Each year, growers in Washington replant about 10,000 acres of apples. Not only do growers incur tree replacement costs in affected orchards, but trees struck by the disease begin bearing fruit 2 to 3 years later than normal, and their yields don't compare to those achieved in the absence of the disease. Replant disease of apple was thought to occur primarily at sites where growers replanted after removing very old fruit trees. However, we have documented the onset of the fungal complex within 3 years of establishing the orchard on new ground. When apple replant affects an orchard, young trees grow unevenly, and eventually, because of disease pressure, growth of most of the trees in the orchards will decline.
Trees can also die.
Severe stunting, shortened internodes, rosetted leaves, and reduced fruit production are all symptoms of apple replant disorder, he says. Affected trees have small root systems with lots of fibrous roots that don't function properly because of decay caused by the disorder. And pasteurizing or fumigating that same soil dramatically increases growth. This proves that the disease is primarily a biological phenomenon. In a recent study in nine commercial and experimental orchards in Washington, Mazzola's data clearly showed that several species of fungi from the genera Cylindrocarpon, Phytophthora, Pythium, and Rhizoctonia are the primary causes of apple replant disease.