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Mary Joy M. Abit and Bradley D. Hanson

Producers of fruit and nut tree nursery stock need effective weed control for maximum production of vigorous, high-grade planting material. Current weed practices include methyl bromide fumigation, preemergence herbicides, hand labor, and multiple tillage operations. As methyl bromide use is phased out due to air quality concerns, and fuel and labor costs continue to increase, herbicides are likely to become even more important for weed management in the nursery industry. Before new herbicides can be registered and used in stonefruit (Prunus sp.) tree nurseries, weed control efficacy and crop safety data are needed under local conditions. Eleven experiments were conducted from 2007 to 2011 in California tree nurseries to evaluate the crop safety of preemergence (PRE) and postemergence (POST-directed) applications of various herbicides on commonly grown peach (Prunus persica), plum (Prunus domestica), and peach/plum hybrid rootstocks budded to almond (Prunus dulcis) scions. Rootstocks grown from cuttings generally were more tolerant to herbicides than those grown from seed. Crop safety was adequate in seeded and vegetatively propagated rootstocks with oryzalin, pendimethalin, and isoxaben, all of which are labeled for use in tree nurseries. The unregistered herbicides, dithiopyr, rimsulfuron, oxyfluorfen, and foramsulfuron; as well as lower rates of indaziflam and penoxsulam; applied PRE and POST-directed can provide good to excellent weed control in some stonefruit rootstocks. However, because slight crop injury was occasionally observed, additional work on application rates, timing, and method of application, especially on nonlabeled herbicides is needed before these materials can be considered for registration and broad scale use in tree nurseries.

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Sally M. Schneider and Bradley D. Hanson

Nursery producers of perennial fruit and nut plants rely on preplant fumigation to meet regulatory requirements designed to ensure nematode-free planting stock. In the past, preplant treatments with methyl bromide or high rates of 1,3-dichloropropene were the preferred treatments. However, the phase out of methyl bromide due to environmental concerns and evolving regulations on the use of 1,3-dichloropropene has increased the need for effective and economical alternative fumigation treatments in open field nursery production. A field trial was conducted in a commercial nursery to test weed and nematode control with several tarped and untarped preplant applications of 1,3-dichloropropene, chloropicrin, and iodomethane in comparison with methyl bromide. Crop safety and nematode infestation were evaluated on a wide range of tree, vine, and berry nursery stock. No fall fumigation treatment in this study resulted in measurable injury to spring-planted nursery stock. There were few statistical differences between methyl bromide and the other fumigation treatments in crop establishment, crop quality, or nematode level at planting 5 months after treatment, although some untarped treatments had detectable levels of the root-knot nematode (Meloidogyne spp.). At grape (Vitus vinifera) and bramble (Rubus spp.) harvest after the first growing season, few statistical differences were noted in the number of nematodes isolated from roots; however, only methyl bromide had nondetectable levels in all varieties. The highest nematode levels were usually found in untarped iodomethane:chloropicrin and untarped chloropicrin plots. At tree harvest 26 months after fumigation, root-knot nematodes were isolated from the roots of highly susceptible tree varieties in several iodomethane:chloropicrin treatments and in chloropicrin alone plots. Untarped applications did not provide commercially acceptable control of weeds or root-knot nematode in this trial. Tarped applications of 30:70 and 50:50 iodomethane:chloropicrin provided nematode control similar to 1,3-dichloropropene, although not as good as methyl bromide. Iodomethane:chloropicrin combinations have been registered in other states and should be considered for use in California perennial fruit and nut crop nurseries as an alternative to methyl bromide.

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Gary S. Bañuelos and Bradley D. Hanson

New plant-based products can be produced from seed harvested from Brassica species used for phytomanaging selenium (Se) in the west side of central California. Se-enriched seed meals produced from canola (Brassica napus) and mustard (Sinapis alba) plants were tested as potential bioherbicides and green fertilizers in strawberry production under controlled and field conditions for two seasons. Treatments consisted of adding each meal (containing 2.2 mg Se/kg dry mass) to the soil at rates equivalent to 0, 2, and 6 t/acre, respectively, 7 days before planting. In growth chamber studies, the highest rates of either meal lowered berry yields by a high of 30% compared with no application (control). Among the nutrient accumulation, berry Se, calcium (Ca), manganese (Mn), and zinc consistently increased with most Brassica meal treatments and most significantly with the mustard meal. In the field studies, mustard treatments lowered the emergence of summer-germinating and resident winter annual weeds more than canola and control treatments. Strawberry fruit yields increased with all Brassica treatments, except a 42% fruit yield reduction was observed at a 6 t/acre rate of mustard meal. Increases in fruit Se concentrations and increases in Ca, phosphorus, and Mn were often observed for all Brassica treatments. Amending soils with Brassica seed meals may have more practical viability in organic agriculture as a potential bioherbicide and green fertilizer.

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Mariano F. Galla, Bradley D. Hanson and Kassim Al-Khatib

Walnut (Juglans regia) and rice (Oryza sativa) are among the most important crops grown in the Sacramento Valley of California. Because rice herbicides are often applied by air, there are occasional allegations of rice herbicide drift onto walnut trees. This study was established to investigate bispyribac-sodium residues on walnut leaves after simulated drift treatments. The objectives were to determine whether bispyribac-sodium can generate visual symptoms on walnut trees without leaving detectable residues in leaf tissues and to evaluate the subsequent impacts on walnut yield. Two experiments were conducted in a 3-year-old walnut orchard. In the first experiment bispyribac-sodium was applied to walnut trees at 0.125%, 0.25%, 0.5%, and 1% of the normal use rate in rice (45 g·ha−1). In the second experiment, rates were 1%, 3%, 10%, and 100% of the normal use rate in rice. Bispyribac-sodium caused general leaf chlorosis and discrete yellow spotting on walnut leaves even at very low concentrations; symptoms were recorded on trees exposed to rates as low as 0.125% of the normal use rate in rice. However, based on high-performance liquid chromatography analysis, the lowest simulated drift treatment from which bispyribac-sodium could be detected 10 d after treatment was 1% of the rice use rate. In general, visual injury symptoms remained constant over time, or even worsened, whereas bispyribac-sodium residues decreased or became not detectable. There was no measurable impact on walnut yield from any of the simulated drift treatments in these experiments.

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Christine M. Rainbolt, Jayesh B. Samtani, Steven A. Fennimore, Celeste A. Gilbert, Krishna V. Subbarao, James S. Gerik, Anil Shrestha and Bradley D. Hanson

Methyl bromide (MB) has been widely used in California cut-flower production for effective control of a broad range of soil pests, including plant pathogens and weeds. However, MB is an ozone-depleting substance, and its availability to growers is limited according to the Montreal Protocol guidelines. Steam has been suggested as a nonchemical option for preplant soil disinfestation. Five trials were conducted in protected greenhouse structure or open-field cut-flower nurseries in Monterey, San Luis Obispo, and Ventura counties to evaluate the effect of steam application, alone or in combination with solarization, on soilborne plant pathogen populations, weed densities, and crop growth. Several steam application methods were used including steam blanket, spike-hose, buried drip irrigation lines, or drain tile, and these varied among trials. Calla lily (Zantedeschia aethiopica) nursery trials initiated in 2007 and 2008 showed that steam alone or with solarization was similar to or more effective than MB:chloropicrin (MBPic), applied via drip lines, in controlling weeds and Verticillium dahliae at 6-inch depth. Trials conducted in Spring and Fall 2009 in an oriental hybrid lily (Lilium sp.) nursery showed that, 112 days after steam treatment (DAT) in the spring, the steam (spike-hose) treatment had fewer Fusarium oxysporum propagules than the MB treatment. Lily plant growth in the steam-treated plots was similar to MB-treated plots and taller than in control plots. In the fall trial, fewer lily plants emerged by 44 DAT in the untreated control than in steam- and MB-treated plots and steam was not as effective as MB in reducing Pythium populations. In the 2010 sunflower (Helianthus annuus) and bupleurum (Bupleurum griffithii) trial, all steam treatments reduced Pythium and Phytophthora cactorum survival compared with the untreated control plots, whereas weed densities were reduced only in the spike-hose steam-treated plots. These trial studies showed that steam appeared as effective as MB in suppressing pathogens and weeds and improving crop growth in cut-flower nurseries. However, additional information on fuel consumption, treatment time efficiency, and long-term effects of steam treatment on soil health are needed before steam can be recommended as a viable alternative to MB in California cut-flower nurseries.