Effects of Cylindrocarpon and Pythium species on apple seedlings and potential role in apple replant disease

The nature and cause of apple replant disease has been under investigation and discussion for nearly 300 years. This study on apple replant disease investigated the pathogenicity of Cylindrocarpon and Pythium spp. alone and in combination on apple roots and their effect on apple, pear, plum, and peach roots. Six isolates of P. irregulare collected from different hosts had varying degrees of pathogenicity on apple roots. Pythium and Cylindrocarpon spp. alone and in combination caused a range of effects on apple roots and shoots. However, the combination of P. irregulare and C. lucidum consistently caused the greatest amount of root damage and root and shoot growth suppression. Also, C. lucidum and P. irregulare caused replant disease-like symptoms on apple and pear but not on plum or peach shoots and roots.