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Project description

Biotic and Abiotic Control of Walnut Blight. (90BC051)
Program UC IPM competitive research grants program
M.N. Schroth, Plant Pathology, UC Berkeley
A.H. McCain, Plant Pathology, UC Berkeley
Host/habitat Walnuts; Tree Crops
Pest Walnut Blight
Discipline Plant Pathology
Biological Controls
Start year (duration)  1990 (Three Years)
Objectives Develop and employ a new control strategy for walnut blight which integrates biological and chemical control. Exclude walnut blight bacteria from buds and catkins (the overwintering sites and reservoirs of blight bacteria) by inoculating them with antagonists that naturally colonize these plant parts.

Monitor the populations of buds and catkins for blight bacteria to determine when infestations of newly formed buds occur; this will help to determine when control procedures should be initiated.

Enhance the toxicity of copper ions to blight bacteria by amending fixed copper compounds with iron.

The purpose of this project is to improve the control of walnut blight by the integration of three different practices: use of biological agents, enhancement of fixed copper by amendments of iron, and improved timing by monitoring the population dynamics of the blight bacteria. With respect to biological control, a mixture of biological agents were field tested on walnuts. The results were encouraging in that by July 18, a 64% reduction in blighted nuts were observed in one field plot. Monitoring studies were done to determine how biologicals should be used. The data indicated that they must be applied the year before as well as the present season because overwintering, untreated buds already have a substantial population of blight bacteria. Thus, biological control agents will have little affect in excluding blight bacteria from buds since most are infested from the previous season. This also applies to the use of fixed coppers; coppers can reduce the incidence of infestation of newly formed buds, but appear to have little effect when buds are already colonized. The work on enhancing the efficacy of the copper ion was very successful and a submission was made for a patent. With the addition of iron, a reduced amount of copper is more effective than the standard amounts. Growers are aware of the field testing and are using the new mixture despite the fact that we have not published or made a recommendation. Field studies comparing iron-copper mixtures with copper (two field tests) showed that the addition of iron to copper was superior to copper alone (P=.05) in reducing the percentage of buds infested with blight bacteria. On the walnut leaf surface, increasing dosages of iron resulted in up to a 30% increase in the amount of the free copper ion. No phytotoxicity was detected in any of the field experiments including some conducted by independent growers using high concentrations of iron (300 ppm).

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