Multiplication and movement of Xylella fastidiosa within Citrus sinensis and its efficiency of acquisition and inoculation by sharpshooter vectors

Author(s)

Rodrigo Piacentini Paes de Almeida

• Document type: Master's Dissertation
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 1999-08-10
• Advisor: João Roberto Spotti Lopes

Abstract

The main goal of this research was to investigate possible factors determining the low transmission efficiency of Xylella fastidiosa by sharpshooter leafhoppers (Hemiptera, Cicadellidae) in citrus. Initially, to techniques to isolate and quantify X. fastidiosa population in citrus and vectors, and to needle inoculate the pathogen in the host plant. Studies were then carried out to evaluate the acquisition and inoculation efficiencies of X. fastidiosa by leafhoppers, as well as the multiplication and movement of this bacterium in citrus. The isolation and quantification procedure was successfully adapted, being efficient to detect the bacterium in citrus. ln the specific media for X. fastidiosa growth, PWG and PW, colonies were observed 7-8 days after plating, with a diameter achieving 1 mm after 14 days of incubation, these results are superior to those obtained with BCYE medium, in which colonies were observe 14-15 days after plating with smaller size, this medium also was 60-70 % less efficient in bacterial recovery than the previous two. Homogenized citrus tissue did not inhibit X. fastidiosa bacterial growth on solid medium, in fact, it stimulated more colony recovery when compared to pure suspensions of the pathogen. None of the assayed leafhoppers by the diagnostic tests used acquired X. fastidiosa in citrus, therefore the acquisition might be an inefficient step of the transmission. About 10 % of the tested insects in the inoculation experiment had X. fastidiosa isolated in medium from their heads, but no inoculation was observed, also suggesting inefficient inoculation. The multiplication and movement of X. fastidiosa in citrus observations were done after needle inoculation of the pathogen in stems and leaves of test plants, this method had a 45 - 100 % efficiency. Only 1 week after inoculation the bacterium was recovered in areas above and below the inoculation points, as wells as in the first leaf above it. The amount of positive plants 1 week after inoculation remained constant until the end of the assays (16 weeks), therefore, the infection success rate can be early identified. The bacterial population in the first weeks after inoculation varied from 103 to 106 colony forming units (CFU) per gram of tissue, after 4 weeks this value remained between 105 and 106 CFU/g. Symptoms were observed 2 months after inoculation in all tests done. Based on the Pierce’s disease of grapevines pathosystem, possibly the limiting factor to X. fastidiosa transmission in CVC is the low acquisition efficiency by vectors, due to the low bacterial population found in symptomatic plants. (AU)