ABSTRACT Bacterial fruit blotch (BFB) of cucurbit plants is caused by Acidovorax citrulli and represents a serious concern to melon (Cucumis melo L.) growers worldwide, including those in Brazil. Thirty-four A. citrulli strains from different melon production areas of northeastern Brazil were characterized for their virulence on melon fruits and their substrate utilization and molecular profiles. Based on the analysis of BFB severity on melon fruits, the A. citrulli strains were divided into three groups, classified as mildly, moderately or highly virulent. Although host-related groups were not observed, the watermelon and ‘melão-pepino’ strains exhibited only low or moderate virulence on melon fruit. Substrate utilization profiles revealed that 94 % of the 95 tested compounds were used by A. citrulli strains as a carbon source. Overall, based on substrate utilization, low variability was observed with no relationship to host of origin. The formation of one group of A. citrulli strains based on Repetitive Sequence-based PCR (rep-PCR) analysis confirmed the low variability observed in the substrate utilization analyses. Bayesian inference based on the analysis of 23S rDNA partial sequence data resulted in one well-supported clade and clustered the strains with the A. citrulli-type species with high posterior probability support. Based on the markers used, the Brazilian A. citrulli strains belong to a single group, which corresponds to the previously described Group I for this bacterium in the United States.

This study aimed to evaluate the effect of silicon (Si) on the resistance of tomato plants (cv. Santa Clara) to bacterial speck, caused by Pseudomonas syringae pv. tomato (Pst). Tomatoes were grown in soil without calcium silicate (control), in soil without calcium silicate and sprayed with Supa Sílica® (2 mL/L) (SS) and in soil with calcium silicate (0.16 g/kg of soil) (CS). The effect of SS on the growth of Pst was evaluated in vitro. There was no significant difference among the treatments for foliar Si concentration and incubation period. No significant differences were observed between the control and CS for the number of lesions per plant (NLP) and bacterial speck severity estimated by the software QUANT. The NLP was significantly reduced with SS spray. There was a negative linear response of in vitro Pst growth to the SS rates. Peroxidase, polyphenoloxidase and glucanase activities were significantly higher for plants sprayed with SS and grown in soil with CS compared to the control treatment. Phenylalanine ammonia-lyase and lypoxygenase activities were significantly higher for plants grown in soil with CS compared to the control. The concentration of malondialdehyde was significantly higher for plants sprayed with SS compared to the control at 0 and 7 dai. The results of this study indicate that the symptoms of bacterial speck are reduced when plants are sprayed with SS, which can be linked to the direct effect of this product against Pst rather than the potentation of host defense responses.

A mancha-aquosa, causada por Acidovorax avenae subsp. citrulli (Aac) causa grandes prejuízos à cultura do melão. O controle dessa doença foi estudado in vivo, com microbiolização de sementes de melão Amarelo infectadas, com líquidos fermentados de Bacillus subtilis R14, B. megaterium pv. cerealis RAB7, B. pumilus C116 e Bacillus sp. MEN2, com e sem células bacterianas. O mecanismo de ação dos isolados foi estudado in vitro pelo método de difusão em ágar e os compostos bioativos parcialmente caracterizados por testes de hemólise e atividade surfactante. Nos testes in vivo, não houve diferença significativa entre os tratamentos com e sem células, indicando que o controle ocorreu devido à presença de compostos bioativos produzidos durante as fermentações. Todos os tratamentos diferiram da testemunha sem diferir entre si (P=0,05%). B. megaterium pv. cerealis RAB7 proporcionou redução da incidência (89,1%) e do índice de doença (92,7%), elevou o período de incubação da mancha-aquosa de 9,8 para 11,9 dias e reduziu a AACPD de 3,36 para 0,17. In vitro, todos isolados apresentaram antibiose contra Aac e os compostos bioativos foram parcialmente caracterizados como lipopeptídeos.

The bacterial fruit blotch, caused by the bacterium Acidovorax avenae subsp. citrulli (Aac), is responsible for great losses in melon production. The control of this disease was investigated in vivo by treating infected yellow melon seeds with fermented broths of B. subtilis R14, B. megaterium pv. cerealis RAB7, B. pumilus C116 and Bacillus sp. MEN2, with and without bacterial cells. The mechanism of action of the strains was studied in vitro by the agar diffusion technique. The bioactive compounds produced were partially characterized by hemolysis test and surfactant activity. Regarding the tests conducted in vivo there was no statistical difference between the treatments with and without bacterial cells, which indicated that the control was due to the presence of bioactive compounds produced during fermentations. All treatments were different from the control (P = 0.05%) but they did not differ among them. The B. megaterium pv. cerealis RAB7 reduced disease incidence (89.1%), disease index (92.7%), increased the incubation period from 9.8 to 11.9 days and reduced the area under disease progress curve from 3.36 to 0.17. In the in vitro tests, all strains showed antibiosis against Aac and the bioactive compounds were partially characterized as lipopeptides.