Written By Greg Browne, USDA-ARS UC Davis When orchards are replaced, growth and productivity of the succeeding generations of trees are often suppressed by “replant problems” unless precautions are taken. Replant problems can result from interacting physical, chemical, and biological factors, but the biological aspects usually dominate. Growers can minimize physical and chemical contributions to replant problems by pre-plant ripping and other site remediation practices and amendments to insure good soil water drainage, good soil structure, and optimal soil chemical properties (e.g. pH, soil extract electrical conductivity, etc.). Of course, appropriate soil tests are needed to evaluate the latter properties. Dealing with biological contributions to replant problems should also start with some homework—sampling for nematodes. Although plant parasitic nematodes (ring, lesion, and, on some rootstocks, root knot nematodes) probably contribute to replant problems in less than one third of California’s replanted almond and stone fruit orchards (rough estimate), it is important to sample your soil to check for them before replanting. Root damage caused by the ring nematode in sandy soils predisposes almond and other stone fruit trees to bacterial canker disease. In addition, although not common at most sites, Phytophthora species, Armillaria mellea, and Verticillium dahliae infest some orchard soils and can cause tree stunting, decline, and death in young replanted orchards as well as in established ones. It is difficult to detect and assess populations of these pathogens by soil sampling, and therefore a history of disease caused by them in the previous orchard tends to be the best predictor of future replant problems they may incite. Although it varies in severity, the most common replant problem is Prunus replant disease (PRD). In our experience, it occurs nearly universally in replanted almond and stone fruit orchards in California unless precautions are taken. Figure 1: Almond trees affected by