Nematicides are used to control plant parasitic nematodes. However, there is health and ecological hazards associated with the use of nematicides, hence the need for risk free, economical, and ecologically desirable alternative methods of managing nematodes . Another simple and practical alternative is the use of nematode suppressive cover crops . Cover crops may suppress nematodes by being a poor host, by having a nematicidal effect, by enhancement of nematode antagonists or beneficial nematodes or serving as a ―dead end‖ trap crop . However, cover crop nematode suppression is dependent on cultivar of the cover crop, soil temperature , nematode species , and how the cover crop is managed. For example, growing marigold as a cover crop consistently lowered root-galling by M. incognita on tomato , while incorporation of marigold crop residue failed to do so . Cover crops grown before the maincrop can also suppress nematodes and protect a subsequent and susceptible vegetable crop . Wang et al. on the other hand observed that incorporated cover crop residues had no effect on parasitic nematodes, but enhanced population densities of bacterivorous nematodes. This research is aimed at assessing the effects of summer cover cropping systems on plant parasitic and free-living nematodes in a subsequent vegetable crop. The approach was to use French marigold and cowpea as summer cover crops and compare them to a summer fallow treatment. Effects of cover cropping were assessed on nematode species composition and density in broccoli during the winter growing season. Managing nematodes with summer cover crops would provide vegetable growers, particularly organic farmers with an easy and acceptable method for pest management and improve traditional nematode management approaches.A three-year field study was conducted from 2007-2009 at the University of California South Coast Research and Extension Center in Irvine, CA on a loamy-sandy soil. The field site was loamy sand with a history of root-knot nematode infestation.

Three summer cropping treatments were employed: 1) French marigold , 2) cowpea , seeded at 56 kg/ha, and 3) a summer dry fallow as the untreated control. Cowpea was chosen because it is a drought hardy legume, weed drying rack resistant to weeds and enhances some beneficial organisms . Marigold was chosen because it is known to control nematodes . Each treatment plot was 12 m long x 10.7 m wide and laid out into 14 planting rows. The cover crops were direct-seeded in the last week of June in the center of the planting rows of each plot, watered through drip-tubing and grown for three months. The fallow control plots did not receive water during the summer. Each cover crop treatment plot was planted with the same cover crop in each of the three years of study. Plots were separated from each other with a 3 m wide buffer bare ground. The three treatments were replicated four times in a completely randomized design. At the end of the summer cropping period , the cover crops were mowed at the soil line, chopped, and the residues left on the ground. Concurrently, alternate rows of each of the cover crop treatments were incorporated into the soil at about 0.4 m intervals using a hand-pushed rotary tiller in preparation for broccoli transplanting. The fallow plots were not tilled. Plots for cover crop and broccoli planting are shown in Figure 1a. At the beginning of the subsequent cropping season , broccoli seedlings were transplanted in double rows into the tilled strips of the summer cover crop and fallow plots at an inter and intra-row spacing of 13 and 35 cm, respectively . Broccoli transplants were drip irrigated and fertilized with emulsified fish meal at 5 gallons/acre rate. Broccoli was chosen because it is a high-value vegetable crop that is sensitive to weeds, insect pests, nematodes , and requires high soil nutrients . All plot treatments were maintained in the same location for all three years of study in order to assess a cumulative effect of cover crops over time. During the third year of the trial I included testing nematode response with susceptible tomato plants. In three of the existing treatment replications, 5-6 tomato seedlings were inter planted into broccoli to observe if cropping treatment differences can be seen on tomato.

At broccoli harvest time , all tomato plants were uprooted and evaluated for tomato root nematode and assayed for gall index.Soil nematode population densities were determined by collecting 14 soil cores from 5-25 cm depth from each plot using a 2.5 cm-diameter Oakfield Model L and LS Tube-Type Soil Sampler. Soil samples were collected at cover crop planting , at cover crop incorporation and at broccoli harvest . In each of these years, the 14 soil cores were pooled for nematode analysis. Nematodes were extracted from a 100 gram sub-sample on modified Baermann funnels for 5 days and the number of nematodes was counted. Ten broccoli plants were removed at harvest from each treatment plot and rated for root-knot nematode galling on the 0 to 5 scale outlined by Taylor and Sasser . One hundred grams of broccoli roots were placed in a misting chamber for 5 days fornematode extraction and the number of second-stage root-knot nematode juveniles was counted. All data were analyzed using a one-way ANOVA analysis and means separated used the student T-test.Soil nematode population levels in the experimental field were generally low in all treatments during all years and all sampling periods. Observed plant parasitic nematodes were root-knot , cyst , and pin nematodes only. On any of these nematodes, there were no significant differences among cropping treatments at any sampling period or trial years . The huge variability of data among replications of each treatment and hence a high standard error made most of the differences statistically insignificant. However, there were some relative variations among the cropping treatments. The root-knot nematode population densities were relatively higher for the ACCI sampling of all years in the cowpea treatment compared to either marigold or the fallow treatment . The sugarbeet cyst nematodes were higher at the ABH sampling for the cowpea, relative to the other cropping treatments . When pooled for the three sampling periods, only RKNs were significantly greater in the cowpea plots for 2007 and 2009, but not 2008 . Neither the SCN nor the pin nematodes were significant for year or cropping treatments . If pooled for the cropping treatments , the RKN were denser for the cowpea treatment compared to marigold or fallow treatments .

The population density of RKN for the cowpea treatment was about 14 times higher than in the RKN population in the fallow treatment . The pooled mean population densities for the other nematode species were not significantly different among the cropping treatments . For the broccoli root analysis, neither of the broccoli nematodes nor the broccoli root gall index was significantly different among the cropping treatments or experimental years . Nematode root-gall formation on broccoli was generally very rare and only appeared during the first year and none during the subsequent vegetable growing years . When data were pooled for the sampling periods, and years, there were more RKN population levels on broccoli roots grown on the summer cowpea field than those grown on either marigold or fallow treatments .The last year trial using nematode-susceptible tomato plants inter-planted did not show any significant variation among the cropping treatments on the population density of any of the nematodes , although there were relatively more j2 RKN in the fallow plots than in either marigold or cowpea cover crop treatments. In general, rolling bench the results reveal that contrary to the hypothesis, the use of cowpea and marigold as an off-season cover cropping do not provide suppression to parasitic nematodes, at least to these observed within this experimental field. In most cases the cover cropping treatments had the same effect on parasitic nematode population densities as the fallow treatments. In rare cases, the cover crops enhanced the population densities of some parasitic nematodes compared to fallow treatment.While the effects of summer cover cropping treatments were not significant on the crop parasitic nematodes, they had significant effects in enhancing saprophytic nematode population densities . Enhancement of saprophytic nematodes started at the ABH sampling in the first year , with no significant differences among cropping treatments for the ACCI sampling. Data on saprophytes was not collected for the ACCP sampling in 2007. At the ABH sampling of 2007, saprophytes were about double on the cowpea treatment compared to the fallow , indicating the stronger enhancement of saprophytes with cowpea cover crop. Population densities of the saprophytes continued to increase in the second and third year compared to the 2007. Higher population were observed in both cover cropping treatments at the ACCP sampling of 2008 compared to the fallow , probably accounting for the previous year cover crop and broccoli crop residues. At this sampling period, saprophyte population were about 5 and 4 times higher in plots that had summer cowpea and marigold, respectively compared to the summer fallow . Regardless of the huge differences in saprophyte population densities among cropping treatments for the ACCI and ABH samplings of 2008, there were no significant differences among the treatments. Saprophyte population densities reached highest peaks following ACCI sampling in 2009 than any other sampling times of all years. At this sampling saprophyte populations were by far greater on the cowpea compared to either marigold or fallow treatments . However, there was a sharp decline in those nematodes for the ABH sampling of 2009 compared to the same time sampling in 2008 . When pooled for the three sampling periods , saprophytic nematode population levels were enhanced by both cover cropping treatments in 2008 and only by the cowpea cover crop in 2009 compared to the fallow system. Over all, cropping treatments had no significant effect in 2007 , indicating that the influence in cover crops on saprophytic population densities cannot be realized within one year of cover cropping rotations. If mean data are pooled just for the cropping treatments , both cowpea and marigold significantly enhanced saprophytes over the fallow treatment.Plant-parasitic nematode population densities in the experimental field were generally low during all years. There were only few species of plant parasitic nematodes, the root knot , cyst , and pin nematodes observed. The root knot and cyst nematodes are classified as the most dominant and economically damaging groups of plant parasitic nematodes . At all sampling times there was a huge variability in nematode population densities within the treatment replications, resulting in a large standard error and often leading to a non-significant effect in spite of large differences in mean values. Accordingly, the cropping treatments did not show significant differences on most nematode responses. Such problem may probably be minimized by using higher replication treatments . Furthermore, both cowpea and marigold cover crops are non-susceptible crops to nematodes and hence the reason for no significant differences among the cropping treatments. Broccoli crop is also a poor host to most nematodes and if planted late in the season when soil temperature is low, the nematode populations would also be low. Although a nematode susceptible crop was introduced by intercropping with broccoli at the third year, nematode population densities were still not variable among the cropping treatments. While cowpea and marigold cover crops are generally regarded as nematode resistant or suppressing plants, their use as off-season cover crop did not guarantee this value under this particular trial conditions. In some cases, RKN population densities were rather higher following cover crop incorporations, particularly cowpea, than under a fallow system. The relatively higher RKN following cowpea residue incorporations may indicate that cowpea still allows some level of nematode multiplication and thus is not resistant against RKN. It is also possible that the cover crops may have suppressed nematodes, but the initial low nematode population level in the field may have made it difficult to make a clear demarcation whether the cover crops suppressed nematode populations or not. Therefore, these responses basically contradict the previous findings that generally regarded cowpea and marigold as resistant and a potential means by which parasitic nematodes can be managed . Wand and McSorley observed that ‗Iron Clay‘ cowpea was susceptible to the same species of nematode it was once identified as suppressing. Chen et al. also showed that an increase in SCN population density in a former nematode suppressant perennial ryegrass treatment.