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Identification
Symptoms of replant disease on affected trees include stunting, short internodes, rosette leaves, small root systems and decayed or discoloured roots which results in poor productivity. Often affected trees have fewer lateral roots and root hairs. Vigorous young trees affected by apple replant disease often stop growing in early summer. Affected trees leaf out in the spring but often produce little or no shoot growth. Leaves are generally small and light green in colour as compared to the leaves on vigorous trees. Few new lateral or feeder roots are produced, and existing roots become discoloured and deteriorate. Severe replant disease results in the death of young trees and entire orchards. Trees in orchards not killed by replant disease have delayed fruit bearing and reduction in overall yield.
Biology
The disease and causal agent of apple replant disease are not well understood. A complex of various fungi, bacteria and nematodes are associated with replant disease. In addition to biological factors, other non-biological factors such as soil pH, moisture stress (too much and too little), soil compaction, toxins, soil structure, heavy metals and insufficient availability of nutrients (particularly phosphorous) are also implicated as part of the complex of factors contributing to replant disease. Although abiotic factors play an important role in this disease, research showing dramatic tree growth in response to soil pasteurization and fumigation suggests this disease is primarily a biological phenomenon. The current theory is that a complex of several fungal pathogen combinations including Cylindrocarpon, Phytophthora, Pythium spp. and Rhizoctonia, as well as parasitic nematodes and bacteria, are involved with replant disease.
Period of Activity
Symptoms appear during the growing season.
Scouting Notes
During regular scouting in orchard blocks make note of sections where young trees show stunted growth, short internodes and rosette leaves.
Thresholds
None established.
