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- Author or Editor: William W. Coates x
The ability to remove the pistillate flowers and young nuts from precocious lateral-bearing English walnut (Juglans regia L.) cultivars during the first several years following planting would be useful both in reducing competition with vegetative growth and eliminating potential infection sites for blackline disease (cherry leafroll virus). Applications of ethephon shortly after full bloom were shown to effectively remove all or most pistillate flowers depending upon spray timing and ethephon concentration. Moderate phytotoxicity and reduced seasonal growth limit the usefulness of this technique in the field. Removal of staminate flowers (catkins) prior to pollen release may reduce the excessive pistillate flower abscission of the `Serr' cultivar. Applications of ethephon shortly before the onset of pollen shedding were shown to be ineffective in catkin removal.
Walnut husk fly (Rhagoletis completa) is a major pest of English walnuts in California. Research was initiated in 1995 to evaluate reduced-risk pesticides. The first effective control measures were found in 2001—spinosad plus bait and kaolin (Surround). Spinosad was registered for organic use in 2003 as GF-120 NF (a formulation of spinosad and bait) and Entrust (a spinosad formulation to be mixed with existing baits). GF-120 is recommended for low to moderate populations while Entrust + bait is recommended for higher populations. Entrust + bait is price competitive with the standard malathion + bait. Kaolin is also effective for walnut husk fly control; however, control is dependent upon adequate coverage. In 2004, varietal susceptibility was assessed. The cultivars `Payne', `Serr', `Pedro', and `Hartley' were found to be highly susceptible while `Chandler' and `Howard' were somewhat resistant. Infestation in `Chandler' began later in the season than in other cultivars. Quality parameters were also measured. Samples of 100 infested and uninfested `Tulare' walnuts were compared at harvest. There were significant differences in adhering hull, percentage of edible yield, and relative value between infested and uninfested nuts. The relative value of infested nuts was 28% less than uninfested nuts. There were minimal differences in percentage of mold and kernel color. Walnuts sprayed with kaolin had larger nuts, fewer external defects, and an earlier harvest timing.
Moderate California winters often result in delayed, erratic or extended bloom, inadequate overlap with pollenizers, poor leafing-out, low fruit set, and irregular fruit maturity. In recent years, use of rest-breaking agents has become commonplace in California sweet cherry (Prunus avium L.) culture, mainly to promote earlier bloom and fruit maturity, but also to promote uniform flowering and overcome lack of marginal chilling. Variable responses by different cultivars and in different seasons may be due to different chilling requirements, despite little variation in genetic background for chill requirement in California's commercial cultivars. Other sources of variation include the activity of the rest-breaking agent used, concentration and carrier volume. A minimum amount of effective chill appears to be required for a given cultivar before rest-breaking agents can be effectively applied. This threshold, as exhibited by degree of response to treatment, can be an important indicator of when to spray. Method of measuring chill accumulation, and thus, timing of applications, varies by region and historic acceptance. California's tree fruit industry typically uses the 45 °F “chill hour” model. The Utah Chill Unit Model and the Modified 45 °F Chill Hours Model had not been thoroughly tested under California conditions nor with the rest-breaking chemicals that are in use today in California. We tested our research results against these models and the Dynamic Model developed in Israel and concluded that the Dynamic Model provided the best explanation of responses in our experimental trials. We have been developing recommendations for application of rest-breaking chemicals based on Dynamic Model chill portion accumulation.