Crop Protection

1.1.1        Planting

Planting time

The time of planting is a very critical step in maize production. Planting should be done within the first two weeks of the onset of rains. Early planted maize allows the germinating seed to benefit from nitrogen flux effect, warm soil temperatures and good aeration and escape from insect pests and diseases. Maize planting depth ranges from 2.5-5cm.

Spacing and plant population per hectare

The recommended spacing and planting density of maize for different zones is as shown in Table 3.

Table 3: Recommended spacing and planting density of maize for different zones

Region

Spacing

Density (plants per ha)

Highland

75x25cm 1 plant/hill (pure stand)

75 x 50cm 2 plants/hill (intercrop)

53,333

53,333

Medium

75 x 30cm 1 plant/hill (pure stand)

75 x 60cm 2 plants/hill (intercrop)

44,444

44,444

Dry land

90 x 30cm 1 plant/hill (pure stand)

90x 60cm 2 plants/hill (intercrop)

37,850

37,850

1.1.2        Weeding

Weeds reduce maize yields by competing for moisture, nutrients, space and light. Weeds are also an alternative host to pests and diseases. The most critical stage of weed competition in the life of a maize plant is during the first four to six weeks after emergence of the crop. The most common practice in weed management are hand weeding and use of herbicides. Some of the recommended practices include: hand weeding that should be done at least three weeks after emergence of the plants followed by a second weeding at knee high. Other recommended approaches include the use of a dense legume cover crop to suppress weeds for example lablab (Lablab purpureus), velvet bean (Mucuna pruriens) or sunhemp (Crotalaria juncea) and desmodium. Farmers can regularly scout their fields and uproot Striga weed early enough before the seeds are produced.

1.1.3        Crop rotation

Crop rotation is highly recommended to reduce build-up of maize diseases and insect pests. Rotation is mainly done with beans, cowpeas and peas. Rotating of maize with other cereal crops like sorghum and millet should be avoided, especially in case of Maize Lethal Necrosis Disease (MLND).

1.1.4        Controlling pests and diseases

Diseases and pests are largely responsible for the low maize yields, however, the incidence and severity vary between seasons. Integrated pest and disease management is recommended. Some examples of common maize pests and diseases and their control measures are shown in Tables 4 and 5, respectively.

Pests

Table 4: Field pests, destructive stage, damaging symptoms and control

Pest

Symptoms

Control

FIELD PESTS:

 

 

Maize stem borers

 

 

A  B   D

Caterpillars feed on young plants (a)and cause  dead hearts (b). They later burrow into stems (c and d) affecting water and nutrient flow.

  • Intercrop with non-host crops.
  • Intercrop with green leaf desmodium.
  • Plant trap crps around the plot.
  • Biological control with predators and parasites.
  • Use recommended insecticides.

Maize leafhoppers

 

 

  • Slender hoppers with two small black spots between eyes and hop away on disturbance
  • Hoppers feed on maize plants and transmit maize streak virus.Plants turn chlorotic and streaked and damage may be up to 100%
  • Plant away from grassland or a previous irrigated crop.
  • Plant early to reduce risk of virus transmission.
  • Keep the fields free from grass weeds.
  • Remove residues of cereal crops.
  • Use recommended MSV resistant maize varieties.
  • Catch them with sticky green traps

Maize Aphids

  • Small, 1 to 4 mm long, soft-bodied dark green to bluish-green in color with two long antenna.
  • Feed on young leaves in dry periods.
  • Mottling stunted growth and dieback. Sooty black mold becomes evident.
  • Mixed cropping, trap crops.
  • Use predators and parasites like ladybirds and hover fly.
  • Sprays with recommended Chemical in severe infestations.

Bollworms

 

  • Polyphagous pest that damages many crops.
  • Bollworms feed on leaves, buds, growing points, flowers and fruit.
  • Damage reduces leaf area, which slows plant growth. Also feeds on flowers and fruit causes the main damage.
  • Intercrop maize with other cropsin heavy infestations.
  • Spray with approved insecticides

Cutworms

 

  • These are caterpillars that are found in the soil and damage young seedlings.
  • They cut off the plant at or below the ground level.
  • Seed dressing with approved seed dressers.
  • Spread bait on the ground.

 

Diseases

 

Table 5: Selected maize diseases, symptoms and control strategies

Disease

Symptoms

Prevention/control

Downy mildew

 

 

Leaves and leaf sheaths have white and yellow stripes, stunting of the whole plant, which produces no yield.

  • Early planting.
  • Crop rotation.
  • Resistant varieties

Northern Corn (Turcicum)

Leaf Blight

A chlorotic “halo” develops into a necrotic lesion that grows into mature cigar-shaped lesions about 2 cm wide and 15 cm long. The symptoms appear first on lower leaves and increase in size and number as the plant develops.

  • Planting resistant varieties.
  • Crop rotation.
  • Fungicides in the early stages of the disease.

 

Southern leaf blight

 

Light brown leaf spots appear with a brown margin, of up to 25 mm long and 2-6 mm wide. The spots are at first restricted by the leaf veins, but later leaves dry out and die prematurely. Survival in soil occurs for up to 12 months.

  • Use disease-free seed or treated seed with fungicides.
  • Destroy crop residues and volunteer plants.
  • Practice crop rotation.
  • Use tolerant/resistant varieties.

Gray leaf spot (GLS)

 

Mature lesions, about 5cm long and 0.3 cm wide start from the lower leaves and increase in number usually after silking. The colour changes from light to gray lesions, which may grow together and kill the entire leaves.

  • Observe field hygiene;
  • Use resistant/tolerant varieties;
  • Early planting;
  • Proper tillage;
  • Crop rotation for at least 2 years with non-host crops like potato, beans or pea;
  • Use recommended rates of fertilizers.

Common rust

The disease is characterized by elongate raised bumps (pustules) scattered or clustered together on both leaf surfaces that are red to dark brown in color. The symptoms are mainly observed in the mid and upper canopy of the crop, especially during tasseling. Later the epidermis is ruptured and the lesions turn black and spores are released as the plant matures.

  • Use resistant varieties.
  • Foliar application of fungicides.
  • Cultural practices.

Common smut

The fungus attacks all parts of the plant that is leaves, stalks, tassels and ears even below the soil surface. It gains entry through wounds or thin walled cells of actively growing maize. Galls that replace individual kernels and are covered with white membranes characterize common smut. Later, the galls break open releasing black masses of spores that will infect maize in the following season.

  •  Observe field hygiene
  • Plant resistant varieties
  • Spray with fungicides
  • Avoid injuring the plants.

Head Smut

The fungus penetrates the seedlings and grows inside the plant without showing symptoms, until the tasseling and silking stage.

 

  • Plant resistant varieties.
  • Plant early, when temperatures are unfavorable for spore germination.
  • Treat seeds with systemic fungicides.
  • Maintain balanced soil fertility.
  • Where feasible, remove and burn smutted tassels and ears as they emerge to reduce inoculum spreading.
  • Crop rotation.

Maize lethal necrosis disease