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Role of Soil Microflora and Pratylenchus penetrans in an Apple Replant Disease. B. A. Jaffee, Department of Plant Pathology, Cornell University, Ithaca, NY 14853, Present address: Department of Plant Pathology and Physiology, Clemson University, Clemson, SC 29631; G. S. Abawi(2), and W. F. Mai(3). (2)Department of Plant Pathology, New York State Agricultural Experiment Station, Geneva, 14456; (3)Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 72:247-251. Accepted for publication 9 June 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-247.

Consistent reproduction of apple replant disease (ARD) was developed under controlled conditions. A small quantity of untreated field soil (FS) obtained from an orchard with a history of ARD was incorporated into a steamed (75 C for 30 min) portion of the same soil (SS). Ten-day-old apple seedlings (Northern Spy) were transplanted into SS or 5% FS (5 parts FS + 95 parts SS) and maintained for 6 wk in a growth chamber. The addition of 5% FS reduced plant dry weight by 50% and induced orange and black root discoloration. Treatment of the FS (prior to incorporation in SS) with gamma radiation, broad-spectrum fumigants, and heating (60 C or higher for 30 min) eliminated the stunting and root discoloration. Pratylenchus penetrans was the most abundant parasitic nematode, but its highest density in 5% FS was only four per 100 cm3 soil. This density was considered to be below the damaging level. Addition of 140 or more P. penetrans per 100 cm3 SS resulted in significant stunting and root necrosis. Our data suggest that at least two agents contribute to the ARD occurring in the field: an unknown organism(s) inducing stunting and root discoloration in 5% FS, and P. penetrans.