Plot Experiments at USDA-ARS Parlier, Results
Replanting all the vines in USDA ARS field block 24 southwest was done in spring 2010 because of a significant number of non-surviving vines after the first replanting in spring 2009. The high mortality was likely caused by the extended high temperatures, exceeding 38 ºC, right after the replanting in April/May 2009. Replanting in block 24sw was delayed to April/May because of unusual low temperatures in February/March that delayed growth of the mustard cover crop (Treatment 2) to similar biomass to the 2007/2008 experiment.
Live citrus nematodes were found only in the non-fumigated field plots (Treatment 1 and 2) in both the native soil and buried bags. The 100% citrus nematode mortality achieved in all fumigation treatments indicated the effectiveness of nematode control with both MB and Telone C35 at low rates. Also, no other species of live nematodes were found in any fumigated plots. Except for the MB treatment, minimal control was found for total Fusarium or Pythium spp. Soil gas measurement clearly showed a progressive reduction of 1,3-D concentration over time. The VIF tarp significantly reduced fumigant emissions compared to the no tarp bare soil treatments.
Figure 1 shows the yield data collected from USDA ARS field 24 southeast, which was replanted in spring 2008. The data clearly indicates a positive fumigation effect on the yield. Relatively low yields were found in the non-fumigated field plots (Treatment 1 and 2).
Figure 3 is the soil nematode populations sampled on January 24, 2011. It is clear that 3.5 years after soil fumigation the MB control still exhibited the lowest nematode populations. Variable results were found for citrus, pin, and root-knot nematodes among the alternative treatments, but generally the fumigated plots had lower citrus and root-knot yet higher pin nematode numbers.
Prior to fumigation treatment, one set of four nematode bags were buried near the center of each treatment plot. The individual bags were placed at 15, 30, 60, and 91 cm depths, respectively, and each bag contained approximately 4000 citrus nematodes (Tylenchulus semipenetrans). After the fumigation treatment, these bags were retrieved from the soil and live citrus nematodes were counted under a microscope. Soil samples were also taken to 91 cm depth for assessment of natural populations of nematodes. For fungal assessments, soil samples were collected on 3 December 2007 and 18 April 2008 with a 2.5-cm diameter push auger to 15 cm depth from six randomly selected locations within each plot to produce a composite sample. Total Fusarium oxysporum and total Pythium spp. were determined using laboratory plating. Fumigant gas movement in the soil and emissions into the atmosphere were measured during both plot fumigation experiments at selected treatment plots. For both plot experiments, the same type of grape seedlings (Vitis vinifera var. Cabernet Sauvignon) were planted in the following spring, e.g. block 24se in 2008 and 24sw in 2009 and replanted again in 2010, respectively. Pruning weights and caliper vine diameter readings were made at the end of first season after planting and yield was measured from the second year on.