), ENF19 (not identified), ENF14, C11 and C21 inhibited the pathogen growth “in vitro” by antibiosis. In relation to iron competition the isolate EN5 showed reversion of inhibition of the pathogen mycelial growth at 20 ppm of FeCl3. The conclusions are: (i) mixtures of the strains C210, ENF16, RAB9 and ENF10 applied by root dipping are able to increase biomass production of micropropagated pineapple plantlets, reducing the acclimation period; (ii) E. cloacae ENF14 is a potential biocontroler for pineapple fusariosis. However experiments by leaf spraying with mixtures of isolates ENF14, EN5, C210 e C21 that act by distinct mechanisms are recommended.