Role of Soil Microflora andPratylenchus penetransin an Apple Replant Disease

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Abstract
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.degree. 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.degree. C or higher for 30 min) eliminated the stunting and root discoloration. P. penetrans was the most abundant parasitic nematode, but its highest density in 5% FS was only 4/100 cm3 soil. This density was below the damaging level. Addition of 140 or more P. penetrans/100 cm3 SS resulted in significant stunting and root necrosis. At least 2 agents contribute to the ARD occurring in the field: an unknown organism(s) inducing stunting and root discoloration in 5% FS, and P. penetrans.