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Xylem Dysfunction in Peach Caused by Cytospora leucostoma. M. C. Hampson, Department of Plant Pathology, Cornell University, Ithaca, N. Y. 14850, Present address of senior author: Canada Department of Agriculture Research Station, P. O. Box 7098, St. John’s West, Newfoundland, Canada; W. A. Sinclair, Department of Plant Pathology, Cornell University, Ithaca, N. Y. 14850. Phytopathology 63:676-681. Accepted for publication 11 December 1972. DOI: 10.1094/Phyto-63-676.

Observations of the Valsa canker disease in New York peach orchards revealed wilting and defoliation of infected branches not girdled by cankers. A greenhouse study involving inoculation of potted peach trees with Cytospora leucostoma, the incitant of the disease, gave evidence of xylem dysfunction as an important cause of symptoms. Trees inoculated during active growth secreted gum within and outside of tissues at loci of infection and displayed symptoms of acute and chronic water stress in parts distal to sites of inoculation. Movement of eosin dye through xylem of segments cut from infected stems was interdicted at loci of infection. Foliage distal to nongirdling cankers showed symptoms of Ca deficiency and contained significantly lower concentrations of A1, B, Ca, Mg, Mn, P and Zn than comparable foliage from uninfected branches of the same trees. Transpiration rates of detached peach shoots standing in viscous solutions including culture fluids of C. leucostoma were 4 to 15% as great as those of comparable shoots standing in water. A 0.2% solution of gum from peach cankers was more viscous than the fluids tested on peach shoots. It was inferred from observations of gum in cambial and xylem tissues in and adjacent to cankers that gum is a major cause of the xylem dysfunction. It is hypothesized that gum may plug vessels directly or, when relatively dilute, impart viscosity to xylary fluid and thus impede cross transfer of the fluid among functional vessels.