PBL 4: Weeds and Herbicides: Plant diseases/pesticides Flashcards
Weeds
Compete with crops and can transmit diseases and pests. Can result in more than a 30% loss of yield on good land:
1. Annual weeds
2. Biennial weeds
3. Perennial Weeds
Annual weeds
plants that complete their life cycle in one growing season.
-Thrive on frequently disturbed sites (i.e. corn+soybean production).
-Often have high levels of seed production
-Tillage+herbicide are effective at killing annual plants but often develop from soil seed bank.
Summer annual weeds: germinate+grow spring/summer
Winter annuals: germinate fall/vegetative in winter, germinate in spring.
Biennial weeds
Plants that require two growing seasons to complete their life cycles.
-Are vegetative in the first year, flower in the second year.
-Intolerant of tillage in the first/second year
-Mowing in the second year before they flower can eliminate seed production.
-Persist in the permanent pastures, waterways, and roadside areas.
Perennial weeds
Plants that live for 3+ years. They have some specialised reproductive structures that allow for:
-overwintering
-long-term persistence (crowns rhizomes, stolons)
-Once established, difficult to control except by herbicides.
-Best adapted to survive under a diversity of cropping systems.
Grasses and Broadleaves
Monocot weeds: grasses (seed remains underground, only leaves emerge)
Dicots: Broadleaves, may have compound leaves. Most seeds energy from below ground during germination and are easier to kill compared to grass.
Impacts of Weeds
-Reduce agricultural productivity and profitability
-Reduces yield: competing with crops
-increase production costs during their control
-Some are allelopatheic
-Some are parasitic
-May contain toxins that are harmful to livestock
Allelopathy
the adverse effect of one plant on another due tot production of a chemical inhibitor.
-Living or decaying plant tissue may excrete the chemicals
Grain contamination
Mixing weed seed with grain can result in economic loss if separation is not possible at harvest or later during processing.
-may decrease its value for livestock feeding or human use: wild oat contamination of wheat, barley, wild garlic, etc.
Unique weed traits: Seed production
Many weeds produce small seeds and very high numbers of seed.
-provides an opportunity for weed populations to rapidly increase.
-ensures the survival of species
Unique weed traits: Seed dormancy
Weed seeds can have a long dormancy period and a staggered rate of seed germination.
-enables weed seed to persist and remain viable in soil for long periods.
Seed bank: total dormant+nondormant seed accumulated in the soil, provides a reserve of seed.
Unique weed traits: Seed dispersal
-Dispersed by numerous means: wind, water, animals, humans, appendages
-e.g.: purple loosestrife, aquatic weed that attaches to feet of ducks/geese
Unique weed traits: negative (asexual) reproductive:
-enables spreading the weed from the original plant without using seed.
-Rhizomes
-creeping roots
-stolons
Biotypes
Populations of weeds that develop special traits like herbicide resistance.
Herbicides
-primary method of weed control in conventional agriculture.
-used for killing or injuring plants: modern herbicides are synthetic chemical compounds with specific mechanisms for killing plants.
Herbicide Advantages
-Production costs per acre reduced vs mechanical weed control/hand weeding.
-Herbicides provide greater flexibility in timing of weed control than mechanical approaches.
-Selectively can remove weeds from turf+forages without damaging the crop.
-Reduce/eliminate the need for mechanical tillage
-Can persist in soil to provide long-term weed control
Herbicide Disadvantages
-Can potentially kill/injure non target plants
-Contamination of water supplies
-Can lead to development of resistant weed species
-Human health.
Glyphosate
A nonselective herbicide, most widely used herbicide.
-Chemical fallowing
-Resistance built from GMO (Roundup Ready crops).
-Low-cost herbicide.
-when used properly, has minimal environmental impact.
LD50
Lethal dose: amount ingested where 50% of test mice die. Acute toxicity.
Acute toxicity: instant result
Chronic toxicity (more dangerous for humans: accumulation of long-term result
Routes of entry:
Exposure to: mouth, nose, eyes, skin
Modes of Action
Biochemical mechanism that herbicides use to kill plants. Can be selective or or nonselective related to the weeds they affect:
- Growth regulators
- Amino acid synthesis inhibitors
- Lipid synthesis inhibitors
- Nitrogen metabolism inhibitors
- Seedling growth inhibitors
- Photosynthesis inhibitors
- Cell membrane disruptors
- Pigment inhibitors
Modes of Action: Growth regulators
Disrupt plant growth by interfering with hormones+protein synthesis. Mostly taken up by the foliage but can be absorbed by the roots. Translocated in the plant’s vascular system
Modes of Action: Amino Acid Synthesis inhibitors (ANNUALS)
disable enzymes reponsible for specific amino acid synthesis: therefore proteins important for plant growth are not formed.
-instead, taken up by the foliage and translocated in plants.
Modes of Action: Lipid synthesis Inhibitors
prevent the formation of fatty acids (key roles in cell membranes and also energy storage)
Modes of Action: Nitrogen metabolism inhibitors
impede enzyme that helps produce glutamine, which is required for nitrogen metabolism.
Modes of Action: Seedling growth inhibitors
Affect growth of roots+shoots of germinating seedlings. Active at multiple sites in a plant and affect mitosis+photosynthesis, etc. Must be applied before germination
Modes of Action: Photosynthesis inhibitors
prevent plants from conducting photosynthesis and producing energy by binding to the chloroplasts
Cell membrane disruptors
destroy plant foliage by directly rupturing cell membranes: quick-acting contact herbicides that require sunlight for activation
Pigment inhibitors
prevent plants from forming the chlorophyll that is essential for photosynthesis. Plants turn white and often become translucent
Application of herbicides: Preplant herbicides
-Applied to the soil before planting; target germinating weed seeds
Application of herbicides: Preemergence herbicides
Applied to the soil after the crop has been planted but before the crop/weeds have emerged. Need to be leached into the soil by rainfall/irrigation
Application of herbicides: Postemergence herbicides
Applied after the crop/weeds have emerged. Most often depend on foliar contact for activity. Most widely-used types (glyphosate)
Application of herbicides: Postharvest herbicides
Applied after harvest but before tillage, foliar contact is necessary.
Types of herbicides
Contact herbicides: Annual weeds
Systemic herbicides: translocation, perennial weeds.
Herbicide resistance
tolerance of herbicides by weed species that were once susceptible. Occurs because of repeated exposure to a specific herbicide.
Avoiding:
-Rotate herbicides over years and to use herbicides with different modes of action
-Use alternative control strategies (crop rotation/tillage)
Biological weed control
employs biological agents such as diseases, insects, animals to reduce/eliminate populations of weed species.
-Consume foliage
-Destroy roots+crowns
-Prevent flowering+reproduction.
-Natural, nonpolluting, target
Mechanical weed control
use of tillage/mowing implements drawn by horses or tractors for weed control
-Tillage: uproots and dismembers mature plants, burying/stimulating germinating/dormant seeds
-Mowing: removes aboveground shoot, leaves, and flowers of plants.
-Covercrops
-Intercropping: allelopathy
Plant disease
a progressive deviation from a plant’s normal development, appearance, function.
3 factors must be present:
1. a susceptible host
2. pathogen (disease-causing agent)
3. favourable environment for infection.
Biotic plant diseases
caused by living organisms (fungi, bacteria, mycoplasma-like, viruses, nematodes, parasitic flowering plants)
Abiotic plant diseases
caused by nonliving factors (air pollution, nutrient imbalances, water excesses/deprivation, extreme temperatures, soil acidity/alkalinity, herbicides)
Pests
Agents of disease/damage
IPM Program: 5 Basic methods that can be applied for pest management
- Cultural
- Biological
- Mechanical
- Genetic
- Chemical
Cultural pest management
attempts to manipulate the environment in which plants are grown, includes:
-Crop rotation: prevents buildup of soil-borne pathogens. Must be diverse enough.
-Irrigation+fertilisation: well hydrated crops with no nutrient deficiencies are more resistance to pathogens
-Sanitation: Equipment, tools, soil residues.
Biological pest management
uses one living organism to control another by predation, parasitism, or competition to prevent overpopulation of pests.
-Beneficial insects: prey on plants without harming the crop plant
-Bacteria: Bt gene can be injected or applied topically to kill various insects.
-Green manures: crops grown to be plowed under to provide an environment that will increase soil populations of organisms that are antagonistic to plant pathogens
-Plant chemicals: (i.e. nicotine from tobacco, pyrethrum from chrysanthemum), or phytoalexins.
Phytoallexins
Other plants produce toxins called phytoalexins when they are damaged/injured, which are poisonous to other pests.
Mechanical pest management
attempts to manipulate the environment around the crop by using mechanical equipment or some other physical means to make it less favourable
-Handpicking: labour-intensive
-Tillage
Genetic pest management
Uses plant breeding and genetic engineering to develop plants that are more resistant to important plant pests.
-A process that can take years to complete (Bt Corn)
Chemical Pest Management
-disinfectants (sterilisation), fungicides, nematicides (nematode populations), antibiotics, insecticides (modes of action).
-smart: identify pest life cycle to apply when most vulnerable
-may cause environmental, animal and health risks
Insects
-six legs, exoskeleton, 3 main sections, outer-shell coverings (wings+antennae)
No metamorphosis: insects emerge from eggs as miniature adults
Incomplete metamorphosis: emerge from eggs and gradually develop into adult
Complete metamorphosis: separate life cycle stages: egg, larva, pupa, adult (larva+adult=danger)
Chewing insects
Have mandibles for chewing plant tissues while feeding
Piercing/sucking insects
Have a piercing mouthpart called a labrum that it uses to puncture plant tissue and suck the plant fluids.
-Can cause additional damage because they can inject toxins/transmit bacteria (aphids)
Aphids
Extremely common pests in both field and greenhouse situations
-reproduce many times in one year and can overwinter as eggs/adults.
-ladybird beetles are natural predators. ants eat aphid excretion.
Stages of IPM
- Prevention and Suppression: combination of tactics into preventive strategies
- Monitoring: Observation, Forecast, Diagnostic
- Decision-making: Use of threshold+multiple criteria
4-7. Intervention: Non-chemical methods, least side effects, reduced pesticide use, anti-resistance strategies
- Evaluation: Assessment of entire process, adoption of new standards.
