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.
PRD causes slight to severe growth suppression in almond and stone fruit orchards planted after one another. Instances of severe PRD can kill or prevent growth in more than half of the trees in a replanted orchard (such cases have occurred repeatedly on some soils in Butte County). More commonly, PRD stunts trees, especially during the first year after planting. In any case, cumulative crop production of trees affected by PRD may never fully catch that of trees planted where PRD prevention practices were used. Although the cause of PRD is still being unraveled, it has been associated with a complex of soilborne fungi, oomycetes, and bacteria left from the preceding crop.
There are not currently soil tests available to predict severity of PRD, but such tests may be useful once validated. In the meanwhile, local experience obtained by growers and from field trials such as those described below can be very useful in predicting risk and severity of PRD on a given soil series with a given crop history.
For the past several years, a team involving the authors, the Pacific Area-Wide Pest Management Program for Integrated MB Alternatives, commercial growers, TriCal, Inc., and others has been testing and optimizing fumigant- and crop-rotation based approaches for preventing PRD. Some key aspects we have examined include:
— Testing efficacy of chloropicrin (CP), 1,3-dichloropropene (1,3-D, trade formulation Telone II), and iodomethane (IM, trade formulation Midas), and several mixtures these fumigants, as alternatives to methyl bromide (MB)
— Developing efficient spot fumigation methods with potential to reduce fumigant costs and emissions
— Examining contributions of a single year of fallow or short-term crop rotations with sudan grass or mustard for management of PRD
— Testing effects of irrigation intensity (e.g., from 70 to 120% of ET) on severity of PRD in fumigated and non-fumigated soil.
Below, in summary form, we highlight some of the recent results from this work. Key findings to date:
— In general, chloropicrin (CP) and mixtures of it with IM, 1,3-D, or MB are more effective for preventing PRD than 1,3-D or MB alone. (IM products are not registered in California at this time)
— Rates of 300 to 400 lb/per treated acre of CP or mixtures of it with 1,3-D or IM appear optimal for prevention of PRD. (See Tables 1,2). (Product labels and Ag Commissioner must be consulted for appropriate rates).
— GPS-controlled shank-applied spot treatments (applied to tree sites before planting), with CP or Telone C35 (CP:1,3-D) or drip-applied spot treatments (also applied to tree sites before planting) with Inline (drip formulation of CP:1,3-D) appear nearly as effective as strip or broadcast treatments with the same fumigants (See Tables 1-3).
— Short-term rotations with sudan grass, wheat followed by sudan grass, or mustard, or a single season of fallowing can improve growth or replanted trees, thereby reducing effects of PRD (Table 4).
— It appears important not to over or under water almond trees replanted without pre-plant soil fumigation after removal of almond on peach rootstock; doing so can make PRD worse.