A Cover Crop System for Vineyard Pest, Weed & Nutrition Management

This report describes the 1993 results of continuing, large replicated trials in three San Joaquin Valley vineyards where we are studying the impact of three vineyard floor management systems on pests, weeds, and vine nutrient status. The experiments consisted of three main treatments: (1) an oat/vetch (two vineyards) or a merced rye/vetch mixture (one) vineyard planted in row middles, with row berms treated with preemergence herbicides, (2) the same as in 1, but the cover was cut and blown on berms for weed suppression (with only postemergence herbicides applied during late winter), and (3) cover crops were not planted, but row middles were clean-cultivated (control), and berms were treated with herbicides for weed suppression. An additional trial was established on a two-acre vineyard at the UC Kearney Agricultural Center. This vineyard included only treatments 2 and 3, and a treatment where we attempted to evaluate the impact of berm mulching on weed suppression without the use of preemergence and postemergence herbicides. During the 1993 season, the presence of cover crops did not affect the number of leafhoppers on vines in any of the four vineyards, but there were large differences between vineyards in the size of leafhopper infestations, levels of leafhopper-egg parasitism, and abundance and species composition of spiders. In general, two vineyards (7 and 10 years old) supported larger numbers of leafhoppers (11-15 nymphs per leaf) compared to the other two vineyards (more than 30 years old) where leafhopper numbers were lower than one nymph per leaf during the peak of the second generation. We do not really know the cause of these differences, but it appears that factors associated with vine age may have a significant impact on leafhopper abundance. The proportion of leafhopper eggs parasitized by Anagrus were also higher in the ‘young’ compared to ‘old’ vineyards, probably resulting from higher densities of leafhopper eggs in these vineyards. Parasitism in cover plots was similar to control plots, but there was a trend toward higher egg parasitism in cover plots during the second leafhopper generation. There were also large differences between vineyards, and between cover treatments within vineyards, in the abundance and species composition of spiders occurring on grapevines. In the ‘older’ vineyards, there were significantly higher populations of spiders on vines in cover crop and mulch plots compared to vines in control plots. Leafhoppers were too low in these vineyards for any effect to be observed during the 1993 season. The differences in spider abundance between cover and control treatments were only obtained in the old’ vineyards where spiders were predominantly of the web-building species. In the two young’ vineyards, where spiders were predominantly of the non-web-building (or hunting) species, we did not observe a significant effect of cover crops on spider abundance on vines. Initial 16 results from marking studies indicated that the cover crops are increasing the spider’s food abundance, and are providing an alternative foraging habitat for the spiders, but it is possible that not all spider species are similarly affected in this manner by the cover crops. Further results of these marking are pending. Results from experiments on weed suppression with dry mulch are variable. However, our studies and those conducted by C. Elmore in north coast vineyards indicated that yearly accumulation of biomass in vine rows should provide sufficient weed suppression to minimize the use of herbicides. The data on the nutritional status of vines showed that petiole tissue analysis for nitrate-nitrogen and potassium varied considerably between vineyards. In this first year of comparing fertilized and unfertilized plots (in the two ‘old’ vineyards) in the presence and absence of cover crops, we did not observe any significant impact of cover crops on nitrate-nitrogen status; but there was a trend toward higher potassium levels during veraision in the cover plots compared to control plots in the two ‘old’ vineyards. In the two ‘young’ vineyards, nitrate-nitrogen levels were reduced in the presence of cover crops, but no changes occurred in potassium levels. These effects were probably caused by factors related to the cover crop stand and weeds growing on berms. It is also possible that young’ vineyards are less competitive with cover crops than are ‘old’ vineyards. We are continuing to monitor changes in vine nutrient status which may not be reliably detected until the following year. Our initial budget for the three floor management systems indicates that the lowest cost is incurred in the absence of cover crops and when the commonly applied herbicides are used to control weed in the vine rows. We expect, however, that the overall benefit of using cover crops in vineyard production will be increased the cost of insecticides and fertilizers are incorporated into the cost/benefit analysis. Our findings are being disseminated to the scientific community and farmers in meetings and field days.