CROP DISEASES
Plant disease is an abnormal condition in the plant caused by an internal disorder
(nutrition and stress) or infection (pathogens).
OR
Plant disease is a condition of plant deterioration and damage caused by various abiotic
(non living) and biotic factors (living).
Effects of diseases on crops
❖ Reduced Yields: Diseased plants often produce fewer fruits, grains, or vegetables, leading
to lower overall yields.
❖ Crop Losses: Severe infections can result in complete crop loss
❖ Stunted growth- when diseases attack a plant, it will not reach its full size.
❖ Discolouration- When a plant is attacked by disease, the leaves and stems may become
yellow, reducing photosynthesis rate leading to poor growth.
Classification of crop diseases
Causative Agent
Disease group
Examples
Bacteria
Bacterial Diseases
Bacterial wilt of Tomatoes &
Potatoes, Bacterial blight of cotton
& cowpeas, Bacterial blight of Soya
bean
Fungi
Fungal Diseases
Damping-off of seedlings, Downey
mildew of peas, Sorghum & Maize
smut, Powdery mildew of beans,
Early & late leaf spot of groundnut
Viruses
Viral Diseases
Maize streak virus, Cowpea aphid
born mosaic virus, Cassava mosaic
virus, Groundnut rosette virus
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1. Maize Streak Virus
Pathogen: Virus
Mode of infection: The virus is introduced into the plant tissue of the host plant by sapsucking leafhoppers. Leafhoppers acquire the virus when feeding on infected plants and then
feed on healthy ones thus transmitting the virus to them.
Once in the plant, the virus multiples and moves in the sap and reaches plant leaves causing
characteristic streaking.
Harmful effects of maize streak virus on crops
❖ Leaves turn light green with yellow stripes running parallel to the leaf veins, reducing the
photosynthetic area, growth, and yield of plants.
❖ Cobs are small and partially filled leading to reduced crop yields.
❖ Inflorescence may become sterile leading to reduced yield.
Control methods of maize streak virus
Cultural control:
❖ Use resistant varieties of maize available
❖ Remove infected plants and burn them.
❖ Early planting
Chemical control of maize streak virus
❖ Spray leafhoppers with dimethoate to kill them
2. Maize Smut
Pathogen: Fungus
Mode of infection: Fungal spores remain in the soil for several years, these spores spread to
young maize plants through wind, soil dust, and rain splashes. Fungal spores will germinate
in moist air or in irrigation or rainwater that collects on the plant leaves, ears, tassels and
other structures of the maize plant.
Spores enter plants through wounds/cuts caused by insects, animals, and poor cultivation
practices. This leads to infections that are visible within about 10 days.
Harmful Effects of Maize Smut
❖ Large blackish lumps appear on the stems leaves and cobs. These break
open to release a fine black powder (spores that spread diseases to other plants).
❖ Spores end up covering the whole grain, leading to a complete loss of Grains/cobs.
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Control methods of Maize Smut
❖ Avoid injury to roots, stalks, and leaves when cultivating, spraying, and fertilizing the
fields.
❖ Pull up and burn infected plants
❖ Plant-certified seed, if using own seeds collect them only from disease-free plants
❖ Practice crop rotation
3. BACTERIAL WILT OF TOMATOES
Pathogen: Bacteria
Mode of Infection: The pathogen is soil-borne and enters through root wounds. It Can also
spread through splash irrigation water, contaminated planting materials, and soil or
implements and feet of workers.
Harmful Effects of Bacterial Wilt Of Tomatoes
❖ Block the vascular bundles especially the xylem thus interrupting water uptake hence
temporary or permanent wilting of affected plants.
❖ Death of infected plants.
❖ Yellowing of leaves.
❖ Discolouration of the xylem vessels.
❖ Decay of the pith which may cause extensive hollowing of the stem.
Control and Prevention of bacterial wilt of tomatoes
Cultural Control Methods
❖ Use certified seeds and seedlings from the disease.
❖ Avoid using contaminated land.
❖ Leave planting land fallow for several seasons.
❖ Crop rotation between non-susceptible and susceptible crops.
Chemical Control methods
❖ Spray infected plants with antibiotics (streptomycin, ampicillin, tetracycline and
penicillin)
❖ Fumigate soil using chloropicrin or application of phosphorous acid to suffocate the
Pathogen.
Common Weeds Of Botswana
❖ Black jack (Bidens pilosa)
❖ Mexican poppy (Argenome mexicana)
❖ Thorn apple (Datura stramonium)
❖ Couch grass (Cynodon dactylon)
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❖ Witch weed (Striga asiatica)
❖ Devil’s thorn (Tribulus terrestris)
Safety measures and their importance when handling farm
chemicals
1. Before application
❖ The operator should wear suitable protective clothing such as overall, breathing mask,
gloves, boots and helmet/hat to reduce poisoning.
❖ Read the instructions on the packet carefully, and follow them to ensure the right
dilution before spraying
❖ Ensure anyone using chemicals is suitably trained to promote a safer work environment
and avoid injury.
❖ Prepare enough amount needed for application to reduce wastage and save money.
❖ Use the proper equipment suited for the chemical to ensure accurate application.
2. During application
❖ Avoid drinking or eating when applying the chemicals to prevent poisoning.
❖ Spray on a calm day to prevent pesticide particles from being carried away from the
target area by wind, potentially affecting unintended areas or harming non-target
organisms.
❖ Spray with the wind at your back /along the wind direction to avoid exposure.
3. Storage
❖ Keep pesticides in well-labelled containers to minimize poisoning.
❖ Store chemicals in a locked shed to keep them away from children, unauthorized people
and animals and avoid accidents.
❖ Keep them in a well-ventilated shed to avoid pollution and suffocation of people.
❖ Store them in tightly closed containers to avoid spillage.
❖ Keep them in high cupboards to keep them away from children and avoid poisoning.
❖ Store them in cool or dry or direct to prevent loss of effectiveness.
4. Disposal of containers
❖ Destroy and break up tins and packets that contained the chemicals to prevent re-use.
❖ Burn any paper or plastic containers in a deep hole to prevent re-use.
❖ Bury tins away from streams or dams to prevent pollution
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Appropriate Tools, Equipment, Machinery, And Materials When Handling
Farm Chemicals
A. Sprayers: There are different types of sprayers used for applying pesticides and
herbicides to crops, such as handheld sprayers, backpack sprayers, knapsack sprayers,
tractor-mounted sprayers, and aerial sprayers.
B. Dusters: Dusters are used to apply dry pesticides or dust formulations to plants or
areas infested with pests.
C. Foggers: Foggers create a fine mist of pesticide often used for indoor pest control in
areas such as greenhouses.
D. Granular spreaders: Granular spreaders are used to apply granular pesticides or
fertilizers to lawns, gardens, and agricultural fields. E.g. handheld spreaders, push
spreaders, and tractor-mounted spreaders.
E. Aerial applicators: Aircraft is equipped to spray pesticides or chemicals over large
agricultural fields.
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Knapsack Sprayer
Name the parts of the knapsack sprayer labelled above.
Calibration of a knapsack sprayer
The aim is to determine the Spray Volume (SV), the rate at which water is applied to a
known area.
Calibration steps:
❖ Select and measure a sample/test area:
❖ Fill the spray tank with a known volume of water(l)#
❖ Proceed to spray at a normal speed and constant height until the test area is completed.
❖ Measure the amount of water remaining in the tank
❖ Determine the amount of water used by subtracting the final water volume from the
initial water volume
*Calculate the spray volume (SV) of the sprayer using the formula.
SV=Initial water volume (Vi)- Final water volume (Vf)/ Test Area = Litres/m2
*To calculate amount of spray mixture needed to spray a given field area.
Quantity of Spray mixture needed (Qm) = SV x Area of field (m2)
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Appropriate protective clothing
Name the protective the clothing worn by the applicators on the picture above.
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CROP PRODUTION
MAIZE (Zea Mays)
A) Climatic requirements
Maize requires hot sunny climate with an evenly distributed rainfall of about 450mm during
growing season and average temperatures of 240 C.
B) Soil type and Soil pH requirements
Maize can be grown in a wide range of soils from sand to clay, provided they are well
drained. High yields are normally obtained from deep loam soils that are rich in organic
matter.
pH range of 5.5 and 7.0 is suitable.
C) Seedbed preparation
❖ Soil is prepared by ploughing to a depth of 25 cm to 30 cm.
❖ Disc the soil by breaking soil clods that may have formed.
❖ Then the seedbed is levelled and worked to medium tilth by harrowing.
D) Time of planting
In Botswana, the growing season generally occurs between October (onset) and March (end).
E) Planting of maize: maize is planted directly by seeds
❖ Seed rate: 22kg per ha though it may vary from 17 – 35 kg/ha depending on variety and
spacing adopted. One seed per planting station and at stated spacing plant population
would be 40 000 plants per hectare.
❖ Depth of planting: Maize seeds should be planted about 5 cm to 8 cm deep.
❖ Spacing: spacing between rows (inter-row spacing) is 90 cm.
Spacing within a row (intra-row Spacing) is 30 cm apart.
F) Application of manures
❖ Basal dressing
-Adding 2:3:2(22) + 1% Zn at an application rate of 1000kg/ha. The fertilizer is applied
with a planter in rows during planting.
- Kraal manure is broadcast and dug in 5 weeks before planting at the rate of 30 000kg /ha.
❖ Top dressing
Apply LAN (28%) 100kg/ ha 4 weeks after planting. The fertilizer is applied with a planter
in rows.
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G) Prevention and control of common Pests and Diseases
Common Pest: Stalk borer
Common disease: Maize streak virus
REFER TO NOTES ON STALK BORER AND MAIZE STREAK VIRUS
H) Prevention and control of weeds
1. Mechanical weed control
Control of weeds using physical means such as digging/tillage, cultivation, hoeing, slashing
and hand pulling.
2. Cultural weed control
Include all the aspects of good crop husbandry to minimise weed interference with crops e.g.
mulching, crop rotation, early planting, use of clean seeds etc.
3. Chemical Weed Control
The use of herbicides- these are chemicals used to kill weed.
Selective Herbicides – these are post-emergence herbicides that kill a specific group of
weeds and do not harm other plants. e.g. Duron and 2-4 D herbicides will kill only broadleaf
weeds but not maize plants.
Non selective herbicides-these are pre- emergence herbicides that kill any plant. They are
applied before maize is sown. E.g. Atrazine and glyphosate.
I) Recognition of crop maturity
For dry grains
❖ leaves are yellow
❖ Husks are dry and papery
❖ Grains are hard
❖ Moisture content is at about 20%
J) Harvesting of yield
Maize can be grown for fresh cobs or for dry grains. It takes 13 to 15 weeks before the fresh
cobs are ready for harvesting and at least 18 weeks before dry cobs can be harvested.
De-husking maize cobs is done traditionally by hand and knife, more especially if it is still
green. Self-propelled maize combine harvesters are also used for direct harvesting and
threshing of maize with husk.
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It is harvested and stacked for several weeks to hasten the drying out of cobs. When moisture
content is at about 12% it can be threshed and stored safely in bags or silo. In good harvests,
it may yield 4.8 – 5.5 ton/ha.
K) Storage methods- different structures made with different materials are used.
Storing maize in a crib on the ground
Open-basket storage
Bins roofed with brick
Metal Silos
Bag storage
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L) Marketing- the following activities are involved in the marketing of maize
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Harvesting and stocking
Transport from the field
Temporary storage
Drying
Husking
Shelling and winnowing
Storage, sorted and graded
Weighing and putting in bags and pricing.
Sold to individuals as fresh or dry and local traders through co-operatives and
BAMB
M) Relevant Legislation
1. Maize (Classification and Grading) Regulations:
These regulations apply to maize bought or sold by the Botswana Agricultural Marketing
Board (BAMB) starting from the 1974/75 crop season.
They cover aspects such as:
❖ Classification and Grading: Maize is categorized into different classes and grades based
on quality criteria.
❖ Weevil Maize: Guidelines for handling maize infested with weevils.
2. Control of Goods (Import and Export of Agricultural Products) Regulations:
These regulations govern the import and export of agricultural products, including maize.
❖ Agro chemicals act of 1999 provides for registration, licensing, promotion, advertising,
labelling and handling of agrochemicals.
❖ All imported germplasm to be registered.
❖ Local production to be protected from imports.
❖ Imported seeds should come with Phyto-pathological report.
N) Record keeping
To operate a successful and profitable production enterprise a farmer or farm manager must
maintain a set of records.
A farm Diary: A document in which important events are recorded daily. Some events (data)
may later be transferred to other record types.
There are two types of records
ii) Production records: Production records involve the activities involved in manufacturing
or producing goods. They focus on the physical aspects of production.
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a.
b.
c.
d.
Labour use records.
Machine/tractor use record.
Water, fertilizer, fuel use record, etc.
Maize crop records including areas, yields, planting and harvesting times, etc.
iii) Financial records. Financial records involve activities related to financial transactions.
These may include:
a.
b.
c.
d.
Profit and loss account
Sales account
Income account
Expenses account
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