REDUCING PESTICIDES USE IN CANTERBURY WALNUT FARMS

McNeil,D.L., Romero, S., Stark, C., Kandula, J.
Soils, Plants and Ecological Sciences Division, Lincoln University, Canterbury, New Zealand

Walnuts are a new crop in New Zealand with considerable production potential and possessing many characteristics which make them a healthy alternative in our diets.

Walnut blight symptoms on New Zealand grown walnuts

Twenty years of research and grower collaboration has resulted in a package of how to grow them as a crop. However, walnut blight (Xanthomonas campestris pv juglandis) is the dominant disease limiting the expansion of walnut production in New Zealand. Control of walnut blight in New Zealand is still based on very old technology requiring extensive sprays with copper products. Application rates are high, with American recommendations of 8lb/acre of Kocide applied as often as 8 times per season. This has lead to the development of copper resistant strains in both France and the USA as well as unacceptably high levels of copper in the environment and the potential to adversely affect applicators and stock. It has also lead to walnuts being the ninth highest user of pesticides in California. Blight caused as much as 40% of the harvested walnut damage observed in the walnut Biologically Integrated Orchard Systems (BIOS) programme in California. This is a route we do not wish to travel in New Zealand Thus alternative control methods are needed.

Our proposal was motivated by concern over development of resistance to copper sprays, and over environmental and safety issues arising from the excessive use of copper sprays. We are therefore evaluating biological control options that are effective for controlling walnut blight bacteria. The method we selected to pursue was to use Bacteriophages as a natural control mechanism. Bacteriophages are viruses that only infect unicellular organisms and have considerable host specificity. We have found phages against walnut blight bacteria relatively common in the soil and plant canopyand easy to isolate. They have substantial host specificity but variable pathogenicity. We believe that the walnut blight/bacteriophage system being developed by Lincoln University has great potential for disease control.

Table 1. Variation in the bacterial population and phage number in three different reading times (Spring 2000 - Summer 2001)

With the help of the Brian Mason fund our lab is now capable of expertly handling and counting the bacteria and phages. Our results have demonstrated that canopy phages may be the natural limitation on bacterial blight spread in walnut orchards as they reach large populations late in the season when the bacteria populations collapse (Table 1). This year our group hopes to prove it is a cause and effect relationship. We are thus poised to test possibly more effective canopy phages and to approach other organizations with some very positive results to seek future funding. We are the first group in the world to demonstrate this natural biocontrol system for walnut blight acting in the canopy & hope to extend it to a commercially viable, toxin free, sustainable disease management system.

Photos on this page are courtesy of Lincoln University

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