Suppression of weeds and other pests in fresh market vegetables using wild radish cover crop
Bioassays and a field experiment were conducted in 2004 to determine the level of pest suppression that can be expected from a wild radish cover crop. Banded cucumber beetle larvae and eggs were suppressed by a wild radish extract. In the field experiment, a wild radish cover crop provided early-season weed suppression in sweet corn and had no detrimental affect on sweet corn based on visual estimates of crop vigor. In cover crop fallow plots, sweet corn showed signs of nitrogen deficiency early in the growing season, while symptoms were generally absent in wild radish plots. Mycorrhizal colonization in corn was greater in wild radish plots compared with rye or fallow plots. Insect and soil-borne pathogens were similar among cover crops, except for Rhizoctonia which was lowest in the rye cover crop prior to planting sweet corn. Marketable sweet corn ear number in wild radish plots was often superior to rye or a fallow cover crop, regardless of herbicide program. Early-season weed suppression and lack of nitrogen deficiency in wild radish plots likely contributed to the higher yields.
1) Determine the effect of wild radish residues on soil-borne insect pests in bioassays.
2) Evaluate the effect of wild radish on growth and yield of sweet corn in field studies along with mycorrhizal colonization and weed, pathogen, and insect populations.
Insect Bioassays (Objective 1): To determine the effect of wild radish residues on insect pests associated with vegetable crops in bioassays
Banded Cucumber Beetle Larval Test 1 - Banded cucumber beetle larvae were reared on sprouted wheat at room temperature. Six-day-old larvae were placed in 30 ml clear plastic cups, each of which contained filter paper treated with either water or wild radish/water extract, 100% or 50%. Each of these three treatments had 50 replicates. The 100% extract was made by mixing cold water with oven dried ground wild radish in the ratio of 9 ml to 1 g (v/w) and leaving the mixture to stand for 15 minutes before straining off the coarse particles with a piece of fabric. The finer particles were removed by straining with filter paper and the filtrate was then used to saturate the piece of filter paper in each cup. The 50% extract was then made by diluting the left over 100% extract with an equal amount of water, and the procedure was repeated for the next set of the cups. It took 1.5 ml of fluid (water, 100% extract, or 50% extract) to saturate each filter paper and special care was taken to drain off any excess that might drown the larvae. The larvae were then placed in the cups, followed by two sprouted wheat seeds before fitting the cups with plastic lids and placing them in an environmental chamber at 25C and 80% relative humidity. Mortality data were recorded 4 days after treatment (DAT).
Banded Cucumber Beetle Larvae Test 2 - We also tested the effect of wild radish on the banded beetle larvae on sand with a constant soil moisture level in the 30 ml plastic cups. The control medium consisted of only sand saturated, but not flooded with water, while the other two treatments consisted of sand blended with dry wild radish powder and then moistened with 1.5 or 3 ml of water. The objective here was to keep constant the amount of moisture in each cup and each treatment during the experiment. The wild radish treatments were made by mixing 20 g of wild radish powder with 80 g of sand and then blending them thoroughly. Three grams of this mixture was then placed in each of forty 30 ml plastic cups were then divided into two sets, A and B each with 20 replicates. To set A, 3 ml of water was added; 1.5 ml was added to set B. The cups in each set were weighed daily to maintain the same weights throughout the experiment. Each cup in the control set contained 3 g of plain sand and 0.9 ml of water which saturated the sand without flooding it. The contents of cups were not blended after water was added. A banded beetle larva and two sprouted wheat seeds were then placed in each cup prior to covering with plastic lids and were placed in the environmental chamber at 25 ± 1º C and 80 ± 10% RH.
Banded Cucumber Beetle Egg Test - Studies were also conducted to observe the effect of wild radish extract on freshly laid eggs of the banded cucumber beetle. The extract was made by mixing 3g of dried ground wild radish with 30 ml of water and letting the mixture stand for 30 minutes before straining off the coarse particles with a piece of organdy fabric and removing the finer particles by using filter paper. Treatments were:  Eggs soaked in wild radish extract for 18 hours.  Eggs soaked in tap water for 18 hours,  A control in which eggs were transferred directly from the egg pad to the moistened filter paper strip,  Eggs soaked in tap water for 30 minutes, and  Eggs soaked in wild radish extract for 30 minutes. The eggs were removed from their respective treatment fluids onto the moistened filter paper strips using a fine camel’s hair brush.
Preliminary tests included a third control [3 above] of undipped eggs to determine the importance of moisture in hatching. The tip of a cotton swab was placed in a translucent plastic micro tube and moistened with water. Eggs were removed from the respective treatments and placed on a moistened filter paper strip inside the micro tube. The micro tubes were then placed in an environmental chamber at 25 ± 1 C and 80 ± 10% RH and monitored for hatch over the next 10 days.
Mortality rate of larvae was highest in the 100% extract, followed by the 50% extract, and then the control sample. Results have been inconsistent, and further testing will be conducted. There was no significant mortality in the test with sand and the sand-wild radish mixture, regardless of the amount of moisture. In the egg experiment, hatching was first observed 7 DAT in the control and 18 h water treatment. Moreover, by day 8, the 18 h water treatment and control yielded significantly greater percentage hatch than either of the wild radish extract treatments. Exposure to wild radish extract for 18 h resulted in the least amount of hatch (5.0%) at 9 DAT compared to all other treatments. Eggs soaked in the extract for 18 h yielded a very small hatch percentage, while those soaked in the extract for 0.5 h yielded a similar percentage hatch to the eggs soaked in water for 0.5 h.
Black Cutworm Test 1 - Black cutworms were maintained by placing neonate larvae on artificial diet (multi species insect diet). Upon pupation, they were placed in approximately 3.8 liter wide mouth jars with cheese cloth hanging from the rim for oviposition substrate. Generally, third instar larvae were used. The tested treatments included: water, 100% extract, and 50% extract. The 100% extract was made by mixing cold water with dried ground wild radish in the ratio of 9 ml to 1 g (v/w) and leaving the mixture to stand for 15 minutes before straining off the coarse particles with a piece of fabric. The finer particles were removed by straining with filter paper, and the filtrate was then used to saturate the piece of filter paper in each cup. The 50% extract was then made by diluting the left over 100% extract with an equal amount of water and the procedure was repeated for the next set of the cups. Larvae were then placed in the cups followed by a gram of multi species insect diet, after which the cups were covered and placed in the environmental chamber at 25C and 80% RH.
Black Cutworm Test 2- Soil was amended with the wild radish. Instead of water, a 0.6% solution of methyl paraben was used to inhibit mold. The four treatments were as follows:  The control, which was sand moistened with methyl paraben solution in the ratio of 4 to1 (4 g sand + 1 ml methyl paraben);  Sand on bottom, wild radish paste on the surface (4 gm sand, 0.5 g wild radish, and 1 ml of methyl paraben solution). The sand and wild radish were each moistened separately;  Sand-wild radish mixture (4 g sand, 0.5g wild radish and 1ml of methyl paraben solution; and  Sand –extract mixture (4 g sand, 1.5 ml extract). To each treatment was then added 1g of multi species diet and a black cutworm (six days old) before fitting the cups with lids and placing them in an environmental chamber at 25C and 80% RH.
The eggs of the adult black cutworm were also exposed to the wild radish by soaking them in its extract for 10 seconds before air drying them and placing them in gelatin capsules. They were then monitored during the next few days to see if they hatched and how long it took them to do so versus those that had been exposed to water.
The black cutworms exhibited great tolerance to the wild radish and even seemed to flourish. When filter paper was used as the fumigation substrate, they were alive after 4, 6, and 10 d and when sand was the substrate, the black cutworms were also alive after 4, 6, and 10 d. Both groups of cutworms were then left in the chamber in order to observe what, if any, effect the wild radish would have on them. They pupated in another 3d, and adult moths emerged 12 d after pupation. Hence the wild radish had no apparent effect on development of the black cut worm.
The eggs hatched at different intervals, but they all did so within the next 48 hours. Hence the wild radish extract had no apparent effect on the eggs of the adult black cutworm. This may be because they were almost fully incubated that the larvae may not have been affected. Further testing will be conducted on the eggs of the black cutworm.
Sweet Corn Field Experiment (Objective 2): Determine the effect of wild radish on growth and yield of sweet corn in field studies along with mycorrhizal colonization and weed, pathogen, and insect populations.
The experiment was conducted at the Edisto Research and Education Center near Blackville, SC to evaluate the effect of wild radish and rye cover crops in conjunction with reduced herbicide rates on weed control, and sweet corn vigor and yield. The experimental design was a split plot arrangement of cover crops (main plot) and weed control treatments (subplot), with four replications. Cover crop treatments included wild radish, rye, and the natural weed population, excluding wild radish. Weed control treatments consisted of a weedy control, hand-weeded control, atrazine at 0.84 kg/ha plus S-metolachlor at 0.44 kg/ha (half rate) applied after planting, and atrazine at 1.68 kg/ha plus 0.87 kg/ha (full rate) applied after planting. Florida pusley, large crabgrass, spreading dayflower, and ivyleaf morningglory were the predominant weed species infesting the test site. Weed densities (weedy plots only), visual estimates of weed control by species, and visual estiamates of sweet corn vigor (estimate based on growth and coloration) were recorded 2, 4, 6, and 8 wk after planting (WAP). Sweet corn ears were harvested once at 70 d after planting.
Above ground biomass differed among cover crops, with wild radish and rye producing 486 and 323 g/m2, respectively. Weed biomass in wild radish and rye plots was 104 and 161 g/m2, respectively, compared with average weed biomass of 210 g/m2 in weedy plots. Nine glucosinolates, potential natural weed suppressants were identified in wild radish, with these being glucoiberin, progoitrin, glucoraphanin, glucoraphenin, gluconapin, glucotropaeolin, glucoerucin, glucobrassicin, gluconasturtin.
The wild radish cover crop with the one-half and full rate of atrazine plus S–metolachlor provided 79 to 90% and 81 to 97% weed control, respectively, 4 WAP. Weed control 4 WAP in the rye cover crop ranged from 64 to 83% and 78 to 95% following the one-half and full rates, respectively. In the absence of herbicides, wild radish provided 51 to 66% weed suppression, while rye provided 28 to 49% suppression. Weed densities in the absence of herbicides were 151, 4, 1, and 1 plants/m2 for Florida pusley, large crabgrass, spreading dayflower, and ivyleaf morningglory, respectively, 4 WAP. Wild radish and rye cover crops were suppressive of weed emergence based on Florida pusley densities of 37 and 45 plants/m2 in wild radish and rye plots.
Weed control declined by 8 WAP in all treatments relative to control at 4 WAP. One-half rates generally failed to provide effective weed control through 8 WAP, regardless of cover crop. The full rate of atrazine plus S-metolachlor in wild radish plots provided superior control of all weed species compared to control in the rye and no cover crop systems, except for spreading dayflower control which was comparable in the no cover crop system.
The use of a wild radish cover crop did not have any detrimental effect on the mycorrhizal status of corn grown in hand-weeded or herbicide treated plots. In un-weeded plots, the use of a wild radish cover crop was associated with greater mycorrhizal colonization of four weed species. It should be noted that, in all cases, colonization of the weed species was relatively low, i.e. less than 15%. In general, mycorrhizal colonization is not thought to provide significant nutrient benefit at levels less than 20%. It is therefore unlikely that changes in mycorrhizal colonization associated with the wild radish cover crop would have marked effects on weed nutrition and performance in this system.
Soil samples were collected for pathogen evaluations from the control (non-sprayed, hand-weeded) and 1X herbicide subplots in all main plots on 4 May, 1 day prior to incorporation of the cover crops, and again on 20 May 2004, 2 weeks after seeding. Soils were assayed for Pythium, Fusarium, and Rhizoctonia species. The population level and activity of Rhizoctonia were measured simultaneously using a beet-seed colonization assay. The only difference observed between cover crops at either sampling and for any population was for Rhizoctonia at the first sampling. Colonization of beet seed was significantly lower for rye (2.7%) than for fallow (9.2%) or wild radish (10.3%). At 2 weeks after seeding, the population of Rhizoctonia had increased significantly from the initial sampling. A mean of 42% of the beet seeds buried in the soil samples were colonized by Rhizoctonia species, primarily R. solani. All of the isolates (N = 18) examined microscopically were multinucleate, i.e., pathogenic species. Neither cover crops nor herbicide use had a significant effect on levels of soilborne pathogens at the second sampling. There also were no significant cover crop-by-herbicide interactions. Mean population levels of Pythium decreased significantly after cultivation. There were 1.6 x 103 colony-forming units (CFU) per gram oven-dry soil initially and 390 CFU/g at the second sampling. However, population levels of Fusarium did not change significantly between samplings: 5.6 and 4.1 x 104 CFU/g. Population estimates of Fusarium were very similar with Komada’s and Nash-Snyder media, 4.16 and 4.22 x 104 CFU/g, respectively. Approximately 19% of the colonies recovered on Komada’s medium, or 7.9 x 103 CFU/g, resembled F. oxysporum, one pathogenic species of Fusarium. Because of the lack of treatment effects at 2 weeks after planting, soil samples were not collected at 6 weeks after planting, when levels of ITC in soil would have been much lower.
For the insect component of the experiments, data were also collected from plots at Tifton, GA. In this study, one-half of each plot contained Bt sweet corn. No earworms were detected in the ears harvested from the Bt corn in Tifton. The highest average per ear from the non-Bt corn was approximately 0.23 worms per ear. Hence it can be inferred that as Bt inhibited the presence of earworms, its effect may have been seen in the entire study in Tifton. In addition, factors such as weather and geographic location as well as plant fitness may also have had an impact on the low population density of corn earworm in Tifton. With regard to the cover crops used, there was no consistent pattern with any of the treatments. The wild radish seemed to be effective in Tifton, where population density was less than 0.1 corn earworms per ear of corn. This trend was not detected in Blackville.
Sweet corn vigor differed among cover crop treatments, with stunting from weed competition and yellowing from nitrogen deficiency prior to side-dressing plots. Sweet corn vigor was 43% less in weedy plots with no cover crop compared with handweeded wild radish plots 4 WAP. Nitrogen deficiency and stunting was most prominent in the no cover crop treatments, especially in the absence of effective weed control. Wild radish plots often had more vigor (absence of deficiency symptoms) than other cover crop treatments. Nitrogen deficiency symptoms rarely occurred in wild radish plots which may be due to high utilization of nitrogen by wild radish plants prior to planting sweet corn, with this nitrogen becoming available following planting through mineralization. It is likely that a large portion of the nitrogen was leached from the sandy soil in the no cover crop treatment, whereas the high C:N ratio of rye likely contributed to deficiency symptoms in corn 4 WAP. Atrazine plus S-metolachlor appeared to have no negative affect on corn nor did wild radish appear detrimental to corn development based on the handweeded treatments. Side-dressing plots with nitrogen at 5 WAP alleviated all deficiency symptoms by 8 WAP. Reductions in vigor at 8 WAP were mainly due to stunting from weed interference and/or the initial nitrogen deficiency.
Marketable ears in wild radish plots treated with herbicide or hand weeded were similar, ranging from 38,000 to 48,000 ears/ha. Marketable ear number in rye plots ranged from 33,000 to 35,0000 ears/ha in hand weeded or herbicide treated plots. Marketable ears produced in the absence of herbicide or hand weeding were 5,000, 12,000, and 29,000 ears/ha in no cover, rye, and wild radish treatments, respectively. Neither rye nor wild radish alone provided sufficient weed suppression to prevent yield reduction relative to either cover crop when treated with herbicides. Insect damage to ears and small ear size as a result of weed interference and early-season nitrogen deficiency reduced marketable ear numbers relative to total ear numbers for all treatments. Marketable ear number was 39 to 83% less than total ear numbers.
Impacts and Contributions/Outcomes
Insect bioassays showed that larvae and eggs of some insects are sensitive to an extract from wild radish; however, larvae and eggs of others may not be negatively impacted.
Weed suppression was superior in wild radish plots treated with the 1X herbicide rate compared with other cover crop treatments (rye or fallow).
A positive attribute of the wild radish cover crop was that it allowed for superior mycorrhizal colonization of sweet corn compared to rye or no cover crop; however, colonization was still minor (10%) and likely did not contribute significantly to the enhanced yields in wild radish plots. Additionally, the 1X herbicide rate had no detrimental affect on colonization in corn.
Colonization of Rhizoctonia was lower in rye compared with fallow or wild radish, which were both similar.
There was no suppression or stimulation of any insect species in wild radish plots.
Wild radish provided early-season weed suppression in field grown sweet corn while having no detrimental affect on corn.
Sweet corn yields in wild radish plots treated with a 1/2X herbicide rate were comparable to the full herbicide rate.
Sweet corn yields were often higher in wild radish plots compared with similar weed control programs in the fallow or rye cover crops. Early-season weed suppression along with soil fertility benefits from the wild radish cover crop are likely the major contributors to the higher yields.