The Sacramento Valley of California is a diverse cropping region where rotational field crops and fruit and nut trees are grown in close proximity. Walnut is one of the most important crops with a total of ≈143,000 acres for a gross dollar value of ≈$800 million reported in 2015 in the region [California Department of Food and Agriculture (CDFA), 2016; U.S. Department of Agriculture (USDA), 2016]. In addition, nearly all of the California rice production area is based in the Sacramento Valley, with 410,000 acres harvested in 2015 for a gross dollar value of about $732 million (CDFA, 2016).
Weeds can greatly reduce rice yields (Hill et al., 2006); in California, weed management programs almost entirely rely on the use of herbicides (Fischer et al., 2010). Generally, California rice growers apply herbicides at planting and follow up with one or two additional postemergence applications later in the season. The majority of rice herbicide applications are made by airplane between May and early July (California Department of Pesticide Regulation, 2016). During this time of the year, walnut trees are actively growing, initiating and differentiating buds that will produce vegetative shoots and flowers in the subsequent year (Sabatier et al., 2003).
In recent years, complaints of damage from rice herbicides allegedly drifting into young walnut orchards have been reported in the Sacramento Valley (J.W. Beauchamp, personal communication). Symptoms observed are consistent with acetolactate synthase (ALS) inhibitor herbicide damage: leaf chlorosis, chlorotic spots, and internode shortening (Al-Khatib, 2015).
The effects of ALS inhibitor herbicides on off-target crops have been widely studied in annual and perennial crops (Al-Khatib and Tamhane, 1999; Al-Khatib et al., 1992, 1993; Boutin et al., 2000; Fletcher at al., 1996; Hensley et al., 2012; Rana et al., 2014). For example, high concentrations of chlorsulfuron during reproductive growth phase reduced sweet cherry (Prunus avium) fruit production (Bhatti et al., 1995).
Bispyribac-sodium is widely used in California rice fields for the control of grass species such as early watergrass (Echinochloa oryzoides), late watergrass (Echinochloa oryzicola), barnyardgrass (Echinochloa crus-galli), and broadleaf species such as arrowhead (Sagittaria montevidensis), redstem (Ammannia sp.), monochoria (Monochoria vaginalis), and ducksalad (Heteranthera limosa) (Fischer et al., 2004). Although many rice herbicides in California are applied in a granular form to minimize drift, bispyribac-sodium is applied in liquid spray mixtures. Evaluating simulated drift rates of three of the most commonly used rice herbicides in the Sacramento Valley (bispyribac-sodium, bensulfuron-methyl, and propanil), Galla et al. (2018a) identified bispyribac-sodium, an ALS inhibitor herbicide, as the herbicide with higher impact potential on walnut trees. Bispyribac-sodium simulated drift rates slowed walnut shoot growth and were negatively correlated with walnut kernel quality (Galla et al., 2018a). These results were confirmed in a subsequent study on the effect of multiple exposure of simulated drift rates of bispyribac-sodium on walnut (Galla et al., 2018b).
Laboratory analysis of walnut leaf samples displaying ALS-inhibitor symptoms usually does not detect bispyribac-sodium residues (J.W. Beauchamp, personal communication). Lack of detection could simply mean that the observed symptoms are not caused by bispyribac-sodium. However, it could also be related to the analytical limits of detection and the low exposure levels because typical downwind drift ranges from 1% to 8% or even lower (Al-Khatib and Peterson, 1999). It is also possible that bispyribac-sodium residues are metabolized by walnut leaves before symptoms develop sufficiently for a grower or consultant to notice and collect samples for analysis.
Previous research on bispyribac-sodium drift on walnut trees was based on visible injury symptoms (Galla et al., 2018a, 2018b), but to our knowledge, there are no data available on the detection of bispyribac-sodium on walnut leaves after a drift event occurs. Therefore, the objectives of this research were 1) to estimate the level of drift exposure necessary to generate detectable bispyribac-sodium residues in walnut leaves and 2) to determine whether there is a correlation between symptoms, yield, and bispyribac-sodium residues on leaf tissue.
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