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Cytology and Histology

Indicator Hosts for Pear Decline: Symptomatology, Histopathology, and Distribution of Mycoplasmalike Organisms in Leaf Veins. Henry Schneider, Plant Pathologist, Department of Plant Pathology, University of California, Riverside, CA 92502; Phytopathology 67:592-601. Accepted for publication 27 October 1976. Copyright © 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-592.

The Pyrus cultivars Variolosa, Magness, and Precocious all were susceptible to pear decline, and each exhibited a distinctive type of browning of leaf veins. In Variolosa, the abaxial ribs of major lateral veins became brown; in Magness, the minor veinlets discolored; and in Precocious, a new and unusual indicator host, the mesophyll bordering adaxial ribs of framework veins became yellow and later brown. Pathological features common to the three hosts were: (i) a tendency for mycoplasmalike organisms (MLO) to be abundant in finer minor veinlets and sparse in sieve tubes of the secondary phloem of framework veins; (ii) the occurrence of lesions in vein encasing tissues; and (iii) necrosis of sieve tubes of the secondary phloem. In Variolosa and Precocious, sieve-tube necrosis occurred early and abundantly in the secondary phloem of framework veins and was accompanied by excessive phloem formation. The occurrence of sieve-tube necrosis in Magness was erratic, and no replacement phloem formed. There appears to be an anomaly among the pathological features. Pathosis was absent or rare in fine minor veins where MLO occurred most abundantly; conversely, pathosis was severe in tissues that encased veins where MLO did not occur and in secondary sieve tubes where MLO were rare or absent. One possible explanation is that MLO do not cause pear decline. A more plausible hypothesis is that toxins are produced by MLO in the specialized phloem of fine minor veins or that host tissues are stimulated to produce them. The toxins then are translocated to other sites where they cause pathosis.