In recent decades, air quality has improved near most cities but not in rural areas such as the San Joaquin Valley. Many studies using diverse exposure techniques have shown that ground-level ozone air pollution reduces plant growth and yield, from negligible impacts in some species to over 30% losses in others. We studied the interaction of ozone with weed competition from yellow nutsedge in Pima cotton and tomato in open-top field-exposure chambers at the UC Kearney Research and Extension Center in Fresno County. Ozone impacts on cotton (which is relatively sensitive) were compounded by weed competition, whereas tomato (which is less sensitive) competed well at all ozone concentrations. Our data suggests that crop-loss estimates obtained in single-factor experiments accurately reflect the serious risk of ozone to agriculture, but that more accurate yield predictions will require the consideration of interactions between the components of complex crop production systems, including weed competition.

While glyphosate-resistant horseweed has not previously been reported in California, we suspected that it might exist, especially in noncrop areas. We collected horseweed seeds from two locations in the San Joaquin Valley and treated greenhouse-grown plants at different stages with different amounts of glyphosate. This study showed that a glyphosate-resistant biotype of horseweed exists in the noncrop areas of Dinuba, in Tulare County, and that the level of resistance may be influenced by the plant’s growth stage at the time of glyphosate application.

In recent years, growers and pest consultants have reported poor control of the weed hairy fleabane in some areas of the Central Valley. Hairy fleabane seeds were collected from Esparto, Fresno and Reedley, Calif., and greenhouse-grown seedlings were treated at several different glyphosate rates and compared with an untreated control. None of the Esparto or Fresno plants survived glyphosate rates greater than 0.78 pounds acid equivalent per acre (lb ae/ac), while some of the plants from Reedley survived even the highest rate of glyphosate tested (12.4 lb ae/ac). The dose required to reduce plant dry weights by 50% (GR50) of the Esparto plants ranged from 0.28 to 0.30 lb ae/ac, whereas the GR50 of the Fresno and Reedley plants ranged from 0.26 to 0.61 and 0.92 to 2.88 lb ae/ac, respectively. This study showed that the hairy fleabane plants from Reedley were much more tolerant of glyphosate than either of the other two biotypes and, based on the GR50, the level of resistance ranged from 3- to 10-fold greater.

Organic farmers and limited-resource growers in the San Joaquin Valley and other agricultural areas in California — many of whom are ethnic minorities — encounter limited options and environmental constraints when seeking economically viable pest management methods. Over the past 8 years, we have conducted weed research and implementation projects on soil solarization at the UC Kearney Research and Extension Center and on farms in the surrounding San Joaquin Valley. In the Kearney studies, small-scale solarization in parsley reduced weed biomass 94% to 99% over the untreated control. Furthermore, in an on-farm study, solarization provided effective weed control for strawberries at a much lower cost than methyl bromide, with comparable yields. This research has provided guidelines and technical support for growers wishing to implement solarization and related techniques for nonchemical soil disinfestation in a wide variety of specialty crops.

Overhead systems are the dominant irrigation technology in many parts of the world, but they are not widely used in California even though they have higher water application efficiency than furrow irrigation systems and lower labor requirements than drip systems. With water and labor perennial concerns in California, the suitability of overhead systems merits consideration. From 2008 through 2013, in studies near Five Points, California, we evaluated overhead irrigation for wheat, corn, cotton, tomato, onion and broccoli as an alternative to furrow and drip irrigation. With the exception of tomato, equal or increased yields were achieved with overhead irrigation. Many variables are involved in the choice of an irrigation system, but our results suggest that, with more research to support best management practices, overhead irrigation may be useful to a wider set of California farmers than currently use it.