The disease is often observed in low-lying areas of orchards with heavy, poorly drained soils. The importance of this disease has increased with the introduction of more dwarfing rootstocks. The first symptoms to appear in the spring are delayed bud break, leaf discoloration, and twig dieback. These symptoms indicate that crown infection is advanced. While infected trees may survive the growing season, they show symptoms of leaf and bark discoloration and premature leaf drop in the fall.
Infected bark becomes brown and is often slimy when wet.
Close examination of the roots often reveals reddish brown, water-soaked areas of necrotic tissue located at the base of the root where it attaches to the rootstock. The entire underground portion of the stem is usually water-soaked and brown, and the necrotic area usually extends upward to the graft union. The disease is caused by fungi in the genus Phytophthora , which belongs to a group of fungi known as the water molds. These spores are resistant to drought and are somewhat resistant to chemicals. Of the rootstocks preferred by growers, none are completely resistant to crown rot.