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Etiology

Differentiation of Syndromes Caused in Apple by Graft-Transmissible, Xylem-Affecting Agents. M. F. Welsh, Horticulturist, British Columbia Ministry of Agriculture, Summerland, B.C. Canada, V0H 1Z0; J. K. Uyemoto, associate professor, Department of Plant Pathology, Kansas State University, Manhattan, KS 66502. Phytopathology 70:349-352. Accepted for publication 31 September 1979. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-349.

Transmission tests in woody and herbaceous hosts demonstrated infection of apple clones by at least three graft-transmissible agents that can induce yxlem aberrations in Virginia Crab apple indicator plants. The syndromes that they induce are stem pitting (SP), brown line decline (BLD), and junction necrotic pitting (JNP). They differ in ability to persist when plants of infected apple clones are heat-treated. SP is expressed on the Virginia Crab stem as xylem pits matched by protrusion of symptomless phloem tissue. Consistent differences in severity of pitting among isolates suggest occurrence of strains, or that different agents can cause pitting. BLD is characterized by a groove in the xylem encircling the union between Virginia Crab and its rootstock, matched by a protrusion of necrotic phloem and sometimes accompanied by scattered longitudinal grooves on the Virginia Crab stem. When severe, the trees decline and die. All BLD-affected trees are infected with apple stem grooving virus. JNP is a syndrome that includes a line of pits encircling the junction matched by necrotic phloem pegs, not accompanied by stem grooving, and not leading to tree decline. The causal agent is not sap-transmissible. Flute fruit symptoms are associated with SP and JNP, but not BLD.