Calag Archive

Yellow starthistle is the most widespread invasive weed in California. An exotic that invaded California in the Gold Rush days, starthistle was once a minor annoyance but is now out of control. One reason is that few herbicides are registered for use on California rangelands, pastures and wildlands. Of the registered herbicides, the majority are effective only when applied to the foliage of target plants; season-long control of yellow starthistle is not provided because seedlings that emerge after application of the herbicide escape injury. In a comparison of herbicides, the newly registered herbicide clopyralid provided excellent control at low use rates and worked eaually well whether applied to leaves or to soil to control germinating seedlings. In soil, it showed residual activity throughout the season. Complete yellow starthistle control was achieved with applications made from December through April, but treatments in February maximized desirable forage production, particularly grasses. Late-season applications of glyphosate and clopyralid were effective for control of starthistle plants in late rosette and bolting stages.

Successful control of yellow starthistle by mowing depends on both proper timing and the plant's form of growth and branching. The branching habit of yellow starthistle is highly variable and is in part dependent on the level of competition with other species for light. Field studies were conducted in five Northern California counties to examine the effects of growth form, timing of mowing and number of mowings on yellow starthistle growth and seed production. Erect, high-branching populations were effectively controlled by a single mowing at early flowering, while sprawling, lowbranching plants were not satisfactorily controlled even by multiple mowings. Mowing should provide an effective tool for yellow starthistle control in an integrated approach with clopyralid treatment, prescribed burning or biological control.

The glassy-winged sharpshooter (Homalodisca coagulata), an insect that has recently invaded California, and the smoke tree sharpshooter (Homalodisca lacerta) are creating serious new economic problems in both agricultural and ornamental plantings. The greatest threats surround their ability to spread the plant-pathogenic bacterium Xylella fastidiosa, which induces Pierce's disease in grapevines, almond leaf scorch disease, and a new disease known as oleander leaf scorch. Because of the potential distribution and large host range of the glassy-winged sharpshooter, other diseases caused by different strains of X. fastidiosa may have an even greater impact on the state's agricultural and ornamental landscape industries in the future.

As it moves through California, the new pest known as glassy-winged sharpshooter (GWSS) may significantly increase the spread of plant diseases caused by the bacterium Xylella fastidiosa. The bacterium causes Pierce's disease of grapevines, almond leaf scorch and the newly recognized oleander leaf scorch disease. Currently, almond leaf scorch is a minor problem for California's almonds, but if GWSS becomes established in significant numbers in or near almond orchards, it might increase the incidence of almond leaf scorch. Our studies show that GWSS and two other sharpshooter species can transmit X. fastidiosa from diseased oleanders to healthy oleanders. GWSS also can transmit Pierce's disease strains of the bacterium from grapevine to grapevine and to almond trees. The oleander strain of the bacterium did not infect grapevines, but the ability of GWSS to transmit Pierce's disease strains may increase the spread of this lethal grapevine disease in vineyards bordering citrus orchards or other habitats where invading GWSS may establish permanent populations.

Sterile medflies are being released at a weekly rate of 125,000 to 200,000 flies per week per square mile over a 2,155-square-mile area of urban Southern California to help prevent the development of new medfly infestations. This areawide approach reduced the annual number of infestations found from 1994 to 1998 in the treated area by 93.3%, compared to infestations detected between 1987 and 1993.

California's native perennial grasses have been largely replaced by alien annuals. Interest in restoration of native grasslands is strong, but genetic differences among the available collections of some grasses may affect the survival of plantings and remnant native stands. In collections of the four native grasses examined, differences in phenology, growth form and forage quality suggest the existence of geographic races.

In the Arbuckle area of the Sacramento Valley, a 22-acre orchard was planted in 1990 with four almond varieties (‘Nonpareil’, ‘Butte’, ‘Carmel’ and ‘Monterey’). The orchard was irrigated with three types of microirrigation — surface drip, subsurface drip and microsprinklers. The orchard soils are 3 to 4 feet of gravelly, loamy sand overlaying a restricting clay layer. The coarse-textured soil with its low water-holding capacity allows little lateral movement of water from the microirrigation emission device. Under these soil conditions, microsprinkler-irrigated trees produced larger almond yields and showed greater tree growth. In addition, irrigation system evaluations show that all three microirrigation systems provide excellent irrigation uniformity levels after 8 years of operation with only routine maintenance.

In our evaluation of three microirrigation systems used in greenhouse cut-flower production, the systems' ability to apply water uniformly varied widely. The most common irrigation system, the perimeter system, generally applied water less consistently than other tested systems. Center riser and drip irrigation systems generally applied water more uniformly than the perimeter system. And while many perimeter irrigation systems could be retrofitted to improve water distribution, all irrigation systems could be improved with a regular maintenance program consisting of flushing rust and other particulates from irrigation pipe, and chemically controlling biological growth in irrigation pipe.