Manejo da mancha aquosa do meloeiro: fontes de resistência genética e extratos aquosos de plantas da Caatinga

Abstract

The bacterial fruit blotch (BFB) caused by the bacterium Acidovorax citrulli is one of the main and important diseases of the melon culture in the producing areas of Northeast Brazil and the world due to its destructive potential and absence of effective management measures. In view of this, the main objectives of the work were: (a) to identify accessions of melon with source of genetic resistance to the fruit blotch in different phenological stages of the plant and to test the stability of the accessions to multiple isolates of A. citrulli; (b) to evaluate the efficacy of aqueous extracts of plants of the phytogeographical domain Caatinga in the control of the fruit blotch of the melon by the treatment of seeds and seedlings. In addition, the colonization of A. citrulli in leaf tissues of the seedlings after infection was examined under optical microscopy and scanning electron microscopy SEM. In the selection of resistance sources, A. citrulli CCRMAc1.12 was inoculated in seeds (55 accessions), seedlings and plants before flowering (39 accessions), with severity of the fruit blotch estimated with the aid of descriptive scales. After the stability test of six accessions to three strains of the bacterium, the resistance of the accessions I-136 and A-43 was evaluated in plants in the flowering and fruiting stages, transmissibility test and microscopic analysis. The fruits of these accesses did not show symptoms of the watery spot and the transmissibility of A. citrulli by the seeds was only 0,6% (I-136) and 2.5% (A-43); microscopic analyzes showed little structural and temporal changes in the colonization of A. citrulli in the leaf mesophyll. Among the eight aqueous extracts tested in the control of the fruit blotch, the extracts of Croton heliotropiifolium, Ximenia americana and Ziziphus joazeiro were selected as promising in gross concentration, as they were efficient in both seed treatment and seedling spraying, with levels of control from 56.76 to 94.14% respectively. Optical microscopy showed little colonization of A. citrulli on cotyledonary leaves treated with these three extracts, and mesophilic cell structures (palisade and lacunar parenchyma) were structured and organized. Little aggregation of bacterial cells, with little or no colonization of the stomata was also verified in SEM. With the results obtained it is suggested that accessions A-43 and I-136 may be donors of resistance genes to the fruit blotch and the aqueous extracts of C. heliotropiifolium, X. americana and Z. joazeiro can be exploited as a new technology for management of bacterial fruit blotch of melon.