Epidemiologia da podridão vermelha do sisal no Estado da Bahia

Abstract

Abstract: Brazil is the world's greatest producer of sisal (Agave sisalana Perrine), and the State of Bahia is the main producer, contributing with 94% of Brazil's production. Sisal red rot disease is the main phytossanitary problem of this crop. This disease causes symptoms of pale green to yellowish and wilted leaves, and a reddish discoloration of internal stem tissue, which later becomes rotten and causes plant death. The present work is a pioneer study on epidemiology of sisal red rot disease in the State of Bahia, Brazil, which aimed to: identify the etiologic agent of lesions observed on sisal stolons in the field, the disease incidence in sisal plants with and without stolons, submitted to injury and inoculation, the probability of disease occurrence in plants submitted to stress conditions: injury and drying period; the minimum number of plant sampling for quantification of sisal red rot disease; disease prevalence and incidence in the field; to establish the spatial and temporal patterns of sisal red rot disease in order to characterize the dynamics of this disease in sisal producing areas of the micro-region of Serrinha, in the State of Bahia. Prevalence and incidence study was conducted in 46 areas in the micro-region of Serrinha, in Bahia, for three consecutive years (2006 to 2008). Disease spatial and temporal analysis was performed in nine sisal producing areas, located in the municipalities of Araci, Barrocas, Conceição do Coité, Retirolandia, São Domingos and Valente, by monitoring the evolution of symptoms in affected plants and the appearance of new symptomatic plants, during the period from January of 2008 to march of 2010. For mapping the areas, 400 plants were marked in each area and the positions relative to the other was recorded, along with monitoring of plant phytopathological status, according to the disease diagrammatic scale, at each evaluation period and area. The spatial distribution of disease was performed with the aid of software 2DCORR, and also maps of isopath areas of areas with highest incidence were elaborated. In addition, the area under disease progress curve (AUDPC) and the absolute rate of disease progress were estimated to evaluate the influence of initial disease incidence on disease progress in the field. Probability of disease incidence was of 100% and 5% for sisal plants with and without injury, respectively, and severity was also higher in injured plants. Sisal stolons are susceptible to infection by Aspergillus niger. Disease prevalence was 100% in studied municipalities. Average incidence of sisal red rot disease in studied municipalities varied between 5 and 40%. Maps of isopath areas revealed a tendency for agglomeration of areas with highest disease incidence in the Northeastern and Central region of the studied sisal micro-region of Serrinha, for the three years evaluated. This result was confirmed by the dispersion index, which indicates that, although it presents a widespread occurrence, disease spatial distribution is aggregated in the micro-region of Serrinha. However, disease occurs randomly within the planting area, alternating with periods of increase in incidence and periods of stability. The initial disease incidence in sisal fields responds for 82% of the differences observed for the disease progress curves.