California Agriculture

Water management practices in Califonia rice production can affect salinity in the field. This is particularly important because rice is one of the most sensitive crops to salinity. We extensively monitored salinity patterns in dozens of rice fields in Colusa and Glenn counties, in order to determine how salinity varies from basin to basin and to compare salinity patterns under different irrigation systems. We found that the fields most vulnerable to salinity damage were those with higher soil salinity and using irrigation water sources initially high in salinity, particularly nondistrict sources that are combinations of well and drain water. Long water holding periods, while effective in reducing pesticide concentrations in rice fields, can contribute to salinity increases in bottom basins. Salinity can increase with either conventional or static irrigation management systems, but the salinity pattern in the field will be different.

Field studies conducted by UC and under controlled greenhouse conditions by the U.S. Department of Agriculture's Agricultural Research Service indicate that rice is more sensitive to salinity than current guidelines suggest. This information is particularly important to rice growers who have experienced salinity problems after holding water on fields for longer time periods to reduce pesticide loading into the Sacramento River. Our field experiments show that an average seasonal salinity of the field water in excess of 1.9 deciSiemens per meter (dS/m) can reduce grain yields; current guidelines indicate that salinity affects rice yield at or above 3.0 dS/m. Salinity had a negative impact on a number of yield components including stand establishment; panicles, tillers and spikelets per plant; floret sterility; individual grain size; and even delayed heading. The emergence and early seedling growth stages were most sensitive to salinity, as was the three-leaf to panicle-initiation stages. Irrigation management practices should be adopted to minimize salinity during these critical growth stages.

UC Davis's Foundation Plant Materials Service (FPMS) maintains the disease-tested, professionally identified collection of grape scion and rootstock varieties, which is the core of the California Grapevine Registration and Certification Program. In 1992, newly developed serological testing techniques revealed the presence of grapevine leafroll­associated viruses (GLRaVs) in previously healthy vines in an older foundation propagating block, indicating active and recent virus spread. FPMS responded by increasing isolation distances and implementing a comprehensive virus screening program using the new methodology. The critical problem was the lack of information on leafroll virus epidemiology. When the distribution of infected plants in the old vineyard was mapped, new infections were frequently adjacent to known diseased grapevines. This study examined the ability of mealybugs, a putative leafroll vector, to transmit this group of viruses. We were able to confirm that four species found in California obscure, longtailed, citrus and grape mealybug can transmit GLRaV-3 isolates. This is the first experimental evidence of grapevine leafroll virus trans-mission by obscure and grape mealybug. In addition, we report for the first time that GLRaV-5 can be transmitted by longtailed mealybug.

Harvest timing has a profound effect on the yield and forage quality of alfalfa hay. Early harvest results in low yield but high forage quality and price, while delayed harvest increases yield but reduces forage quality and price. Since gross revenue is a function of both yield and price, it is important for growers to select the optimum cutting schedule. We quantified a biological relationship among yield, forage quality and day of harvest, using the results from 2 years of field studies at locations in the intermountain alfalfa production region of California. An economic analysis, including a decision model, was developed to enable producers to assess current market conditions and seasonal effects, and in turn select the most profitable harvest timing. Our analysis demonstrated that no single harvest strategy is always best. The most profitable approach depends on the rate of change in yield and quality for that season and the current price differential between the quality market classes for alfalfa hay.