There are over 1400 liner nurseries in the United States with over $395 million in sales in 2009 [U.S. Department of Agriculture (USDA), 2010]. In this production system, small liners (3- to 20-inch tall) are densely planted at a rate of 7000 to 10,000 plants per acre. Many tree species reach a height of 6 to 10 ft during 1 to 3 years (Fare, 2006). The high density of liner plantings can aggravate the severity and incidence of insect infestations and diseases in liner production. In the Pacific northwestern United States, where much of bareroot liner production occurs, over 27 genera of arthropods [e.g., ambrosia beetles (Curculionidae), caterpillars (Limacodidae), leafhoppers (Cicadellidae), etc.] (Hollingsworth, 2010) and more than a dozen diseases [e.g. powdery mildew (Erysiphaceae), anthracnose (Colletotrichum spp.), crabapple scab (Venturia inaequalis), etc. (Ingham et al., 2010)] are common.
Insecticides and fungicides are used routinely to suppress and control pests and diseases. In 2006, chemicals used in liner production in California, Florida, Michigan, Oregon, Pennsylvania, and Texas were ≈21,000 lb for insecticides and 157,900 lb for fungicides (USDA, 2007). To maximize the effectiveness of pest management strategies, confirmation of the actual spray coverage and spray deposition on targets under field conditions is required (Bache and Johnstone, 1992). The spray coverage required to effectively control pests has been studied for other crops (Falchieri et al., 1995; Fisher and Menzies, 1976; Hewitt and Meganasa, 1993; Washington, 1997) but not for ornamental nursery liners. Without definitive guidelines for the requisite amounts of spray to achieve adequate deposition and coverage, the result will be either reduced pesticide coverage from less than desirable spray volume or off-target movement of the pesticide because of excessive spray volume.
No spray equipment or spray application method currently exists that can apply chemicals to the numerous varieties of nursery crops. Several reports on using spray systems for nursery shade tree production are available (Derksen et al., 2004; Krause et al., 2004; Zhu et al., 2006, 2008), but none of them addressed the problem of how to optimize application rates for use in field-grown tree liner production. In response to an onset of pest infestations, liner spray applicators must make calculated decisions within a very narrow time window on how much pesticide and spray volume will be needed for economical control. Accordingly, they often simply use a “best guess” practice that usually applies excessive amounts of pesticides for pest control. This “best guess” practice is to spray liners to the point of runoff or saturated target areas with pesticides. The “point of runoff” method was tested with water as the carrier alone for its effectiveness to insure adequate spray deposition and coverage inside the canopies of nursery trees, and exceeded the amount of pesticide required for economical pest control by at least four times (Zhu et al., 2006, 2008). Consequently, the application of spray is inefficient and crops are oversprayed.
Sprayers suitable for field-grown tree liner applications are few in number. The high ground clearance with vertical booms and over-the-row frame sprayer is commonly used to accommodate the relatively small canopy of liners and narrow row spacing. The spray booms are suspended between two rows of liners and the multiple nozzles from two parallel vertical booms simultaneously apply the spray horizontally to the both sides of the liners. The advantages of this sprayer are that multiple rows are sprayed at the same time and differences in tree height can be accommodated by turning the top nozzles on or off. The disadvantage is the excessive amount of time for applicators to frequently turn on or off nozzles manually to match canopy height across several rows. Also, the adjustments of spray nozzles for tree height are often ignored when the demand for spray applications must be completed in a short time and thus contribute to unnecessary sprays. Lastly, a higher than needed application rate is often used at any one-time spray when applicators are hindered in their determination of the proper rates for application because of the rapid growth of liners. However, if guidelines were available to optimize the application rate for pest control, growers could minimize excessive pesticide use in liner production.
The objectives of this research were to quantify the amount of spray deposition and coverage inside nursery liner canopies from over-the-row frame vertical boom sprayer, to determine its optimal application rates, and to establish a spray rate model for different size liners with similar canopy shapes, in an effort to increase spray application efficiency and achieve real cost savings for liner growers.
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