California nursery operations combined to produce $165.5 million in wholesale value of fruit and nut tree planting stock—a value that is multiplied severalfold by retailers and exporters (U.S. Department of Agriculture, 2007). Fruit and nut tree nursery production practices in California vary among growers, regions, and tree species; however, production cycles usually follow a similar schedule (Hanson and Schneider, 2008). Generally, a rootstock is planted in the field in the fall as either seed or cuttings, a scion variety is grafted or budded onto the rootstock the following spring or fall, and the trees are harvested and sold as bare-root planting stock after one or two growing seasons. Most almond and stonefruit trees produced in this system are grown as 14-month crops on peach, plum, or hybrid rootstocks.
Weed control is an ongoing management problem faced by producers of field-grown nursery stock. Competition from weeds can decrease crop productivity and interfere with field and harvest operations. Weed management techniques include preplant soil fumigation with methyl bromide, preemergence herbicides, hand labor, and multiple tillage operations (Schneider et al., 2003). Methyl bromide and its potential replacement alternatives do not always provide sufficient weed control over the 1–3 year nursery tree cropping cycle. Reasons for incomplete control with fumigants include hard/impermeable weed seedcoats that prevent fumigant penetration, reinvasion of weeds into the field, reduced fumigant solubility and dispersion in response to environmental conditions (e.g., inappropriate soil moisture content) (Hanson and Shrestha, 2006). This problem will likely be compounded by the use of fumigants other than methyl bromide which is being phased out due to environmental concerns (United Nations Environmental Program, 1999; Zasada et al., 2010). Hand labor can be an effective means of controlling weeds within rows of nursery stock but can result in mechanical crop damage, requires a large and expensive labor force, and is subject to stringent worker safety regulations. Although several herbicides are labeled for use in tree nurseries, residual herbicide options are limited during the critical rootstock emergence and early season growth period by crop safety concerns (Zheljazkov et al., 2007). Some herbicides can injure either crop roots (stunting or malformations) or aboveground portions of the plant (meristem damage, stem malformations, stunting, and chlorosis). Plants with visual root or stem defects are unacceptable to the buyers and are usually destroyed.
As other options become more expensive or difficult to use, it is likely that herbicides will become an even more important part of integrated pest management in nursery crops (Altland et al., 2003; Gilliam et al., 1989). Identification of herbicides and herbicide application techniques that provide economical, effective, and safe in-season weed control will provide growers with additional management options for production of fruit and nut tree nursery stock. Several new herbicides have been registered in orchard crops for control of a broad spectrum of weeds; however, these herbicides are not currently labeled for use in tree nurseries and the safety of these materials to common peach, plum, and peach/plum hybrid rootstocks used in production of stonefruit and almond nursery stock is not known. The objective of this work was to evaluate weed control efficacy and nursery rootstock safety of several PRE and POST-directed herbicide treatments in commercial tree nursery production systems.
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