EFFECT OF INOCULATION OF MATURE CITRUS TREES IN THE GROVE WITH XYLELLA FASTIDIOSA ON CITRUS BLIGHT INCIDENCE

HOPKINS, D. L.

Abstract


From June 1989 through June 1993 in a Fort Pierce grove, forty mature 'Valencia' sweet orange trees (Citrus sinensis (L.) Osb.) on rough lemon rootstock (C. jambhiri Lush.) that were free of blight symptoms were inoculated twice yearly with strains of Xylella fastidiosa Wells et al. and an additional forty were inoculated with buffer only. They were inoculated by gravity-flow of dilute inoculum into a hole that had been bored into the trunk of the tree and by submerging two cut roots in vials of inoculum. Since strains of X. fastidiosa vary in virulence and host specificity, several different strains were utilized. Virulence of the isolates was tested yearly on grapevine and newer, more virulent strains replaced some of the original strains to assure that 20 trees were being reinoculated with virulent bacteria. The other 20 trees were reinoculated with the same strains, which had lost virulence to grapevine, throughout the test. Incidence of blight was determined twice yearly, using visual symptoms and diagnostic tests. At the termination of the experiment in December 1993, there was 35% blight incidence in the 40 control trees, 65% blight in the 20 trees reinoculated with virulent strains, and only 20% blight in the trees reinoculated with strains weakly virulent, or avirulent, to grapevine. Thus, X. fastidiosa strains virulent to grapevine caused an increase in citrus blight; whereas, weakly virulent strains resulted in less blight, probably due to cross protection.

Keywords


cross protection; valencia sweet orange; rough lemon rootstock

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Proc. Fla. State Hort. Soc.     ISSN 0886-7283