Submitted to: Almond Industry Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 12/1/2004
Publication Date: 12/31/2004
Citation: Kluepfel, D.A., Browne, G.T. Biology and management of replant disorder and lethal phytophthora canker. Almond Industry Conference Proceedings. 2004
Technical Abstract: This research concerns biology and management of replant disease (RD), a complex problem that complicates an important process--orchard replacement. Orchard replacement forces difficult management decisions, including some related to scheduling (i.e., at what age should replacement occur, whether time in crop rotation or fallowing between almond production cycles is worthwhile, etc.), scion, rootstock, and irrigation system choices, and pre-plant soil treatments (i.e., back hoeing, ripping, fumigation). Although we don not know precisly what causes RD, we know that it retards or prevents establishment of young trees and that it can occur in absence of other replant problems, including plant parasitic nematodes, insects, or poor chemical or physical properties in soil. Several new plantings of almond in the Sacramento Valley have failed to establish (up to 50% or more of the trees have died or failed to grow) when they are planted after removal of old almond orchards on peach rootstock. In addition, significant but less-severe incidence of RD has occurred in young almond and peach plantings in the San Joaquin Valley. Previous research findings highlight some important features of RD and its persistence as an industry problem. In the 1940s, Probesting and Gilmore at UC Davis reported a "peach replant problem". The affected trees, planted where peach trees had been, were severely stunted compared to adjacent trees planted after apple. The problem was not associated with know root parasites. Application of maco and micro nutrients did not help. It was hypothesized that toxigenic peach root residues caused the problem, but this hypothesis, by itself, was not consistent with subsequent experimental data. Multiple lines of evidence from our research and that of others suggest microbial mediation of RD, including the ability of mild soil heating (50 degrees C), a fungicide treatment, and diverse fumigants to alleviate RD symptoms. Our date indicate that RD is not caused by parasitic nematodes, although it is well-documented that some replant problems are. For some plant species, crop yield or growth depression has been associated with elevated populations of deleterious rhizosphere microorganisms that negatively influence plant growth without parasitizing plant tissue. We are continuing research designed to determine the causes of almond RD. Improvement management strategies are needed for RD. As orchard districts age, risk of replant problems tends to increase. Pre-plant soil fumigation with methyl bromide (MB) solves some of the problems, but it is being phased out. Alternative fumigants are available and being developed, but they face increased regulatory restriction. Research is needed to optimize MB alternatives, including cultural and biologically based management approaches.