Production guide for small grains in Africa part 4: Pest and disease management

Birds are the worst nightmare for small grain producers, and can reduce yields significantly. Farmers in some areas have abandoned growing small grains due to losses from birds. Quelea birds, which come in large numbers, affect the crop at soft dough stage whilst doves attack the grain at maturity. In pearl millet and finger millet, quelea birds remain a problem throughout the growth of the crop. 

In order to control the birds farmers should:

1. Plant early to reduce the intensity of crop attacks by birds since by the time the crop starts setting grain there will be a lot of wild fruits and grasses for the birds to eat. Birds tend to prefer these to the field crops.
2. Bird scaring is effective in reducing grain loss from birds. Farmers can build bird scaring structures which help scare birds.
3. Tape ribbon can also be used to scare away birds. It can be tied from one end of the field to the other, such that when it is blown
by the wind it will produce a sound that will scare birds away.
4. Bird sharing reduces the number of birds attacking a field by having many people in a given area planting small grains of the same type.

There are many pests and diseases that affect small grains like:

Common sorghum diseases

Diagnosis – Sorghum smut

plantwiseplusknowledgebank.org

This is the most common disease in sorghum. The disease spores are easily carried by wind and rain, spreading the disease to other plants.

Management/control

Destroy affected plants by burning them or burying them outside the field.

• Whitish growth on the under surface of leaves. Leaves have lighter green or yellow bands which eventually become bleached. Eventually the discolouration covers the entire leaf surface.

Diagnosis – Downy mildew

https://www.cropscience.bayer.us/articles/bayer/sorghum-downymildew

As the infected leaves mature, the bleached portions die, and the tissues break up, such that the leaves look shredded.

Management/control

-Destroy affected plants by burning or burying them outside the field
-If the area is prone to the disease, destroy plant debris by deep ploughing
-Remove all wild sorghums and grasses that have the disease near the fields
-Grow tolerant cultivars

• Droplets of liquid honeydew, varying from colourless to yellow, brown or white, exude from the infected florets. When infestation is severe, the honeydew may drip down and smear the whole head, sometimes dripping down to the soil. Later the head appears as though it has blackened grain.

Diagnosis – Ergot or sugary disease

plantix.net.

This is caused by the ergot disease. It can affect sorghum heads before and after seed set. If infection occurs before seed set, the fungus prevents the development of grain on such heads. If infection occurs after seed set, the disease makes harvesting difficult. It also
reduces the quality of harvested seed, making it unsuitable for both human and animal consumption.

Management/control

-Sow seed that is free from ergot spores. Sorghum heads infected with ergot should be removed from a seed crop.
-Soak seed in a 5% salt solution. Ergot infected seeds will float in the salt solution, making it easy to separate them.
-Treat seed with fungicides like captan or thiram before planting.

• Molded grain with pink, orange, grey, white or black discolouration. Sometimes the seed is not discoloured but is rather small and fails
to mature properly, and shrinks after drying.

Diagnosis -Grain mould

researchgate.net.

This is a major disease, especially in early maturing sorghum cultivars, which flower and mature under high humidity. Moulded grain is unsuitable for human and animal consumption.

Management/control

-Choose planting dates and varieties to grow such that the crop will mature after the end of the rains. Harvesting crop at physiological maturity may reduce mould damage. There is however need to adequately dry the crop after harvesting
-Grow tolerant varieties

• Scattered purple, red or tan speckles on both surfaces of leaves. When fingers are passed on the leaves, a reddish powder can be seen on the fingers.

• In highly susceptible cultivars the entire leaf tissue is destroyed by the disease. The disease may also affect sorghum heads.

Diagnosis – Rust

izumibiosciences.in.

Young plants are rarely affected by rust. The typical symptoms are seen in 1,5 to 3 month old plants.

Management/control

-Only grow rust tolerant sorghum varieties like Macia, SV4, SV1, SV2 and SV3 in areas prone to rust
-Spray crop with fungicides that are effective against the disease.

Common pearl millet diseases

• Instead of grain, some parts of the head may have grain-like structures called sori, which are oval in
shape, and larger than grains.

Diagnosis- Smut disease in pearl millet

Initially the sori are bright green, but later turn black.

Management/control

-Destroy affected plants by burning them or burying them outside the field
-Grow tolerant varieties like PMV 2, PMV 3, and Okashana 1

• Yellowing of the lower part of the leaf with a distinct border between the diseased lower part and the non-diseased upper part of the leaf.

Diagnosis- Downy mildew

abtec.in

Severely affected plants are generally stunted and do not produce heads. When the heads are infected they are transformed into leaf-like structures.

Management/control

-Use disease free seed
-Plant early to reduce the risk of infection
-Destroy affected plants by burning or burying them outside the field
-destroy plant debris by deep ploughing
-Tolerant cultivars like PMV 2, PMV 3 and Okashana 1 can be grown

• Grain covered with pink, orange, grey or black mould. The colour of the mould will vary with the type of fungi involved.

• In some cases the grain may look normal with slight discolouration.

Diagnosis – Head mould

infonet-biovision.org

This is caused by various types of fungi. It is most prevalent when there are humid conditions during the grain filling stage. Infected grain is not suitable for consumption, while infected seed will not germinate well.

Management/control

– Choose planting dates and varieties to grow such that the crop will mature after the end of the rains
-Discard all infected heads

• Cream to pink droplets of sweet sticky liquid (honeydew) oozing out of the infected florets or pearl millet. The honeydew drops onto leaves making them sticky. After some days droplets may dry and harden and form dark brown fungal spores instead of seed.

Diagnosis – Ergot or sugary disease

semanticscholar.org

The pearl millet first turns dark at the tip and then completely black. Grain infected with ergot is not fit for consumption, and infected heads must be removed from a seed crop.

Management/control

-Sow seed that is free from ergot spores. Pearl millet heads infected with ergot. should be removed from a seed crop.
-Soak seed in a 10% salt solution. Ergot infected seeds will float in the salt solution, making it easy to separate them.
-Treat seed with fungicides like captan or thiram before planting.
-Use disease tolerant cultivars like PMV 1, PMV 2, PMV 3 and Okashana 1.
-Destroy affected plants by burning or burying them outside the field.

• Small reddish brown to reddish orange spots on leaves. When fingers are rubbed against the leaves, a reddish powder remains on them. The infection may spread, killing the whole leaf, and eventually leading to permanent wilting.

Diagnosis – Rust

pestoscope.com

In cultivars that are tolerant to rust, the symptoms do not develop any further than the coloured speckles on the leaf surface.

Management/control

-Rust tolerant cultivars should be grown if available
-Removal of grass weeds close to the pearl millet crop will help reduce the incidence of this disease since most rust causing fungi develop on grass weeds.

Common finger millet diseases

• Water soaked lesions appear on the leaf. As the disease progresses these spread, join together and form patches of dead tissue. Eventually whole leaves dry completely, leading to the death of the plant at an early stage.

Diagnosis – Blast

frontiersin.org

This is a common disease of finger millet. The symptoms are usually observed at a very early stage (2 weeks after germination). It can cause more than 50% yield losses in prolonged wet conditions.

Management/control

-Use disease tolerant varieties
-Apply only the recommended amount of nitrogen fertiliser, as excess nitrogen favours the disease

• Infected plants appear pale green and stunted. They eventually fall and dry up. White fungal growth may be seen on the stem and
small tan coloured fungal spores can also be observed.

Diagnosis – Wilt or foot rot

This is a soil borne disease which is more prevalent in swampy areas.

Management/control

-Avoid growing crop on swampy land
-Practise deep ploughing before planting
-Rotate finger millet with crops that are not in the grass family

• Plants have stunted growth, shortened internodes and leaves arising close together, giving the plant a bushy, bunched appearance. The leaves appear pale green but do not shred as they do in sorghum.

Diagnosis- Downey mildew

plantsdiseases.com

This is caused by various types of fungi. It is most prevalent when there are humid conditions during the grain filling stage. Infected grain is not suitable for consumption, while infected seed will not germinate well.

Management/Control

– Destroy affected plants by burning or burying them outside the field
– Keep the field clean
– Plant tolerant varieties

• Black, grain-like structures called sori develop on the grains. The entire head will be small and shriveled, with the head being converted into sac-like smut sori.

Diagnosis – Smut

The disease is prevalent at the time of grain formation.

Management/control

-Destroy affected plants by burning or  burying them outside the field.

In the next edition we shall discuss pests that affect small grain and harvesting. We thank the Zimbabwe government and the Food and Agricultural Organisation of the United Nations for the use of this guide.