California Agriculture

Pierce's disease has caused extensive losses to grapes in the Temecula Valley. The primary vector of Pierce's disease in the region is the glassy-winged sharpshooter (GWSS), which has been found in large numbers in citrus trees. We examined the role of citrus in the Temecula Valley Pierce's disease epidemic and found that citrus groves have influenced the incidence and severity of Pierce's disease in grapes. Because GWSS inhabit citrus in large numbers, California grape growers should take additional care with vineyards located near citrus groves.

Glassy-winged sharpshooter (GWSS) females form white spots on the forewings from secretions of ultramicroscopic bodies known as brochosomes. This occurs after mating of the GWSS and just prior to egg laying. The first published reports of wing spots were made by Riley and Howard in 1893. The behaviors associated with brochosome formation could have important implications for integrated pest management (IPM) programs to control GWSS, an important vector of the bacterium that causes Pierce's disease in grapevines and other crops.

The bacterium that causes Pierce's disease (Xylella fastidiosa) is transmitted to grapevines by the glassy-winged sharpshooter (GWSS). Insecticides were evaluated for efficacy and residual activity against adult GWSS on grapevines. Ten insecticides were tested in the cyclo-chlorinated, carbamate, organic phosphate, pyrethroid and neonicotinoid chemical classes. Results from field trials indicate that the pyrethroids and neonicotinoids are promising control agents. Information on efficacious and environmentally compatible chemical control will be helpful in developing integrated pest management to protect California vineyards from Pierce's disease, as well as insecticide resistance management within crop-management production systems.

In California, first-generation table grape vineyards were planted on their own roots. But in recent years growers have been replanting onto acreage previously in grapes more often. Grafted plants are being sought to overcome nematodes and other site-related problems. We investigated the etiology of the decline and death of ‘Redglobe’ table grape scions propagated on different rootstocks, using a series of greenhouse and field experiments, and reverse transcriptase-polymerase chain reaction (RT-PCR) assays. We determined that a specific graft-transmissible agent was closely associated with declining young grapevines. The ‘Redglobe’ closterovirus that we identified was given the trivial name Grapevine rootstock stem lesion associated virus (GRSLaV).

We conducted a sediment source survey to gain insight into soil erosion on Northern California rangeland watersheds and to provide information to facilitate informed land-use management, conservation prioritization and water-quality regulation decisions. The results indicate that by focusing on erosion associated with natural and historical influences, inventory and assessment efforts on these watersheds can characterize the majority of sediment deliverable to streams. While this volume of sediment does not require mitigation under current water-quality regulations, it none-theless prohibits the ability of instream sediment monitoring to detect water-quality changes. Water-quality regulations require managers to create inventories for all sources with 10 cubic yards or more of potentially deliverable sediment. If a monitoring threshold of 100 cubic yards was used, more than 99% of the deliverable sediment identified in this survey would be inventoried. This would require developing inventories for only 82 of the 117 sites in this study. Overall, we determined that rangeland managers can achieve the greatest reductions in sediment generation by focusing on erosion from roads.

Although alfalfa is the dominant water user in the Imperial Valley and is planted on 40% of its irrigated acreage, data is incomplete regarding its water use under the most common growing conditions: moderately saline, clay soils with a relatively shallow, moderately saline water table. In a study from 1996 through 1998, we determined the seasonal variation in the alfalfa crop coefficient (Kc), based on measurements of applied water and soil-moisture depletion, and the contribution from the water table aquifer. Using chloride mass-balance methods, we estimated that the shallow water table contributed approximately 12% of the total applied water during the study, of which just over 8% occurred during the first year. Correcting the values for water table contributions (WTC) to crop water use removed the increasing trend in values during the study and resulted in an overall study crop coefficient of 0.84. The decrease in WTC was the result of a doubling in soil salinity to 12 deciSiemens per meter at the 3- to 4-feet-depth interval by the second year. For moderately saline growing conditions in the Imperial Valley, the alfalfa crop coefficient is smaller than elsewhere in the southwestern states, where values closer to one are common.

Although sudangrass hay is the second largest forage crop (next to alfalfa), grown south of Sacramento and its planted area has quadrupled in the past decade, little information is available about its water use or relative water value. We would expect the sudangrass yield-water use relationship to be similar to that of alfalfa. Limited data suggest that under moderate soil salinity conditions in the Imperial Valley, sudangrass water-use efficiency (WUE–-the ratio of hay yield to water used) approximates the low end of measured alfalfa WUE. However, when we account for an estimated salinity-induced yield loss of 15% in the Imperial Valley, sudangrass WUE approximates the high end of measured alfalfa WUE. In order to better characterize the relationship between water use and crop production, as well as develop information about the relative water value associated with sudangrass-hay production, we compared hay yields and prices with estimated crop water use from 1988 to 1999 in Merced and Stanislaus, Riverside and San Bernardino, and Imperial counties. Linear regression analysis between yield and crop water use from the five-county dataset resulted in different WUE values depending on whether or not the Imperial Valley data was included. Variability in the crop-yield-to-water-use ratio from year to year was greatest in Imperial County and least in Merced and Stanislaus counties, while irrigation-water values (not including production costs) for sudangrass hay were lowest with the greatest variability in the northern counties.