California Agriculture

Since its emergence in the late 1990s, sudden oak death has killed mature oak trees and tanoaks in 14 California counties. Treatment options are now available to safeguard these trees from infection by Phytophthora ramorum, the aggressive and exotic pathogen responsible for sudden oak death. We provide an update on current knowledge regarding this emergent disease in California, and present results from three controlled experiments of two chemical treatments to manage the disease in oaks and tanoaks. Phosphonate treatments, legally registered in California to control sudden oak death, were effective in slowing both infection and growth rates for at least 18 months. Conversely, an alternative method consisting of an azomite soil amendment and bark lime wash was always ineffective, and did not reduce either growth or infection rates.

California pistachio growers have long relied on a single female (‘Kerman’) and single male (‘Peters’) cultivar. Despite their benefits, these cultivars present important production and marketing concerns. To evaluate new varieties for the pistachio industry, we conducted the first randomized and replicated pistachio variety trials in the San Joaquin Valley, where most U.S. pistachios are grown. After more than a decade of scientific evaluation, several varieties new to California (such as ‘Kalehghouchi’) or the world (such as ‘Golden Hills’) demonstrated commercial potential for the California pistachio industry and would complement the production characteristics of ‘Kerman’.

The navel orangeworm (NOW), a primary pest of almonds and pistachios in California, is controlled in part by sanitation, with a current threshold of two mummy nuts or fewer per tree. However, almond and pistachio acreage has increased dramatically since the tree mummy threshold was established. This study addresses the impact of this expansion and the possible need for a more stringent standard. Beginning in 2002, the Paramount Farming Company conducted a series of large-scale studies reevaluating the current tree mummy threshold in almond orchards, as well as the impact of ground mummies and proximity to pistachio orchards. The data supports a more stringent threshold of 0.2 mummies per tree. In addition, a new threshold for ground mummies of four per tree for ‘Nonpareil’ almonds is supported in Kern County, although this needs to be validated in other regions. Proximity to pistachios was an important risk factor for navel orangeworm damage of 2% or less in almonds. Likewise, the influence of pistachios extended 3 miles from the center of the 10-acre almond orchard sections in our experiments to the margin of the nearest pistachio orchard.

Dairy producers seldom test or examine incoming cattle, although these important biosecurity practices are recommended. This pilot project examined risk management decisions that producers make when faced with test-positive animals in purchased groups of dairy cattle, in order to provide information on disease risks and conditions that could affect animal health and performance. New arrivals to seven herds at dairy farms in four California counties were examined and tested for a range of conditions. The most common findings were bovine leukosis virus (33% of cattle purchased) and male reproductive abnormalities (16% of bulls purchased). Once testing results were known, producers made a variety of risk management decisions. Although testing costs for some conditions outweigh the benefits of finding an infected animal, an individual producer’s decision to test new animals most likely depends on their knowledge of the pros and cons as well as their risk tolerance.

Grass cover crops can be harvested for biomass or used as a surface mulch to reduce erosion, improve soil structure, suppress weeds and conserve moisture. There is concern, however, that such plantings may affect subsequent crops. We studied the effects of sudex, a sorghum hybrid used as a cover crop, on subsequent crops of tomato, broccoli and lettuce started from transplants. Within 3 to 5 days of being transplanted into recently killed sudex, all three crops showed symptoms of phytotoxicity including leaf necrosis, stunting and color changes. There was 50% to 75% transplant mortality in all three species. Plant growth and development, as determined by biomass measurements, were also significantly affected. Yields of mature green tomato fruit and marketable broccoli and lettuce heads were reduced significantly. Tomato, broccoli and lettuce should not be transplanted into sudex residue for at least 6 to 8 weeks, or until the residue has been thoroughly leached.

Many plants that are candidates for refining into biofuels also possess qualities that make them potentially useful for managing soilborne pests, reclaiming polluted soils, supplementing animal feed and other purposes. Phytoremediation with these plants may provide a practical and economical method for managing the movement of trace elements into water tables, surface- and tail-water runoff, and drainage effluent. Mustards (Brassicaceae) are of particular interest for biodiesel, and grasses (Gramineae) for bioethanol production. These plants, as well as others such as certain members of the onion family (Alliaceae), also possess properties that could make them effective natural biofumigants for soil. Some of these crops have high allelopathic activity and must be employed carefully in rotations to avoid damaging subsequent crops.