Submitted to: California Pistachio Commission Production Research Report
Publication Type: Proceedings
Publication Acceptance Date: 2/15/2006
Publication Date: 3/1/2006
Citation: Holtz, B., Hoffman, E., Caesar, T., Caesar, A.J. 2006. The effect of tillage on decomposing mulch, Verticillium, the microbial community, soil nutrients, and soil aggregation. California Pistachio Commission Production Research Report. p. 91-95. Interpretive Summary: Most pistachio growers already shred their prunings as an alternative to burning and are pleased that they are adding organic matter to their soils. Many pistachio growers however regularly disk or till their soils for weed management and the shredded prunings are tilled under, enhancing decomposition. Tillage has been shown to speed up the decomposition of organic matter in many field studies but is generally regarded as less sustainable because of organic matter loss, soil compaction, layering and erosion. Tillage has been shown to adversely affect soil structure, porosity, soil aggregation and basidiomycete populations. Some pistachio growers believe that Verticillium wilt symptoms appear worse in years after tillage when compared to years their orchards were not tilled. Very little research has been conducted to study the microbial community in pistachio soils or how the addition of wood chips or tillage practices might affect these microbial populations along with soil nutrients. The objectives of this study were to study the effect of tillage and shredding pistachio brush on Verticillium dahliae, soil nutrient status, soil aggregating fungi and the pistachio rhizosphere microbial community.
Technical Abstract: In assessing the underlying reasons for the apparent effect of no tillage on causing lower levels of Verticillium symptoms on pistachio trees, we assessed the effects of tillage practices on predominant rhizosphere bacteria. Predominant bacteria were deemed as such when they occurred on the outer periphery (most dilute portions of the plating) of spirally-plated pistachio rhizospheres. Organic no-till treatments had dramatically greater bacterial diversity than other treatments. Bacteria that were found exclusively (under these criteria) in organic no-till treatments were the biocontrol agents Lysobacter antibioticus and L. enzymogenes, both powerful antagonists of fungal plant pathogens. Other bacterial species that include strains which are biocontrol agents such as the Pseudomonas chlororaphis/aureofaciens/auriantica complex were found in organic tilled treatments. The common plant pathogen was found in both the tilled organic and conventional treatments. There appears to be an underlying basis for the apparent lower Verticillium disease incidence associated with no-till treatments that is worthy of further investigation.