BRE 211: Principles of Agriculture and Forestry

1
BRE 211 Principles of Agriculture and Forestry

• Lecture 5

2
Plant Diseases

• Plant said to be diseased when
• Its chemistry or structure has been subjected to
  an abnormal, sustained alteration.
• An injury is caused when a leaf pulled off a tree
• A fungus, bacterium or virus enters a plant and
  deprives the plant of nourishment or alters
  normal functions of the plant.
• There a dysfunction of a plant due to lack of a
  nutrient resulting in undesirable symptoms.
• For a disease to occur it is necessary to have a
  susceptible plant, an agent causing the disease,
  and a suitable environment are all necessary for
  disease to occur.

3
Causes of Plant Diseases

• Plant diseases are caused by
• Non-living agents including low temperatures,
  high temperatures, atmospheric impurities,
  mineral deficiencies and mineral excesses
• Living agents such as fungi, bacteria, a few
  higher plants, nematodes, algae, viruses,
  mycoplasmas, and viroids.
• Fungi and bacteria cause the majority of plant
  diseases.
• Viruses are minor causal agents but their damage
  is severe and they are more difficult to control.

4
Control of Plant Diseases

• The objective is to reduce the disease effect
  below the economic threshold.
• Most important point in controlling a plant
  disease is choosing the best control method or
  series of control methods for
• Use of more than one control measure (systems
  control) is often needed.
• Principle of control is to find the part of the
  life cycle of a living pathogen that will succumb
  to available control methods and utilize the
  control methods early in relation to pathogen
  propagation.
• Most plant disease control methods are
  preventative.
• Some eradicative techniques are available but
  should not be relied upon for consistent control.

5
Conventional Methods of Plant Disease Control

• Include
• Physical
• Growing of crops in green houses where diseases
  are kept out of touch with the growing crops
  especially for high value crops.
• Legislative
• Use of laws and regulation to prevent the
  importation of diseased planting materials into a
  country and to restrict the spread of diseases in
  other areas
• Objective of legislation is to prevent dangerous
  diseases from colonizing new areas.
• Legislative control involves
• Quarantine
• Eradication regulations
• Certification

6
Conventional Methods of Plant Disease Control

• Cultural control
• Manipulation of agronomic practices employed in
  crop production to influence the incidence of
  certain diseases.
• Basic principle is the disruption of the
  development and life cycles disease causing
  organisms either by denying them their food or by
  exposing stages in then life cycle to adverse
  conditions so that they are killed.
• Relatively cheap and effective.
• Poses minimal danger to the environment.
• Methods involve
• Cultivation of the soil
• Crop rotation
• Trap cropping
• Resistant crop varieties
• Mixed cropping
• Good husbandry practices

7
Conventional Methods of Plant Disease Control

• Biological Control
• Deliberate use of organisms (parasites, predators
  and pathogens) to reduce populations of disease
  causing organisms.
• Such natural enemies may be arthropods (insects
  and mites), bacterial protozoan, fungi, viruses,
  nematodes or even vertebrates (birds, toads,
  fish).

8
Conventional Methods of Plant Disease Control

• Chemical control
• Most common and easily applicable method.
• Plays a significant role in solving the food and
  wealth problems of tropical countries.
• Advantages include
• Relatively easy and cheap
• Produces quick results
• Can be repeated as often as desirable
• Broad-spectrum action of many chemicals makes the
  method control many diseases singly or
  combination

9
Conventional Methods of Plant Disease Control

• Disadvantages of Chemical control
• Chemical control is repetitive thus wasteful
• Chemical applied rarely controls the disease
  completely.
• Chemicals are toxic substances and the residues
  that remain in the plants can cause harm
  wildlife, fish and humans
• Cause environmental pollution and ecological
  disturbance.
• Repeated use can cause disease resistance.
• Chemicals provide only a temporary solution to
  disease problems
• Chemicals expensive to manufacture.

10
Weed Plants

• A weed could be defined as
• A plant growing where it is not wanted
• A plant out of place or
• A plant whose usefulness has not been discovered.
• Plants are also considered weeds when they
  interfere with utilization of land and water
  resources or otherwise intrude upon peoples
  welfare.

11
Weed Plants

• Some other plants are weeds because they are
  poisonous to humans and livestock or are
  generally obnoxious.
• Useful plants growing where they are not wanted
  such as volunteer maize in cowpea fields and rice
  seedlings in soybean fields are considered as
  weeds.

12
Characteristics of weeds

• They have a tendency to grow in an undesirable
  location
• They have competitive and aggressive habits
• They are wild and have rapid growth habits
• They are persistent and resistant to control and
  eradication
• They have a high reproductive capacity

13
Characteristics of weeds

• They grow densely or in large populations around
  economic plants
• They are useless and undesirable
• They have spontaneous growth
• They are harmful to humans, animals and crops
• They have unattractive sight

14
Classification of Weeds

• Done according to
• Life form
• Life span
• Growth habit
• Habitat

15
Classification of Weeds

• Life form
• Narrow-leaf weeds
• Leaves have parallel veins and the growing point
  of the plant has protective layers of leaf sheath
  at early vegetative stages e.g. grasses and
  sedges
• Broad leaf weeds
• Leaves of these weeds have net venation

16
Classification of Weeds

• Growth habit
• Independent weeds
• Grow independently
• Parasitic weeds.
• Establish from seed in association with host
  plants which supply food
• Life Span
• Annuals
• Perennial

17
Classification of Weeds

• Annuals
• Complete their life cycle in one growing season.
• They produce large quantities of seeds
• Perennial
• Require more than two growing seasons to complete
  their life cycle.
• They are adapted to long-season crops such as
  cassava and plantation crops.
• Have perennating structures such as tubers,
  rhizomes or stolons.
• Simple perennials live for many years but
  reproduce only by seeds.

18
Classification of Weeds

• Perennial
• If the aerial plant is cut, the basal portion may
  produce new shoots vegetatively.
• True perennials live for many years and will
  produce new plants either by seed or by
  vegetative propagules.
• Many true perennial weeds have either lost their
  ability to produce seeds or produce very few
  viable seeds and thus rely mainly on vegetative
  propagules

19
Classification of Weeds

• Habitat
• Aquatic weeds
• Grow in soils that either have standing water or
  are permanently wet
• True aquatic weeds may be
• Floating hydrophytes if they are in contact with
  water and air only
• Emergent hydrophytes if they are in contact with
  substrate water and air
• Submerged hydrophytes if they root in the
  substrate but do not emerge above the water
• Non-aquatic weeds
• Cannot complete their life cycle in any of these
  high moisture regimes.

20
Economic Importance of weeds

• Are important components of our ecosystem.
• Although they cause greater loss in agriculture
  they also provide food for people and animals and
  protect the soil from erosion.
• Losses Caused by Weeds
• Competition
• Compete with crop plants for light, water and
  nutrients reducing yield and quality of produce.
• Act as an alternative host for other crop pests
  and diseases
• Weeds may harbour many fungal, viral and
  bacterial diseases as well as insect pests. Weeds
  also provide food for birds, rodents and their
  predators.
• Reduce human efficiency
• Some weeds cause allergies and poisoning.

21
Losses Caused by Weeds

• Increase water management cost
• Aquatic weeds interfere with use of water for
  irrigation, recreation and fishing
• Increase production costs
• Weedy crops are prone to insect and disease
  infestation and thus more resources are spent on
  controlling these pests and diseases.
• Weeds increase labour costs
• Increase cost of inputs such as herbicides and
  machinery.

22
Weed Control Methods

• Cultural Methods
• Includes all aspects of good crop husbandry used
  to minimize weed competition with crops.
• The methods include -
• Burning
• Hand-weeding
• Mechanical weeding use of machines
• Mulching
• Tillage cultivation
• Mowing
• Flooding
• Good Cropping system each crop has its
  characteristic weeds
• Preventive weed control use of clean seed
• Cultural weed control methods are often
  laborious, unattractive and ineffective in some
  plants.

23
Weed Control Methods

• Biological Control
• This is the control or suppression of weeds by
  the action of one or more organisms accomplished
  either naturally or by the manipulation of weed,
  control organisms or environment.
• Specific control methods
• Fallowing
• Live mulch
• Use of low growing crops to shade the soil
  surface suppresses weeds.

24
Weed Control Methods

• Specific biological control methods
• Appropriate modification in plant population
• Higher plant populations reduce weed competition.
• Spatial arrangement
• Growing crops with different canopy structures
  will give good shading that suppresses weeds

25
Weed Control Methods

• Chemical Control
• Herbicides kill, suppress or modify weed growth
  to prevent interference with crop establishment,
  growth and production of economic yield.
• Advantages
• Reduces early weed competition in crops thus
  increasing yield.
• Reducing labour requirements for weeding.
• Pre-emergence and post-emergence herbicides
  enhance timely weeding
• Makes it possible for a farmer to cultivate
  larger hectarage with efficient weed control.
• Disadvantage
• Causes environmental pollution.

26
Weed Control Methods

• Integrated weed management
• Suppresses weeds by combining one or more weed
  control methods. Some farmers may still prefer
  to plant seedlings directly in the soil. This
  often means, however, that considerable damage is
  done to the seedling on extraction thus delaying
  its development.
• Environmental and socio-economic constraints make
  integrated weed management one of the best
  options for weed control in the tropics.
• Light soils, high or low rainfall a wide range
  of crop types and cropping systems, abundance of
  persistent weeds all combine with low persistence
  of many herbicides to make absolute reliance on
  chemical weed control difficult.

27
Crop Harvesting

• Done when the useful plant parts reach maturity.
• Maturity usually related to the age of the crop
  when harvestable parts have accumulated maximum
  dry matter.
• At maturity
• Harvestable part ceases to increase in size
• Colour changes due to ripening especially in
  fruits
• Senescence and loss of vegetative parts occur.
• In grains, there is reduction in moisture
  content.

28
Harvesting

• Harvesting at maturity ensures maximum yields but
  delayed harvesting will lead to losses.
• Harvesting is sometimes advisable, particularly
  in grain crops where it may be necessary to
  reduce the moisture content in the field to
  facilitate handling and storage.
• In fruit and leafy crops high moisture content is
  preferred.

29
Harvesting

• Potatoes should be harvested when soil is not
  excessively wet.
• Residue should be properly disposed by burning or
  incorporation in the soil after harvesting
• Helps in preventing a carry over or dissemination
  of any pests and diseases present on the
  harvested crop.
• It also helps in enriching the soil.

30
Harvesting

• Time of harvesting depends on
• Characteristics and quality requirements of a
  crop.
• Harvest produce when foliage is dry.
• Grain crops can be left in the field to dry down
  to 30 per cent moisture content or less in dry
  weather.
• However this may result in
• Losses from shattering and shedding of the grains
  or lodging of the plants.
• the grains may absorb water and sprout on the
  plant
• Grains may be bleached thus reducing quality.
• Insect and fungal attack may become serious

31
Post Harvest Handling

• Rough handling of produce should be avoided.
• Harvest and discard rotted fruit last.
• Do not allow harvested fruit to remain in direct
  sun.
• Soil adhering to harvested fruit, tubers, or
  bulbs should be allowed to dry and then lightly
  brushed off.
• Soaking or washing produce to remove trash or
  soil can result in more storage rots than if not
  done.

32
Post Harvest Handling

• If washing of produce is done
• It must not be done in water that is cooler than
  the produce
• Blemished produce should be discarded prior to
  washing.
• Adding bleach to clean wash water for tomatoes
  reduces bacterial soft rot.
• Fruits should not be allowed to be deeper than 12
  inches or for more than 3 to 4 minutes.
• Storing produce dry and within cooled or air
  conditioned areas will reduce post-harvest rots.
• Discard all produce with rot prior to handling or
  storage.
• Periodically, inspect stored produce for rot and
  discard rotted produce.

33
Post Harvest Handling

• Agricultural produce may be directly consumed or
  sold after harvesting or it may be treated to
  facilitate transportation and storage.
• The treatments of harvested crops include
• Drying Natural drying or Artificial drying
• Aeration
• Crops are dried in order to
• Prevent germination of seeds
• Retain maximum quality in the grain or forage by
  preventive deterioration
• Reduce moisture content in order prevent insect
  attack and microbial infestation.

34
Natural Drying

• Harvesting is done in the dry season exposing it
  to the sun can dry the produce conveniently and
  adequately.
• In humid areas the most practical method of
  drying would involve Partial drying in the field
  followed by further drying either in
• Shallow layers on damp-proof platforms protected
  from moisture in the form of dew or rain or
• Containers that permit dry air movement through
  the materials.
• Excessive or prolonged drying leads to
• Bleaching
• Wrinkling
• Scorching
• Case hardening,

35
Artificial drying

• Use of heat to dry produce in conditions where
  natural drying is not possible or convenient.
• In small-scale peasant agriculture, artificial
  drying is limited to such quantities of produce
  as can be accommodated in the fireplace.
• This usually includes seeds required for planting
  next season.
• Maize, beans etc are hung on horizontal grids
  over the fireplace to dry.
• The produce dried over the fireplace is
  characteristically tainted by smoke oduor
• In some instances, raised grain stores are built
  so that the produce can be fired from below.
• Drying with hot air can be done.

36
Artificial drying

• Low and high temperature drying can also be done
• In low-temperature drying, the air temperature is
  raised to 5-10 ºC above the surrounding
  temperature and drying is completed in 3-4 days
  to avoid deterioration of produce.
• In high temperature drying, the air temperature
  is raised 15-60ºC above the surrounding
  temperature and drying is completed within few
  minutes

37
Aeration

• Aeration is necessary to
• lower grain temperature
• equalize temperature through the bulk of produce
• remove unpleasant oduors and fumigants
• reduce moisture content slightly
• Air can be used to dry stored produce when it is
  dry and warm.
• Humid air is unsuitable unless it is heated to
  slightly above the surrounding temperature before
  it is passed through the produce for storage.

38
Storage of Harvested Produce

• Storage life of seeds varies with
• Species
• Environmental conditions in which the seeds are
  stored.
• Seeds must be stored under moisture conditions
  that retard respiration and enzyme activity
  within seeds.
• Moisture content is crucial for extended storage
  life of most seeds.
• Seeds usually contain 20 moisture or more at
  harvest and must be dried to retain maximum
  viability. Some seeds e.g. citrus however lose
  viability when dried.
• Many seeds dry naturally in the field except when
  conditions are extremely humid.
• Others require drying after harvest.

39
Storage of Harvested Produce

• Most seeds retain viability best at low relative
  humidity.
• A relative humidity of 65 or more is unsuitable
  for seed storage.
• The optimum temperature for long-term storage is
  in the range 18C to 0 C.
• Since moisture and temperature are interrelated,
  it is important to ensure that when one is high
  the other must be low.
• For most seeds, a temperature between 0 C and
  10C and relative humidity of 50-60 is adequate
  to maintain full viability for at least one year.
• Controlling the storage atmosphere, through
  reducing the oxygen content and increasing the
  carbon dioxide content, both of which reduce
  respiration, can extend the storage of seeds.

40
Pest Damage to Stored Produce

• High temperature and high moisture content
  predispose grains to damage by insects and
  disease organism.
• At 13 moisture and 21C insects actively damage
  grains.
• They respire and raise the moisture content of
  the grain leading to spoilage by heating,
  moulding, decay and further damage by the
  insects.
• At 9 moisture and 5C, insects are very inactive
  and spoilage can be avoided.
• Besides insects and rodents, fungi also damage
  stored grains.
• The predisposing factors for fungal damage are
  moisture temperature, physical damage, infection
  and duration of storage.