Common Pests and Diseases OF Cassava

Cassava mosaic

African cassava mosaic virus (ACMV)

                                                  

Cassava leaves showing typical mottling caused by ACMV

                                                 
Resistant cassava variety beside a traditional susceptible variety with symptoms of ACMV

                                                 
ACMV-infected cassava plant

Symptoms

Discolored pale green, yellow or white mottled leaves which may be distorted with a reduced size; in highly susceptible cassava cultivars plant growth may be stunted, resulting in poor root yield and low quality stem cuttings

Cause

Virus

Comments

Disease is spread by infected cuttings and by whiteflies

Management

Varieties of cassava resistant to the virus are available in many countries, most traditional varieties of cassava grown in Africa are susceptible to the virus, seek advice from an agricultural extension on suitable varieties for your region; do not plant cuttings from plants with symptoms of disease; inspect plants regularly for symptoms of disease and remove and destroy any showing symptoms

Cassava bacterial blight Xanthomonas axonopodis

Symptoms

Small, angular, brown, water-soaked lesions between leaf veins on lower surfaces of leaves; leaf blades turning brown as lesion expands; lesions may have a yello halo; lesions coalesce to form large necrotic patches; defoliation occurs with leaf petioles remaining in horizontal position as leaves drop; dieback of shoots; brown gum may be present on stems, leaves and petioles

Cause

Bacterium

Comments

Most important bacterial disease of cassava; spread by water splash and infected tools; disease more severe in wet conditions; particularly destructive in South America and Africa; most important method of spread is probably through exchange of infected plant cuttings

Management

Rotate cassava crop with non-host; plow crop debris into soil after harvest or remove and burn it; prune infected parts from plant; propagate cuttings only from healthy plants; intercrop cassava with corn (maize) and melon

Anthracnose Colletotrichum gloeosporioides

Anthracnose canker on cassava stem

Symptoms of a severe anthracnose infection on cassava stem

Symptoms

Cankers on stems and leaf petioles; leaves drooping downwards; wilting leaves which die and fall from plant leading to plant defoliation; death of shoots; soft parts of plant become twisted and distorted

Cause

Fungus

Comments

Disease emerges at the beginning of wet season (Africa) and worsens; spores spread by wind

Management

Anthracnose usually does not cause large-scale economic damage to cassava and control is usually not necessary; avoid planting cuttings with cankers; if disease does occur crop debris should be removed and destroyed after harvest

Bud necrosis Unknown

Symptoms

Patches of dark brown or gray fungal growth on stems; necrotic areas covering buds on the stem

Cause

Fungus

Comments

Disease most commonly found on cassava grown in humid regions (e.g humid forest zones) and is less common in drier savanna areas; fungal spores are carried by wind to new plants and farms

Management

Only plant cassava cuttings taken from healthy plants which are free from necrotic lesions; space plants widely to allow good air circulation around plants and reduce disease incidence; remove weeds around plants; id disease is present, burn all necrotic stems and crop debris immediately after harvest to prevent spread

Brown leaf spot Cercosporidium henningsii

Symptoms

Circular or irregular brown spots with darker margin between leaf veins on older leaves; centers of lesions may drop out givinf leaves a shothole appearance; if infection is severe, leaves may turn yellow, dry out and drop from the plant

Cause

Fungus

Comments

Disease emergence favored by high temperature and humidity

Management

Remove leaves and crop debris from around plants to prevent disease spreading; remove weeds from around crop

White leaf spot Cercospora vicosae

Symptoms

Large, diffuse white spots on upper surface of leaves; spots with grey center on underside of leaves in humid weather

Cause

Fungus

Comments

Prevalent in wetter, warmer regions where cassava is grown e.g. Brazil and Colombia; common in West Africa

Management

Remove leaves and crop debris from around plants to prevent disease spreading; remove weeds from around crop

Cassava brown streak disease Cassava brown streak virus (CBSV)

Mottled leaves infected with Cassava brown streak

Tuber necrosis caused by Cassava brown streak

Rotting tuber caused by Casava brown streak

Symptoms

Brown elongated necrotic lesions on young stems; 'feathery' chlorosis of on plant leaves associated with leaf veins (no distortion of leaves); necrosis of tubers; roots develop knots; internal tissues of roots and tubers stained brown

Cause

Virus

Comments

Disease is transmitted by whiteflies; disease is prevalent in Africa, particularly in coastal cassava growing regions

Management

Only plant cuttings from healthy plants with no symptoms of disease; plant cassava varieties that are more tolerant of brown streak virus such as Garukunsubire and Seruruseke; remove and destroy any plants which are symptomatic of the disease; harvest crop early to avoid severe losses due to necrosis of tubers

Cassava root rot disease Macrophomina phaseolina
Botryodiplodia theobromae
Fusarium spp.

Rotting tubers cut open to reveal discoloration

White fungal structures visible on stem

Dark tuber discoloration caused by Botryodiplodia

Discoloration on tuber surface caused by root rot

Symptoms

Leaves on affected plants turning brown and wilting and plant has a scorched appearance; leaves may remain attached to the plant or drop to the ground; plant death will occur; examination of roots reveals root dieback and swelling of tubers; tubers may have light brown, dark gray, blue or pink discoloration; rotting roots may be soft and produce a foul odor; infection by Botryodiplodia fungi may cause the appearance of white fungal structures at the base of the stem, particularly during the wet season

Cause

Fungi

Comments

Root rot disease emergence is often favored by waterlogged, poorly-draining soils

Management

Plant cassava in well-draining soils; remove and destroy all crop debris by burning; sanitize all tools after use

African root and tuber scale Strictococcus vayssierrei

Scales on cassava stem

Scales on cassava tubers

Symptoms

Oval shaped scales on stems, roots and/or tubers; infections which occur at an early age kill plants and prevent the production of tubers; plant becomes shriveled and discolored at feeding sites

Cause

Insect

Comments

Insect is indigenous to Africa and is particularly prevalent in forest areas of Ghana and Congo

Management

Improve soil by adding organic matter to make soil more fertile; remove and destroy infested stems; do not plant cuttings with scale

Cassava green spider mite Mononychellus tanajoa
Mononychellus progresivus

Symptoms

Yellow stipping of leaves; chlorotic spots on leaves; chlorosis of entire leaves; if infestation is very high then leaves may be stunted and deformed; terminal leaves may die and drop from plant; pest responsible is a tiny green mite

Cause

Arachnid

Comments

Green spider mites are very common pests in most African growing regions and become problematic during the dry season; can cause significant tuber losses

Management

Plant tolerant cassava varieties where possible; plant at the beginning of the rainy season to encourage vigorous growth which allows plant to tolerate attack; intercropping with crops such as cowpea may reduce damage; introductions of the predatory mite Typhlodromalus aripo have been very successful at controlling the green spider mite in many regions of Africa

Two-spotted spider mite Tetranychus spp.

Symptoms

Leaves stippled with yellow; leaves may appear bronzed; webbing covering leaves; mites may be visible as tiny moving dots on the webs or underside of leaves, best viewed using a hand lens; usually not spotted until there are visible symptoms on the plant; leaves turn yellow and may drop from plant

Cause

Arachnid

Comments

Spider mites thrive in dusty conditions; water-stressed plants are more susceptible to attack

Management

Spraying plants with a strong jet of water can help reduce buildup of spider mite populations; if mites become problematic apply insecticidal soap to plants; certain chemical insecticides may actually increase mite populations by killing off natural enemies and promoting mite reproduction

Grasshoppers (Variegated grasshopper, Elegant grasshopper) Zonocerus variegatus
Zonocerus elegans

Variegated grasshopper

Symptoms

Defoliated plants; bark removed from stems; insects are large brightly colored grasshoppers

Cause

Insect

Comments

Grasshoppers have a wide host range and mainly attack seedlings; cooperation between neighboring farms can help with grasshopper control as the insect tends to lay its eggs outwith the plantation with nymphs migrating to the crop to feed

Management

Hand pick any grasshoppers found on plants; locate any egg pods around cassava field and destroy to reduce grasshopper populations; biopesticides such as "Green Muscle" are available in South and West Africa which are very effective at reducing the grasshopper population; products containing neem have also given good control of variegated grasshoppers

Cassava scale Aonidomytilus albus

Symptoms

Stem surfaces covered with white waxy substance; leaves wilting and dropping from plant; severe infestations may result in stunted plants and poor tuber yields; cutting from infected plants do not sprout; insect is a flattened oval scale with an elongated white cover

Cause

Insect

Comments

Using pesticides to control scale insects reduces populations of beneficial insects such as natural enemies

Management

Plant material that is completely free of scale insects; remove and destroy infested stems from existing plantations; apply organic matter to soil to improve fertility

Root knot nematode Meloidogyne spp.

Symptoms

Galls on roots which can be up to 3.3 cm (1 in) in diameter but are usually smaller; reduction in plant vigor; yellowing plants which wilt in hot weather

Cause

Nematode

Comments

Galls can appear as quickly as a month prior to planting; nematodes prefer sandy soils and damage in areas of field or garden with this type of soil is most likely

Management

Plant resistant varieties if nematodes are known to be present in the soil ;check roots of plants mid-season or sooner if symptoms indicate nematodes; solarizing soil can reduce nematode populations in the soil and levels of inoculum of many other pathogens

Witches' broom Cassava witches'-broom phytoplasma

Symptoms

Plants are stunted with an excessive proliferation of branches; shoots have small leaves and shortened
internodes; no chlorosis is present; shoots from the cuttings and are weak but show no visible symptoms; few shoots successfully grow from cuttings

Cause

Phytoplasma

Comments

Disease has been detected in Vietnam, Thailand, Cambodia, Laos, Indonesia, and the Philippines

Management

Remove and destroy and plants suspected of being infected; remove all cassava debris from filed after harvest; disinfect all tools and equipment between cuttings