Cancro bacteriano (Xanthomonas campestris pv. viticola) da videira (Vitis spp): métodos de preservação e crescimento de isolados;escala diagramática e reação de variedades de videira à doença

Abstract

The Submédio São Francisco is the main producting and exporting region of table grapes in Brazil, where the cultivated area with this fruit crop has been significantly expanded in the last years. However, the intensification of grape production, the use of susceptible cultivars, and the favorable environmental conditions of that region allowed the occurrence of disease and pest problems, with emphasis for the bacterial canker (Xanthomonas campestris pv. viticola - Xcv). Reported for the first time in Brazil, in plantations of the Submédio São Francisco in 1998, the canker is the most important bacterial disease of grapevine in that region. The present work aimed to study: methods of preservation and pathogen growth at different temperatures, pHs and NaCl concentrations; elaboration and validation of a diagramatic key for bacterial canker; and the reaction of grapevine varieties to the pathogen, based on the disease epidemiological components. The preservation methods: dried paper strips (DPS), periodic transfer (PT), sterile distiled water (SDW) and dried leaves (DL) were utilised for storing two Xcv strains during 12 months. The variables viability and pathogenicity were evaluatedmonthly and estimated by bacterial growth and area under disease incidence curve (AUDIC). Both the DPS and SDW methods maintained 100% of cell viability and showed higher AUDIC values during 11 months. PT did not permit growth at 30 days while DL allowed the pathogen isolation for 5 months. The influence of temperature (0, 5, 10, 15, 20, 25, 27, 28, 29, 30, 35, 40 and 45°C), pH (5.0; 5.5; 6.0; 6.5; 7.0; 7.5; 8.0; 8.5 and 9.0) and NaCl concentration (1, 2, 3, 4, 5, 6 and 7%) on the growth of two Xcv strains was studied in NYD medium and evaluated in spectrophotometer. The minimum and maximum temperatures for Xcv growth were respectively 5 and 39°C, while the optimum growth was observed from 27 to 29°C. Xcv does not growth at 0 or 40°C. The optimum pH for Xcv growth ranged from 7.0 to 7.5. The pathogen growth declined from 3.0 % NaCl and was null at 6.0 %. Aiming to standardize methods to quantify bacterial canker severity a diagrammatic key was elaborated including the levels 2, 4, 8, 17, 34, 63 and 91% of diseased leaf area. To validate the diagrammatic key, 50 leaves with different levels of severity, previously measured by the softwareAutoCAD®, were evaluated by 10 raters with and without the use of the key. Two evaluations were performed with the key at 7-day intervals when different sequences of the same leaves were visually estimated by the same raters. The accuracy and precision of each rater were determined by simple linear regression between actual and estimated severity. Without the key, most of the raters overestimated disease severity. With the key raters obtained better levels of accuracy and precision; however, all tended to underestimate severity, with absolute errors concentrated around 10%. Raters showed good repeatability and high reproducibility of estimative by using the key compared to not using it. The reaction of 20 grapevine varieties, 13 scions and seven rootstocks, was evaluated in relation to disease resistance under greenhouse conditions. Plants were inoculated with a suspension of the strain Xcv1 (A570 = 108 CFU mL-1), incubated in a greenhouse and observed daily for epidemiological components of bacterial canker: incubation period (PI), incidence of leaves with symptoms (INC), incidence of leaves with canker (IFC), disease severity (SEV), progress rate of disease incidence (TPID), area under the disease severity progress curve (AACPSD). All varietieswere susceptible to the pathogen, although there were significant differences among them (P=0.05) for most of the analyzed variables. ‘Brasil’ showed the highest disease levels for all variables tested, while ‘Isabel’ and ‘Paulsen 1103’ presented the highest values of PI and the lowest values of INC, IFC, SEV, TPID and AACPSD, suggesting the potential of those varieties in programs of genetic breeding and integrated management of grapevine bacterial canker. The studied variables might be utilized in research including reaction of varieties to this disease. Considering all the epidemiological components, the analysis of the Euclidean distance by UPGMA allowed the separation of scion and rootstock varieties into three similarity groups each.