A wild radish cover crop provided early-season weed suppression in field-grown sweet corn and had no detrimental affect on sweet corn. Sweet corn sometimes showed signs of nitrogen deficiency early in the growing season, while symptoms were generally absent in wild radish plots. Mycorrhizal colonization in corn was not negatively influenced by wild radish or rye compared with fallow plots. Marketable sweet corn ear number in wild radish plots was often superior to other cover crops, regardless of herbicide program. Laboratory bioassays revealed that banded cucumber beetle larvae and eggs are suppressed by a wild radish extract.
1) Determine the effect of wild radish residues on vegetable crops and associated pests in bioassays.
2) Determine if a wild radish extract reduces weed emergence and soilborne insect populations.
3) Evaluate the effect of wild radish on growth and yield of sweet corn in field studies along with mycorrhizal colonization and nitrification and weed, pathogen, and insect populations.
4) Evaluate the economic feasibility of the use of wild radish as a cover crop in vegetable production using data from on-farm trials.
5) Disseminate results to vegetable producers and other clientele.
Glucosinolates, commonly found in members of the Brassicaceae family, are hydrolyzed upon cellular disruption to form thiocyanates, isothiocyanates (ITCs), and nitriles, which possess allelochemical activity on numerous pests (Brown and Morra 1997). These natural phytotoxins are capable of suppressing many weeds (Brown and Morra 1997; Norsworthy 2003a), insects (Borek et al. 1998; Elberson et al. 1996; McCaffrey et al. 1995; Williams et al. 1993), soilborne plant pathogens (Mayton et al. 1996; Muehlchen et al. 1990; Smolinska et al. 1997), and nematodes (Halbrendt 1996; Kirby 1995; Mojtahedi et al. 1991; Smith 1994).
Wild radish, a Brassicaceae, is prevalent throughout the southeastern United States. Wild radish can be found in some field at densities as high as 830,000 plants/ha and can produce 5,190 kg/ha shoot biomass (Norsworthy – Clemson University, unpublished data), which is equivalent to biomass quantities of small grain winter cover crops commonly grown in this region (Bauer and Reeves 1999). Wild radish like other Brassicaceae plants produces glucosinolates (GSLs) (Cole 1976). These GSLs play an important role in weed suppression when converted to ITCs by myrosinase (Peterson et al. 2001). A number of GSLs have been identified in wild radish, namely 4-methyl thio-3 butenyl GSL, 3-(methyl thio) propyl GSL, and the aliphatic 2-propenyl GSL, allyl GSL, and glucioberin (sinigrin) (Cole 1976). These are distributed in roots, shoots, and leaves of wild radish and are stored in vacuoles (Belles 2002). ITCs formed form GSL hydrolysis suppress many soilborne pests including weeds and the process is called ‘biofumigation’ (Brown and Morra 1997).
Wild radish may be used as a cover crop in summer vegetables because of its allelopathic and GSL production potential (Cole 1976; Norsworthy 2003). Field corn (Zea mays L.) has exhibited tolerance to wild radish in greenhouse experiments (Norsworthy 2003); therefore, wild radish may be used as a component of an integrated weed management system in field or sweet corn. Brassicaceae plants have been successfully used as green manure prior to planting potato (Solanum tubersum L.) in the Pacific Northwest, reducing weed density and weed biomass as much as 85 and 96%, respectively (Boydston and Hang 1995). Hence, a natural infestation of wild radish if properly managed may be effectively used as a biofumigant in vegetable crops. Besides weed suppression, wild radish would offer benefits similar to other cover crops such as reduced soil erosion, conservation of soil moisture, and increased soil organic matter (Sojka et al. 1991).
Rye (Secale cereale L.) is widely used as a cover crop because of its allelopathic properties in addition to providing a physical, weed suppressive mulch, resulting in suppression of numerous weeds (Liebl et al. 1992). The use of rye cover crop with a one-half recommended rate of atrazine and metolachlor in sweet corn resulted in excellent control of redroot pigweed (Amaranthus retroflexus L.) and yellow nutsedge (Cyperus esculentus L.) in field experiments in Arkansas (Burgos and Talbert 1996). Allelochemicals released from rye resulted in inhibition of small to medium seeded weed species such as Palmer amaranth (Amaranthus palmeri L.), large crabgrass (Digitalia sanguinalis L.), goosegrass (Eleusine indicia L.), and barnyardgrass (Echinochloa crus-galli L.), whereas large seeded crops such as cucurbits and corn were tolerant (Burgos and Talbert 2000).
Winter cover crops produce plant residues which create an unfavorable environment for weed emergence in early spring (Teasdale 1996). These cover crops provide early season weed control and may reduce the quantity of herbicide needed for effective weed control. Wallace and Bellinder (1992) reported that a no-till rye cover crop system in conjunction with reduced rates of linuron, metolachlor, and metribuzin provided >90% redroot pigweed control and >93% common lambsquarters (Chenopodium album) control in sweet corn, snap beans (Phaseolus vulgaris L.), potato, and tomato (Lycopersicon esculentum L.). Rye alone (absence of herbicide) provided >70% weed control for 6 weeks and then the weed control declined.
Wild radish effectively suppresses weeds common to the southeastern United States. Incorporating wild radish residues (2% by weight) into soil reduced sicklepod, pitted morningglory, and prickly sida emergence and shoot fresh weight >90% (Norsworthy 2003a). Yellow nutsedge showed tolerance to wild radish residues; however, tuber growth was severely inhibited, which reduces its competitiveness with crops and ability to propagate (Norsworthy and Meehan 2004).
Pest insect feeding of Microtheca punctigera (Achard) (Coleoptera: Chrysomelidae) on wild radish was 25% and 28% of that found on mustard [Brassica juncea (L.) Czern. & Cosson] and Chinese cabbage [B. pekinensis (Lour.) Rupr.], respectively, although significant differences were not detected. It is probable that the insect was attracted to the plant, but the presence of arrestants or lack of feeding stimulants caused relatively weak feeding (Menezes et al. 2005). In other research, Williams et al. (1995) and Borek et al. (1995) detected ITCs in most Brassicaceous plants that were toxic to the black vine weevil [(Otiorhynchus sulcatus (F.)]. The relative growth rates of two lepidopterous pests, the diamondback moth [Plutella xylostella (L.)] and southern armyworm [Spodoptera eridania (Cramer)], were lower on lines of B. juncea with high GSL concentrations [6.8 – 21.3 ug/g (FW)] (Li et al. 2000). In toxicity experiments with artificial diets, GSLs and ITCs were lethally toxic to neonate S. eridania.
The plant chemicals also play a role in the development of insect defense mechanisms (Aliabadi et al. 2002). The harlequin bug, Murgantia histrionica (Hahn), is a pentatomid that feeds on toxic crucifer plants, and it can sequester and retain GSLs from its host plant as a defense mechanism against predaceous birds. Larvae of the large white butterfly, Pieris brassicae (L.), feed on crucifers, resulting in the development of toxic adults (Alpin et al. 1975). Moreover, chemicals in some Brassica species can have an impact on the biology of certain arthropods (McCloskey and Isman 1993, Huang and Renwick 1994, Traynder and Truscott 1991). Francis et al. (2001) linked biological parameters of aphid predators with the chemical composition of Brassicaceae. Blau et al. (1978) reported that Pieris rapae, a crucifer specialist, is not affected by artificially high concentrations of allyl GSL. However, larval growth of Spodoptera eridania, a generalist feeder, is inhibited by high, but not by low concentrations of allyl GSL. Our previous research indicates that exposure to flowers of wild radish decreases survival and longevity of a parasitic wasp, Diadegma insulare (Cresson), which attacks and kills the diamondback moth (P. xylostella), a major pest of collard and cabbage (Gourdine et al. 2005).
Southern blight (Sclerotium rolfsii) is a widespread disease throughout the southern US that affects most vegetable crops, peanuts (Arachis hypogaea), tobacco (Nicotiana tabacum), cotton (Gossypium hirsutum), and many ornamentals. This soilborne fungus produces long-lived survival structures called sclerotia. Sclerotia are sensitive to soil fumigants, several biological control fungi, and solarization (Papavizas and Collins 1990; Ristaino et al. 1991). However, these control measures are not widely used because of legal prohibitions on certain crops (fumigants) or cost. Sclerotia respond to certain volatiles and can be induced to germinate when treated with methanol (Ristaino et al. 1991). Residues of several Brassica spp. produce volatiles such as allyl-isothiocyanate that are toxic to fungi (Mayton et al. 1996). Rhizoctonia solani and Pythium ultimum, two widespread soilborne fungal pathogens, were completely inhibited by volatiles from B. juncea (Mayton et al. 1996).
Even though pest suppression by wild radish and rye cover crops would be a positive attribute, it is equally important that this approach not be detrimental to the beneficial microbial processes of nitrification and root colonization by mycorrhizal fungi. When canola (Brassica napus), a non-mycotropic plant species, was used in rotation with corn, mycorrhizal development was delayed (Gavito and Miller 1998). In other research, germination of mycorrhizal spores (Glomus intraradices) was inhibited by two non-mycotropic mustards (Brassica kaber and B. nigra), but not non-mycotrophic redroot pigweed (Schreiner and Koide 1993). Without mycorrhizae, most plants become stunted, show phosphorus deficiency, and eventually die.
Objective 1 -Bioassays for Crop Tolerance and Pest Suppression.
Weed and Crop Bioassay. A replicated greenhouse study was conducted to assess the tolerance of direct-seeded vegetables to soil-incorporated wild radish residues and the degree of suppression of several weeds. Crops evaluated included: ‘Clemson’ okra, ‘Merit’ sweet corn, ‘Henderson’ lima bean, ‘Iron/Clay’ southern pea, ‘Carolina Hybrid’ cucumber, ‘Xena’ bell pepper, ‘Sunny’ tomato, ‘Odyssey’ cantaloupe, ‘Montreal’ watermelon, and ‘Lemondrop’ squash. Weeds evaluated included: broadleaf signalgrass, large crabgrass, Texas panicum, johnsongrass, purple nutsedge, yellow nutsedge, Palmer amaranth, pitted morningglory, sicklepod, wild radish, and common cocklebur.
The screening technique used the methods reported by Norsworthy (2003) where wild radish plant material was harvested, oven-dried, and then ground and passed through a 1-mm sieve. The ground material was mixed with soil at rates of 0, 0.5, 1, 1.5, and 2% (by weight), placed in plastic trays, and moistened with water. All crops and weeds were seeded at known quantities. Percentage emergence and biomass reduction at 3 weeks after emergence was determined relative to a nontreated control. Percentage shoot, root, and total biomass reduction for each crop and weed was regressed against wild radish residue rate to fit a two-parameter exponential function.
Soilborne Pathogen Bioassay. Sclerotia of Sclerotium rolfsii were placed in nylon mesh bags filled with soil amended with 0, 0.5, 1, 1.5 or 2% (w/v) ground, dried wild radish residue and buried in the same trays used for the weed and crop evaluations. After 3, 7 or 21 days, sclerotia were retrieved from soil with forceps and placed on a semi-selective culture medium or on hypocotyls of green beans laid on moist filter paper in Petri dishes.
Objective 2 – Toxicity of Soil-Applied Extract to Pests.
Weed Bioassay. Sensitivity of the weeds evaluated in Objective 1 to an aqueous extract from wild radish was assessed. The extract was obtained by mixing 100 g of ground wild radish plant material with 1,000 ml of distilled water, after which the mixture was kept at 24 C for 24 h without agitation. The mixture was passed through a series of sieves ranging from 1000 to 38 mm and vacuum filtered through standard filter paper (>20 to 25 mm). Distilled water was added to the vacuum-filtrated residue to bring the final extract volume to 1 liter. The aqueous extract was applied to soil at 374 liter/ha and incorporated immediately. The weeds were seeded at known quantities and emergence and growth characterized.
Insect Bioassay. Cold distilled water and oven-dried ground wild radish were mixed in a ratio of 10 ml to 1 g (v/w), using 30 ml water and 3 g wild radish. The mixture was left to stand for 0.5 h. Coarse particles were strained with organdy over a 50-ml flask. Finer particles were removed by straining again with filter paper (grade 36).
Diabrotica balteata eggs that were deposited by moths on paper towels and 6-d-old D. balteata larvae were obtained from a colony maintained at the USDA, ARS, U.S. Vegetable Laboratory in Charleston, SC. Larvae were reared at room temperature on sprouted wheat soaked in Captan® to inhibit fungi. Five D. balteata eggs were transferred to each 30 ml cup onto strips of filter paper (1.0 cm X 2.0 cm) soaked with the designated treatment. Five treatments were tested: 1) eggs soaked in wild radish extract for 18 h, 2) eggs soaked in distilled water for 18 h, 3) control in which eggs were transferred directly from the paper towel substrate, 4) eggs soaked in distilled water for 0.5 h, and 5) eggs soaked in wild radish extract for 0.5 h. The treatments were replicated 24 times with 120 experimental units consisting of 600 eggs in each of 4 trials. Percentage hatch was recorded from the first day of hatching and for each of the next 3 to 4 days.
Eggs of A. ipsilon were purchased from Benzon Research (Carlisle, PA). A laboratory colony was subsequently maintained by placing neonate larvae on artificial diet (multi-species insect diet). Upon pupation, they were placed in 3.8 liter wide mouth glass jars with cheese cloth (20 cm X 8 cm) which was hung from the rim for an oviposition substrate. The eggs were tested in the five treatments as described above. Treatments were replicated 24 times in each of 7 trials.
To conduct larval bioassays, the wild radish filtrate (1.5 ml) was used to saturate a filter paper (grade 36, 9 cm diam., folded in half twice) in each 30-ml cup. The filtrate was made by mixing 100 g wild radish with 450 ml water (100% extract) and 100 g wild radish with 900 ml water (50% extract). A single 6-d-old larva of D. balteata and two sprouted wheat seeds or a second instar A. ipsilon and one gram multi-species artificial insect diet were placed in 30-ml plastic cups that had been treated with either 100% extract, 50% extract, or water for the control. The plastic cups were covered with a plastic lid. The 3 treatments were replicated 50 times with 150 experimental units testing 150 BCB and BCW larvae in each of 4 trials. The test insects were placed in an environmental chamber at 25 + 2o C, 80 + 2% RH, and 14:10 L:D regimen. Larvae were checked daily, and mortality data were recorded.
Objective 3 – Field Level Pest Suppression in Sweet Corn and Impact on Key Microbial Processes.
Field experiments were initiated at the Edisto Research and Education Center near Blackville, SC, and Coastal Plain Experiment Station near Tifton, GA, in fall of 2003 and 2004. The test site near Blackville was a Dunbar sandy loam (fine, kaolinitic, thermic Aeric Paleaquults) with a pH of 6.0 and 0.6% organic matter. The Tifton site was a Tifton sand with pH 5.9 and 1% organic matter in 2004 and Tifton sandy loam with a pH of 5.9 and 1.3% organic matter in 2005. The experimental design was a split plot arrangement of cover crops (main plot) and weed control (subplot), with four replications at both locations.
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 ai/ha plus S-metolachlor at 0.44 kg ai/ha (one-half rate) applied immediately after planting, and atrazine at 1.68 kg/ha plus S-metolachlor at 0.87 kg/ha (full rate) applied immediately after planting. ‘Wrenz’ rye and wild radish were seeded at 90 kg/ha and 34 kg/ha on March 1, 2004 and November 30, 2004 at Blackville. The test area at Blackville contained a natural infestation of wild radish; however, plots were overseeded to ensure a uniform stand. ‘Wrenz’ rye and a natural population of wild radish were used as cover crops at Tifton. Rye was planted at 63 kg/ha on December 9, 2003 and November 5, 2004 at Tifton. Plots were flail mowed to a height of 1-cm before planting sweet corn at Blackville. Glyphosate was applied to kill the cover crops prior to planting sweet corn at Tifton in both years. ‘Silver Queen’ sweet corn was strip-till seeded into mowed plots on May 7, 2004 and April 22, 2005 at Blackville. ‘Summer Sweet’ and ‘Prime Plus’ were planted on April 2, 2004 and April 4, 2005, respectively, at Tifton. The plot size was 7.6 m by 1.8 m at both locations with 0.9 m spacing between rows. Fertilizer was applied at 78 kg/ha N-P-K 1 month prior to planting corn at Blackville. All plots were side-dressed with 80 kg/ha N approximately 5 wk after corn emergence. Over-head irrigation was applied to the test site during extended periods of no rainfall. Fertilizer was applied 92 kg/ha N-P-K at planting and plots were side dressed with 92 kg/ha ammonium nitrate at Tifton. Irrigation was applied once weekly at Tifton.
Weed densities by species in a 1 m 2 quadrant in weedy plots were recorded every two weeks at Blackville. Weed control was visually estimated by species on a scale of 0 to 100 where 0 equals no control and 100 equals complete control at 2, 4, 6, and 8 wk after corn planting (WAP) at Blackville. Sweet corn vigor was assessed on a similar rating scale on the same day of weed control evaluations where 0 equals healthy plants and 100 equals plant death. Sweet corn vigor was estimated based on growth and coloration. Weeds were removed from hand-weeded plots at 2, 4, 6, and 8 WAP at Blackville, whereas at Tifton, weeds were removed whenever weed control within a plot dropped below 85%. Stand counts of sweet corn plants per 1 m row and per 3 m row were taken at Blackville and Tifton, respectively. Above ground heights of sweet corn plants in 1 m row were taken at Blackville, whereas above ground height of 8 sweet corn plants were recorded at Tifton. Root length of sweet corn plants was also recorded at Tifton every two weeks. Weed biomass was collected from 1 m2 quadrants in the no-herbicide plots at 2, 4, 6, and 8 WAP at Blackville. The biomass was oven dried at 66 C for 2 wk and dry weights recorded. Sweet corn ears were harvested once at both locations. Fresh corn ears per plot were counted and yield was recorded as total ears/ha and marketable ears/ha, with marketable ears being full size ears without insect damage. Sweet corn yield was only recorded as marketable ears/ha at Tifton.
Soil samples were collected from the control (non-sprayed, hand-weeded) and 1X herbicide subplots in all main plots on May 4, 2004 and May 10, 2005, prior to incorporation of the cover crops, and again on May 20, 2004 and May 24, 2005, 2 weeks after seeding sweet corn. Soils were assayed for Pythium and Fusarium by plating dilutions of soil suspensions on a Pythium-selective medium and on Komada’s medium, respectively. The population level and activity of Rhizoctonia were measured using a beet-seed colonization assay.
Populations of corn earworm and fall armyworm (Spodoptera frugiperda ), along with crop damage in various treatments were determined by visual examination at 4 weeks after planting and at silking. Marketable ears at Blackville were assessed for insect damage.
Corn roots were collected at 4 weeks after planting and at silking and evaluated for mycorrhizal colonization from wild radish, rye, and no cover subplots for the hand-weeded and 1X herbicide rate. In addition, mycorrhizal colonization on weeds in the test site was determined. A mixture of roots and soil (to reduce dehydration of roots) within a 15-cm radius from each plant and 15-cm deep was collected for each species at specified times. Root samples were placed in plastic bags, transported to Clemson University on blue ice, and stored at 4 C until stained. A sample (1.5 g fresh weight) of fine root tissue were removed from each plant and placed in plastic histological cassettes for staining. Roots from each cassette were monitored on gridded microscope slides using a polyvinyl alcohol-based mounting medium. Slides were examined under a light microscope at 40X magnification, and the percent colonization of each sample quantified using a modified line-intercept technique.
Objective 4 – Economic Feasibility.
See section on “Economic Analysis”.
Objective 5 – Education and Outreach.
See section on “Outreach”.
Weed and Crop Bioassay. Tolerance of vegetable crops to the wild radish-soil amendment differed greatly among crops. Emergence of each crop was generally delayed as the wild radish-soil amendment rate increased. The order of crop tolerance based on biomass production was as follows: squash > sweet corn > cantaloupe = cucumber > bell pepper = watermelon > tomato > southern pea > okra > lima bean. Although some of these crop showed a high degree of sensitivity to the wild radish-soil amendment when direct-seeded, crop production where transplants are used can be an effective means of maximizing the weed suppressive potential of the soil amendment without having a negative influence on the crop, particularly bell pepper and tomato (Norsworthy and Meehan 2004).
Wild radish-amended soil demonstrated little phytotoxicity to emergence of the weed species evaluated. Broadleaf signalgrass, large crabgrass, johnsongrass, purple nutsedge, common cocklebur, jimsonweed, and wild radish were not significantly affected by the soil amendment. However, wild radish-amended soil did significantly suppress Palmer amaranth emergence. Palmer amaranth emergence reached an ED50 at 1.02% wild radish-amended soil. The maximum evaluated percentage of dried wild radish residue incorporated into the soil (i.e., 2%) reduced emergence by 71%.
Above-, below ground, and total biomass were not significantly suppressed by any percentage of wild radish-amended soil evaluated. Abundance of available nutrients and reduction of intraspecific competition may have compensated for any growth suppression of Palmer amaranth.
The results of the soil-incorporated wild radish extract varied greatly depending on species. Broadleaf signalgrass and sicklepod emergence was stimulated 56 and 49%, respectively, by the extract. Large crabgrass, johnsongrass, purple nutsedge, yellow nutsedge, wild radish, and common cocklebur were not significantly inhibited by the extract.
Wild radish extract applied to the soil reduced above ground biomass of purple nutsedge by 40%. In contrast, broadleaf signalgrass above ground biomass increased 55% due to application of the extract. Below ground biomass of broadleaf signalgrass and sicklepod increased 79 and 50%, respectively, following the extract.
Total biomass increased by 64% for broadleaf signalgrass following the extract. Although, above ground biomass of purple nutsedge and sicklepod were affected by the extract, total biomass of neither species was significantly altered. The extract had no affect on large crabgrass, Palmer amaranth, johnsongrass, wild radish, Texas panicum, pitted morningglory, yellow nutsedge, or common cocklebur total biomass.
The results of the wild radish-amended soil study demonstrate a phytotoxic affect toward Palmer amaranth emergence. This finding parallels previous research by Norsworthy (2003), in which it was concluded wild radish-amended soil was more effective on dicots. Furthermore, Petersen et al. (2001) found smaller seeded weeds were more susceptible to ITCs, which may have been a product of wild radish tissue degradation. Palmer amaranth was the smallest seeded weed evaluated.
Lack of significant biomass suppression of Palmer amaranth may have been caused by many factors. First, intraspecific competition was reduced because the number of emerged plants was reduced. In comparison, the control had a greater plant density, which reduces shoot and root dry matter per plant (Deschênes 1974). Secondly, ITCs are very volatile (Chew 1988; Fenwick et al. 1983) and could have volatilized prior to radicle protrusion.
The effects of wild radish-amended soil in this study were not as profound as those in previous research by Norsworthy (2003) and Norsworthy and Meehan (2004) where wild radish-amended soil decreased emergence and biomass of many species. Differences among studies may be a result of the varying glucosinolate concentration within wild radish. Glucosinolate content is affected by such factors as soil fertility (Fahey and Stephenson 1999), soil moisture (Bouchereau et al. 1996); wounding (Bodnaryk 1992), and pathogen attack (Butcher et al. 1974). Other factors, such as plant age and organ can affect the quantity and quality of glucosinolates (Fahey et al. 1997). For instance, lower glucosinolate production occurred during maturation in rape (Brassica napus L.) (Clossais-Benard and Laher 1991). Fahey et al. (1997) notes that late vegetative to reproductive stage plants typically contain very low glucosinolate quantities (i.e., 1-4 µmol glucosinolates per g fresh weight). Since glucosinolate content was not measured in our experiments, both the amount and specific type of glucosinolates remain unknown.
Wild radish was collected in the spring of the year for the residue and extract studies. In other studies by Norsworthy (2003) and Norsworthy and Meehan (2004), wild radish was collected in the late fall or early spring months. Wild radish was collected just before flowering had begun in Norworthy’s (2003) study. Furthermore, in previous studies wild radish was collected from Blackville, SC, whereas our wild radish was collected from Clemson, SC. The elevated phytotoxicity noted in these studies may have been due to higher glucosinolate content based on environmental conditions (Bodnaryk 1992; Bouchereau et al. 1996; Butcher et al. 1974; Fahey and Stephenson 1999), growth stage (Fahey et al. 1997), and/or genetic variation (Kliebenstein et al. 2001).
The products of glucosinolate hydrolysis may also be influenced by environmental conditions, which may inevitably affect the allelochemical toxicity of wild radish. ITCs have very short half-lives in the soil (Borek et al. 1995; Petersen et al. 2001). Organic carbon has been shown to negatively impact the half-life of allyl-ITC (Borek et al. 1995). The soil used in our experiments contained 1.8% organic carbon and it is possible that organic matter within the soil may have bound ITCs, causing them to become inactivate. Total nitrogen within the soil also negatively affects the half-life of the ITCs; whereas nitriles, another product of glucosinolates, are not affected (Borek et al. 1995). Soil used in our experiments was well fertilized. Borek et al. (1995) also noted that soil temperatures from 10 to 25 C negatively influence ITCs, but positively impact half-life of nitriles. In our experiments, the greenhouse remained at 24 to 30 C, which may have contributed to volatilization and loss of ITCs. Therefore, our experimental methods may have negatively impacted ITCs, reducing the allelopathic affects of wild radish.
Soilborne Pathogen Bioassay: Percentage germination on agar and bean hypocotyls was reduced with 1% or greater wild radish residue. Percentage of sclerotia that produced lesions also was reduced, but percentage of viable sclerotia that produced lesions was not affected by wild radish residues.
Insect Bioassay: Hatching of D. balteata eggs was first observed 7 days after treatment (DAT) in the control and 18 h water treatments. Hatching was delayed by the wild radish extract treatment in which eggs were exposed for 18 h in each of the four laboratory trials. Percent hatch of eggs in the 18 h water treatment was similar to the untreated control throughout the study. Percentage hatch of eggs in both the 0.5 h water and 0.5 h wild radish treatments were also similar. Mean percentage hatch of D. balteata eggs exposed to various treatments across all trials ranged from 0 to 63 for 7 DAT, 0 to 81 for 8 DAT, 5 to 83 for 9 DAT, and 62 to 78 for 10 DAT. Percentage hatch was significantly less in the 18 h wild radish treatment than in all other treatments for the duration of the experiment in two of the four trials (t = 12.32 and 12.67, Pr > t = 0.050). When D. balteata eggs are deposited in the field in cracks in the soil, they require 5 to 9 days to hatch (Marsh 1912). This range is affected by environmental factors. There may be opportunities to adjust these factors with the use of cover crops. The effect of wild radish on the hatching of eggs of D. balteata may be important in the pest management decision-making process. To better define the importance of the wild radish factor, a field test should be designed to provide exposure of D. balteata to wild radish extract for 18 h.
Response of black cutworm eggs to exposure to wild radish extract was different from that of the banded cucumber beetle. Hatching generally occurred 4 DAT. Hatching of the black cutworm was significantly delayed in only one of the six trials 4 DAT. However, by the next day (5 DAT) in that trial, percent hatching for all treatments was similar (t = 10.07, Pr > t = 0.050). Larvae of the black cutworm apparently exhibited tolerance to the wild radish and even appeared to flourish. All larvae that hatched from eggs exposed to wild radish were alive after 4, 6, and 10 d in the environmental chamber. They pupated in another 3 d, and adult moths emerged 12 d after pupation. Busching and Turpin (1977) reported that the larval stage of A. ipsilon ranged from 24.6 d on wheat to 47 d on morningglory (Ipomoea spp.) at 26.7 °C (day):15.6 °C (night) in a 15:9 L:D regimen. Hence, the wild radish had no apparent negative effect on development of the black cutworm under the described laboratory conditions. Even though hatching in the wild radish 18 h treatment was significantly less than the control, it was similar to the 18 h water treatment in the first four trials (Table 2). Therefore, there is little evidence that glucosinolate products from wild radish can disrupt the development of black cutworm. Hillyer and Thorsteinson (1969) reported that glucosinolates may stimulate oviposition in the diamondback moth [Plutella xylostella (L.)], a specialist on brassicas. We report a negative effect on hatching of the black cutworm by the extract and by the water treatments when eggs were exposed for 18 h. It is important to recognize that extended exposure to moisture is not a part of the normal rearing process of A. ipsilon. The extended exposure to aqueous treatments was detrimental to the hatching of A. ipsilon eggs. The 18 h water treatment was less than all other treatments in only one trial.
There was no mortality in the A. ipsilon larvae after they were exposed to wild radish extracts. The mortality rate of D. balteata larvae was highest (about three-fold) in 100% extract, as compared with the 50% extract and the control samples. However, these data were not consistent from trial to trial. In the other three trials, there were no differences in mortality rate with the various concentrations of aqueous extract.
Objective 3 – Field Level Pest Suppression in Sweet Corn and Impact on Key Microbial Processes.
Florida pusley, large crabgrass, spreading dayflower, Texas panicum, smallflower morningglory, and ivyleaf morningglory were the predominant weeds infesting test sites in SC or GA. Wild radish or rye in conjunction with the one-half recommended rate of atrazine plus S-metolachlor provided 64 to 97% weed control across locations and years at 4 wk after planting (WAP), whereas weed control ranged from 78 to 100% following the full recommended rate of atrazine plus S-metolachlor. In the absence of a cover crop, weed control with atrazine plus S-metolachlor ranged from 54 to 98% across locations and years. The one-half rate of atrazine plus S-metolachlor generally provided effective weed control through 4 WAP, regardless of cover crop in SC. In GA, both rates of herbicides provided season-long weed control. Wild radish in conjunction with the full rate of atrazine plus S-metolachlor provided superior control of Florida pusley, large crabgrass, and ivyleaf moringglory compared with rye or no cover crop treated with a full herbicide rate in 2004 in SC, whereas in 2005, weed control from the wild radish cover crop was not different from rye. Wild radish or rye cover crops in conjunction with a one-half or full rate of atrazine plus S-metolachlor provided excellent weed control in GA in both years. Sweet corn following wild radish or rye produced 21,000 to 48,000 and 28,000 to 46,000 marketable ears/ha in herbicide treated and hand-weeded plots, respectively, in SC across years. In 2004 in GA, sweet corn following wild radish or rye produced 34,000 to 48,000 and 41,000 to 51,000 marketable ears/ha in herbicide treated and hand-weeded plots, respectively. Only total ears were recorded in GA in 2005. Sweet corn following wild radish or rye in the absence of herbicides produced less marketable ears than herbicide treated plots, indicating that a combination of cover crops and herbicides are required to optimize yields and obtain desirable weed control.
In both 2004 and 2005, the only difference in soil-borne pathogens observed between cover crops at either sampling was for Rhizoctonia at the first sampling. In 2004, colonization of beet seed was significantly lower for rye (2.7%) than for fallow (9.2%) or wild radish (10.3%). In 2005, prior to incorporating cover crops, colonization of beet seed by Rhizoctonia was significantly lower (P<0.02) with fallow (61%) than with wild radish (79%). At 2 weeks after seeding sweet corn in 2004, 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 in either year. There also were no significant cover crop-by-herbicide interactions. In 2004, populations of Pythium decreased significantly after cultivation. There were 1600 colony-forming units (CFU) per gram oven-dry soil initially and 390 CFU/g at the second sampling. In contrast to results from 2004, populations of Pythium spp. increased between the first and second samplings (P≤0.01) in 2005. Pythium populations increased more in rye (P=0.005) and wild radish (P<0.05) plots than in fallow plots. Results with Fusarium were similar to results with Pythium. Population levels of Fusarium did not change significantly between samplings in 2004: 5.6 and 4.1 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. In 2005, populations of Fusarium spp. and F. oxysporum also increased significantly (P<0.03) after incorporating cover crops in wild radish and rye plots and in fallow plots compared with the initial sampling. The population of corn earworm (Helicoverpa zea Boddie) varied at harvest. When population densities ranged from 0.3 to 2.2 corn earworms per corn ear in the plots in 2004, there were more earworms per corn ear in the wild radish cover treated with herbicides than in the weedy checks in rye and no cover crop as well as in no cover crop treated with 0.5X rate of atrazine + S-metolachlor. Nevertheless, in 2005, when population density was much lower (<0.5 corn ear worm per corn ear in all treatments), there were no differences among treatments. Data were inconsistent with corn earworm populations from year to year in the various cover crops. Yield was higher in wild radish treatments in 2004, and there were no consistencies in determining differences among cover crop treatments in 2005 when marketable ears were examined. The history of crops and weeds may be important over a long term and larger test plots may be needed to determine the effect of wild radish on populations of soilborne insects. Mycorrizhal colonization of corn roots in 2004 ranged from 10 to 16% across cover crop treatments, with no differences among cover crops. Similarly in 2005, rye and wild radish had no negative affect on mycorrhizal colonization of corn roots, with colonization ranging from 16 to 21% across cover crop treatments. Hence, it is concluded that mycorrizhal colonization of sweet corn is not influenced by a wild radish or rye cover crop. None of the weeds were highly colonized by mycorrhizae in either year, and there were no differences in colonization among cover crop treatments for any weed species in either year. In 2004, colonization averaged 6% for Florida pusley, 9% for large crabgrass, 8% for ivyleaf morningglory, and 8% for spreading dayflower. Colonization in 2005 averaged 9% for Florida pusley, 7% for ivyleaf morningglory, 10% for large crabgrass, and 8% for wild radish.
Educational & Outreach Activities
A workshop was held on “Brassicaceae Cover Crops: An Effective and Efficient Way to Minimize Pesticide Use in Fresh Market Vegetables” in August 2006 on Clayton Rawl Farm in Lexington, SC. Twenty-three producers were in attendance and the training season focused on research findings and our on-farm demonstration. Three presentations covered various aspects of pest management and vegetable crop production using Brassicaceae cover crops. An evaluation form completed by each attendee indicated that all in attendance gained knowledge from the workshop.
Additionally, findings from this research were included in weed management lectures focusing on cover crops and vegetable production as part of Introductory Weed Science (CSENV 407/607) and Integrated Pest Management (IPM 401), which were taught to students at Clemson University. Additionally, the scientific community was made aware of the research findings through presentations at state and regional meetings.
McCutcheon, G. S., A. M. Simmons, and J. K. Norsworthy. 2007. Effect of wild radish on preimaginal development of Diabrotica balteata and Agrotis ipsilon. J. Sustainable Agriculture in review.
Malik, M. S., J. K. Norsworthy, M. B. Riley, J. Pha, and S. K. Bangarwa. 2006. Wild radish as a cover crop aids weed management in sweet corn. Proc. South. Weed Sci. Soc. 59:150.
Meehan, J. T., IV. 2005. Weed suppression using wild radish (Raphanus raphanistrum) and synthetic isothiocyanates. M.S. Thesis, Clemson University, 99 pp.
Malik, M. S. and J. K. Norsworthy. 2005. Interaction of sweet corn with a wild radish (Raphanus raphanistrum) cover crop. Proc. South. Weed Sci. Soc. 58:220.
Miller, S. and J. K. Norsworthy. 2006. Economic Impact of Biofumigant Cover Crops in Sweet Corn Production. Presented at the workshop on “Brassicaceae Cover Crops: An Effective and Efficient Way to Minimize Pesticide Use in Fresh Market Vegetables” held at Clayton Rawl Farms.
Smith, J. P. and J. K. Norsworthy. 2006. Biofumigation Demonstration: Sweet Corn. Presented at the workshop on “Brassicaceae Cover Crops: An Effective and Efficient Way to Minimize Pesticide Use in Fresh Market Vegetables” held at Clayton Rawl Farms.
Malik, M.S., J. K. Norsworthy, A. S. Culpepper, M. B. Riley, and P. Jha. 2006. Effect of wild radish cover crop on weed suppression in sweet corn. Presented at the workshop on “Brassicaceae Cover Crops: An Effective and Efficient Way to Minimize Pesticide Use in Fresh Market Vegetables” held at Clayton Rawl Farms.
Malik, M. S., J. K. Norsworthy, M. B. Riley, J. Pha, and S. K. Bangarwa. 2006. Wild radish as a cover crop aids weed management in sweet corn. Presented at the 59th Annual Meeting of the Southern Weed Science Society.
Malik, M. S. and J. K. Norsworthy. 2005. Interaction of sweet corn with a wild radish (Raphanus raphanistrum) cover crop. Presented at the 58th Annual Meeting of the Southern Weed Science Society.
Malik, M. S. and J. K. Norsworthy. 2005. “Natural” weed suppression in sweet corn. Presented at the 2005 Clemson University Research Forum.
McCutcheon, G. S., A. M. Simmons, and J. K. Norsworthy. 2005. Effect of wild radish on banded cucumber bettle, Diabrotica balteata Le Conte (Coleoptera: Chrysomelidae) and black cutworm, Agrotis ipsilon (Hufnagel) (Lepidoptera: Noctuidae). Presented at the 51st Annual Meeting of the SC Entomological Society.
Bone, A. J., G. S. McCutcheon, J. K. Norsworthy, and A. M. Simmons. 2004. Effect of wild radish extract on banded cucumber beetle (Diabrotica balteata) larvae. Presented at the Govenor’s School for Science and Mathematics, July 16, 2004, Clemson, SC.
This research established the tolerance of vegetable crops to a wild radish-soil amendment which could be applied to soil for suppression of Palmer amaranth as shown in this research. Wild radish and rye cover crops where found to be effective in suppressing early season weeds, allowing herbicide rates to be reduced in sweet corn without sacrificing yield (marketable yield). As a result of this research, the use of cover crops (natural and seeded) appears to be increasing in South Carolina. There has also been an increase in the awareness of the benefits of cover crops for pest suppression, particularly the use of wild radish and other Brassicaceae cover crops.
The economic feasibility of seeding two Brassicaceae cover crops prior to producing sweet corn was evaluated on Clayton Rawl Farms in Lexington, SC, in 2006. After establishing the Brassicaceae cover crops, the producer decided to use a rye cover crop as his standard practice for comparison. Hence, there was no comparison in the absence of a cover crop. Data on revenues, production practices (e.g., timing of field operations machine and labor requirements, etc.), and production costs (pesticides, fertilizer, seed, fuel, etc.) were collected for each management strategy. These data were used to construct partial budgets for the adoption of Brassicaceae cover crop versus his conventional practice of using a rye cover crop.
Prices and machinery cost data were taken from Clemson University’s BUDSYS master file (http://cherokee.agecon.clemson.edu/Budgets/BudSys_Downnload.htm). The additional costs of seeding a Brassicaceae cover crop was $24.87/acre for turnip and $34.32/acre for ‘Caliente’ (a mustard blend). If wild radish were present, there would be no cover crop costs. When rye was included as a separate cover crop, cost totaled $44.98/acre. The cost for these cover crops includes cover crop seed, cover crop management, and herbicide. Yields were similar in the demonstration plots; therefore, based on the field trial and assumed prices and cost, adoption of a Brassicaceae cover crop would increase the grower’s net income by $10.66 to $20.11/acre. In fields were wild radish is present, the increase would be even greater.
Many farmers became aware of the benefits of cover crops through the outreach efforts associated with this project and other efforts to increase the use of cover crops in South Carolina. Producers routinely called inquiring about ways to manage their seeded cover crops as well as the benefits associated with weedy cover remaining in their fields prior to vegetable crop establishment. Two of the largest sweet corn producers in South Carolina have adopted rye as a cover crop in their production system. This adoption has allowed them to minimize their herbicide inputs and reduce sand damage to their seedling crops. The reason rye was chosen by these producers over that of a Brassicacae cover crop was because the land on which their sweet corn is grown is rotated to Brassica crops following corn harvest.
Areas needing additional study
A greater understanding of the environmental, phenological, and genetic factors that affect the quantity and quality of glucosinolates within wild radish are necessary to improve its effectiveness as a pest suppressant. Future research should focus on identifying the growth stage and environmental conditions that optimize glucosinolate production and use of commercially available glucosinolate-producing cover crops as an additional tool for integrated weed management.