Recent research of the Agricultural Research Service of USDA on the use of natural products to manage pests is summarized. Studies of the use of both phytochemicals and diatomaceous earth to manage ...insect pests are discussed. Chemically characterized compounds, such as a saponin from pepper (Capsicum frutescens L), benzaldehyde, chitosan and 2‐deoxy‐D‐glucose are being studied as natural fungicides. Resin glycosides for pathogen resistance in sweet potato and residues of semi‐tropical leguminous plants for nematode control are also under investigation. Bioassay‐guided isolation of compounds with potential use as herbicides or herbicide leads is underway at several locations. New natural phytotoxin molecular target sites (asparagine synthetase and fructose‐1,6‐bisphosphate aldolase) have been discovered. Weed control in sweet potato and rice by allelopathy is under investigation. Molecular approaches to enhance allelopathy in sorghum are also being undertaken. The genes for polyketide synthases involved in production of pesticidal polyketide compounds in fungi are found to provide clues for pesticide discovery. Gene expression profiles in response to fungicides and herbicides are being generated as tools to understand more fully the mode of action and to rapidly determine the molecular target site of new, natural fungicides and herbicides. Published in 2003 for SCI by John Wiley & Sons, Ltd.
Shoot and root characteristics of four peach tree
Prunus persica
(L.) Batsch (Peach Group) growth habits (compact, dwarf, pillar, and standard) were studied. In compact trees, leaf number ...(1350/tree) was twice, but leaf area (6 cm
2
/leaf) was half that of pillar and standard trees. The number of lateral branches in compact trees (34) was nearly three times more than in pillar and standard trees. Leaf area index (total one-side leaf area per tree divided by the canopy cross-sectional area of the tree) of pillar trees was greater than compact, dwarf, and standard trees (13 compared with 4, 4, and 3, respectively) due to a narrower crown diameter. Dwarf trees were distinct with few leaves (134/tree) and less than half the roots of the other growth habits. Compact trees produced more higher order lateral (HOL) roots than pillar and standard trees. More second order lateral (SOL) roots were produced by compact than standard trees (1.2 vs. 0.8 SOL roots per centimeter first order lateral root). Pillar trees had higher shoot: root dry weight (DW) ratios (2.4) than compact and standard trees (1.7 for both) due to lower root DWs. Root topology was similar among compact, pillar, and standard peach trees but root axes between branch junctions (links) were significantly longer in compact trees. Compact trees had more and longer HOL roots in roots originating near the root collar (stem-root junction) (i.e., more fibrous roots) and this appeared to correlate with more lateral branches in the canopy. These results indicate significant differences in root as well as shoot architecture among growth habits that can affect their use as scion or rootstock cultivars.
The effect of reduced water availability on glyphosate efficacy and adventitious shoot growth in male and female clones of Canada thistle was studied. Water availability was regulated with ...polyethylene glycol (PEG) in hydroponically grown plants or by withholding water from soil-grown plants. In hydroponic media, there was 50% more adventitious shoot growth from male plants than from female plants. Both PEG and foliarly applied glyphosate reduced the number of adventitious shoots, regardless of gender. In soil, the same number of adventitious shoots formed on untreated male and female plants. However, when glyphosate was applied to plants growing under dry conditions, there was 75% more adventitious shoot growth from males than females. Distribution of glyphosate was similar in male and female plants. Approximately 23% of applied 14C-glyphosate was transported throughout Canada thistle plants within 10 d of treatment regardless of gender or moisture condition. Gender differences in capacity for vegetative reproduction may alter the sex ratio of Canada thistle populations in the field following glyphosate application.
A polypropylene fabric containing control-release pellets of the herbicide, trifluralin, can be oriented in the soil to regulate the distribution of plant roots. In 1990, trenches were dug near ...10-year-old red oak (Quercus rubra L.) and 10-year-old yellow poplar (Liriodendron tulipifera L.) and fabric containing trifluralin control-release pellets and polypropylene fabric alone were installed vertically to redirect root growth. Roots grew alongside trifluralin fabric and fabric alone and did not penetrate either fabric 38 months after installation. Shoot growth of yellow poplar was reduced about 47% each year by the trifluralin fabric treatment compared to control. Red oak shoot growth was not affected by trifluralin fabric. Leaf water potential was not affected by treatment in either species. Trifluralin residues in trifluralin fabric decreased from 23.3% to 22.0% from July 1990 to October 1993. During this time, trifluralin levels increased from 0.4 to 3.6 mg.kg-1 in soil sampled 0 to 15 cm below trifluralin fabric. These results suggest that controlled-release trifluralin will provide persistent inhibition of root and shoot growth of some species and will not migrate significantly in the soil
Three experiments were performed to determine the effect of amending the soil surface layer and mulching with hydrophobic kaolin particle on weeds and blackberry (Rubus subgenus Rubus Watson) plants. ...In the first study a processed kaolin material (product M-96-018, Engelhard Corporation, Iselin, N.J.), was incorporated in August into the top 3 cm of freshly roto-tilled field that had been in pasture the previous 5 years. The following spring, dry weight of weed vegetation in the control treatment was 219 g.m(-2) and was significantly higher (P = 0.05) than the 24 g.m(-2) harvested from the treated soil. In two other studies, planting holes for blackberry transplants were either 1) pre- or postplant mulched with a 2- or 4-cm layer of 5% or 10% hydrophobic kaolin in field soil (w/w), or 2) postplant treated with a) napropamide, b) corn gluten meal, c) a product comprised of hydrous kaolin, cotton seed oil, and calcium chloride in water (KOL), d) hand weeded, or e) left untreated. Although untreated plots had 100% weed cover by the end of July, herbicide treatments, 4-cm deposition of hydrophobic kaolin particle/soil mulch, and KOL all suppressed weeds the entire establishment year. Preplant application of hydrophobic kaolin mulch and postplant application of KOL reduced blackberry growth and killed transplants, respectively. In year 2, blackberry plants produced more primocanes that were on average 10-cm taller in weed-free plots (herbicide, 4-cm kaolin soil mulch, and mechanical weeding) than in weedy plots (control and 2-cm kaolin soil mulch). In year 3, yield was significantly lower in control plots (1.5 kg/plant) than in plots that were treated with napropamide and 2- and 4-cm hydrophobic kaolin mulch, or hand weeded during the establishment year (4 kg/plant). The results showed that 4-cm hydrophobic kaolin mulch applied after planting can suppress weeds without affecting blackberry productivity. These kaolin products are excellent additions to the arsenal of tools for managing weeds in horticultural crops.
In growth chamber research, infection by Puccinia punctiformis, a rust fungus, reduced flowering and vegetative reproduction in Canada thistle. Disease symptoms were greatest when Canada thistle ...roots were stored at 5, 10, or 15 C following inoculation. Exposure of roots to different temperatures before inoculation did not affect disease. All Canada thistle clones tested, regardless of gender became diseased although there were differences in susceptibility to P. punctiformis.
Response of clones of Canada thistle from North America and leafy spurge from North America and Eurasia to treatment with chlorsulfuron, clopyralid, and glyphosate was determined. Roots of Canada ...thistle plants from different locations responded differentially to injury from chlorsulfuron and clopyralid but not to glyphosate. Clones of Canada thistle varied significantly in response to all herbicide treatments. Roots of leafy spurge plants from different locations were differentially susceptible to injury from glyphosate. Clones of leafy spurge varied significantly for all responses to each herbicide. The differential effects of chlorsulfuron, glyphosate, and clopyralid on clones of Canada thistle and leafy spurge suggest that genetically-based differences may account for the failure of the herbicides to kill all plants within collection locations or between collection locations. Cross tolerance to the herbicides was not found in leafy spurge or Canada thistle.
Survival, growth and dieback, endogenous auxin and total non-structural carbohydrate (TNC) relations of underplanted northern red oak (Quercus rubra L.) seedlings were investigated. Greenhouse-grown ...red oak seedlings were subjected to 0, 63, or 92% shade and soil water potential was permitted to reach -0.3 or -2.0 MPa before rewatering. In a field study, underplanted northern red oaks were exposed to three levels of canopy removal (0, 70, or 90% residual canopy). White oak (Q. alba L.) and eastern white pine (Pinus strobus L.) were included in the field experiment for comparative purposes.
In the greenhouse, dieback was observed under full sunlight and 92% shade but not in intermediate shading. In the field, dieback was greatest following complete canopy removal; but dieback was usually followed by rapid sprout growth and, as a result, average seasonal height increases generally were unaffected by light availability. Seasonal height increases were small (1-4 cm) and survival usually exceeded 90% in both the field and greenhouse. Field-grown white oaks displayed similar growth and dieback as red oaks. White pines grew 3 cm and 10 cm under 0 and 90% residual canopy, respectively.
Final TNC leves from red oaks grown in drier soils and full sunlight were greater than seedlings grown under full shade and higher soil moistures (27 and 17% of total dry weight, respectively).
Under full sunlight, as occur with complete canopy removal, dieback was probably related to internal moisture/temperature stress. Dieback under 92% shade was likely induced by carbohydrate depletion and weak, succulent growth resulting from the chronically low light levels. Thus, oak dieback appears to be a survival mechanism in which an advantageous root/shoot ratio is maintained and nutrients can be conserved.
High or low TNC was not strongly related to dieback or sprout growth in this study. However, decreased TNC concentrations associated with increased shade indicated that long-term low light intensities may adversely affect growth and survival of red oak.
Quantification of endogenous auxin in oak stems with gas chromatography was impossible due to low IAA levels (less than 0.8 uM) and small amounts of available tissue. Large amounts of IAA were found to be lost to sublimation when IAA was subjected to vacuum.
Ph. D.
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