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 asusceptible 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 people’s 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 18°C 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 10°C 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 21°C 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 5°C, 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.