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A Fusarium Root Rot Problem in Idaho Apple Orchards. A. W. Helton and Richard Dilbeck, Department of Plant, Soil and Entomological Sciences, University of Idaho, Moscow 83843.  Plant Disease 69:1101, 1985.  Accepted for publication 26 August 1985.  Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-1101b.

Growers have experienced replant problems in apple (Malus domestica Borkh.) orchards for many years.  Symptoms range from death of replants within a few weeks, through poor growth and productivity of surviving trees, to slow death of mature trees.  Fusarium oxysporum Schlecht. and occasionally F. solani (Mart.) Appel & Wollenw. emend. Snyd. & Hans. are present in the root systems of affected trees.  Tests of pathogenicity in excised stems in the laboratory and in potted greenhouse trees have shown both species to be aggressive pathogens of clonal rootstocks (EM-7A, EM-26, MM106, MM-111) and McIntosh seedlings.  F. oxysporum usually is more aggressively pathogenic than F. solani.  Symptoms in the greenhouse consist of root decay that advances into the main stems and causes general bark zone necrosis (as in excised stems in the laboratory).  Where inoculated field trees survive the first growing season, feeder roots are destroyed, resulting only in a "slick root" condition and poor top growth.  In mature trees, feeder roots are destroyed and aboveground parts show a thin canopy condition, with too much of the stem system visible.  We suggest that this Fusarium root rot condition is an important component of the apple replant problem.