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Project description

Determining Seed Bank Levels in Citrus Orchards: a Basis for Designing a Weed Control Program. (99CC016)
Program UC IPM competitive research grants program
K.J. Hembree, UCCE Fresno County
T.S. Prather, Statewide IPM Project, Kearney Agricultural Center
Host/habitat Citrus
Pest Unspecified Weeds
Discipline Weed Science
Cultural Controls
Start year (duration)  1999 (Two Years)
Objectives Determine seed density and percentage of viable seeds relationship to duration of herbicide use and to size of citrus tree canopy

Determine weed emergence and density relationship to duration of herbicide use and size of citrus tree canopy by diameter.

In citrus orchards, weeds are controlled primarily with preemergent herbicides. Postemergent herbicide application follows and efficiently prevents a large number of species from replenishing their respective seed banks. Through time, seed banks should diminish but farmers cannot evaluate seed bank depletion since the weeds are controlled prior to their emergence and hence detection. This research investigates seed bank levels in citrus orchards under different duration of herbicide use and size of citrus tree canopy. In objective I, results indicate that total seeds in citrus decrease with increasing duration of herbicide application. Seed banks tended to be higher in the tree row than in the tree middle. Older orchards (> 27 years) that had longer duration of complete weed control had similar seed bank densities for tree rows and tree middles. A clear relationship of canopy size to seed bank has identified. In objective 2, weed emergence and density were evaluated in the field from plots which received preemergent herbicide application and plots that did not receive preemergent herbicide application. Five species were the most common species detected: common grounds (Senecio vulgaris), horseweed (Conyza canadensis), prickly lettuce (Lactuca serriola), annual sowthistle (Sonchus oleraceus) and panicle willowweed (Epilobium paniculata). All five of these species have pappus bearing seeds that allow for wind dispersal and would continually be invading citrus orchards. Weed species were only found on the tree row in the newly planted trees where herbicides had been applied for less than 4 years. These data could help consultants and farmers modify weed management to reduce cost of weed control by changing application rates, skipping applications or relying on postemergent strategies for part of the season. A successful decision support tool could identify fields that could easily be brought into compliance with impending regulatory changes to reduce the risk of environmental impact. A decision support tool would also identify where the consultant and farmer are at greatest risk to changes in current weed management practices.

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