FACTSHEET

Control of Cassava Diseases

Introduction

Majority of farmers and a large number of people in Ghana believe cassava is a very hardy crop that requires little attention when cultivated. A large number of people also think that when you throw cassava cuttings on to any soil in Ghana you will harvest some roots even if no attention is given to the plants. Cassava like any other food crop requires attention if optimum (maximum) yields are to be obtained from a unit area. Cassava is affected by a number of diseases that prevent farmers from getting optimum yields of roots from their farms. Some of the diseases are so devastating  (destructive) that they can give no yield of storage roots if not controlled.

What are the causes of diseases in cassava?

Cassava diseases are caused by very tiny living things that are so small in size that they can not be seen with the naked eyes. Living things that are very tiny and can not be seen with the naked eyes are said to be microscopic. These microscopic living things causing diseases are called pathogens. Pathogens that cause diseases in cassava may be viruses, bacteria or fungi.

The major diseases of cassava are:

·        African Cassava Mosaic Disease  (ACMD) (caused by a virus)

·        Cassava Bacterial Blight (CBB) (caused by bacteria)

·        Cassava Anthracnose Disease (CAD) (caused by a fungus)  

Signs or Symptoms of Diseases

Pathogens or living things that cause diseases in cassava produce certain signs or symptoms in the plants they attack or infect. Cassava plants that show these signs or symptoms listed below may be suffering from one or more diseases. In weaker varieties of cassava the symptoms or signs may be severe indicating that the plant is not healthy.

• Discoloured leaves with patches of green, yellow and white or mixtures of these colours  (a symptom of ACMD)
• Stunted growth (a symptom of ACMD)
• Small distorted leaves (a symptom of ACMD)
• Dark water soaked patches and large brown patches or blights on leaves (symptoms of CBB)
• Presence of yellow or brown gums on petioles, leaf blades or stems (a symptom of CBB)
• Several wilted (dry) leaves even in the wet season (a symptom of CBB or a root rot disease)
• Severe defoliation (leaf drop) even in the wet season (a symptom of CBB or a root rot disease)
• Shoot or stem dieback (a symptom of CBB or CAD)  

What are the Importance of Cassava Diseases?

• Diseases cause low yields of storage roots
• Diseases reduce the earnings of farmers as they prevent optimum yields from being obtained.
• Poor yield due to diseases affect food security (poor yield will leave little or nothing for storage)
• Some cassava diseases are devastating and can bring about total crop failure.
• Total crop failure brings about frustration and hardships to farmers
• Very low crop yield resulting from epidemics (several disease outbreaks) of certain diseases such as CBB may bring about starvation or hunger.
• Cassava diseases that affect stems (those that produce cankers and dieback) can lead to loss and shortages of planting materials
• Loss of leaves through diseases can affect the availability of leafy vegetables.
• Loss of leaves and poor yield of storage roots can affect animal production (cassava leaves and storage roots are used in feeding goats and sheep and are also used in food preparations for pigs and poultry)

Controlling Diseases

Use of healthy planting material

Each cassava disease may have certain specific methods that will make control of that disease very effective. However, there are certain methods of disease control that are effective in all cases. The first important step to effective disease control is starting your farm with healthy planting materials. This means that stems providing cuttings for planting must come from healthy plants. Healthy plants do not contain the viruses, the bacteria or fungi that cause diseases. Stem cuttings from healthy plants sprouts (germinate) better and in most cases plants derived or obtained from healthy cuttings grow vigorously. Often these plants start developing their storage roots before new infections or diseases set in. Late infections (infections that set in 7 months after planting) often has little or no effect on crop yield. It is therefore, very important to start your cassava farm with healthy planting material.

Selection of healthy plants for production of cuttings

Avoid plants:

• with several discoloured leaves with patches of green, yellow and white or mixtures of these colours.

• with small distorted leaves.
• that are stunted or are growing poorly.
• with wilted leaves in the wet season.
• that have shed their leaves in the wet season.
• with dark water soaked patches on their leaves
• with leaf blights and yellow or brown gum discharges on leaves, petioles and stems
• with cankers or ‘sores’ on the stems.
• with shoot or stem dieback.

Other useful methods of controlling diseases of cassava

• Select appropriate varieties that are disease resistant (varieties that can not be attacked by diseases)(get information on these varieties from your Agriculture Extension Officer)

• Avoid soils that are poor in nutrients or make efforts to improve soil fertility with cheap affordable fertilizers or use organic manure such as poultry droppings. Soils that are fertile will support a healthy crop of plants that can stand up better against diseases.

• In varieties that branch, use cuttings from the branches as planting materials as these branches are often free from viruses. This practice can help reduce the spread of ACMD. 

• In cassava varieties that do not branch avoid the basal portion of the stem because cuttings obtained from this zone are rich sources of viruses. This practice can help reduce the spread of ACMD if varieties available for planting are not resistant to the disease.

• Dip stem cuttings with cankers or ‘sores’ in dilute solutions of suitable fungicides before planting if that is the only source of planting material available. Get advise from your Agriculture Extension Officer on the selection and safe use of fungicides.

• Avoid using the top green parts of stems  as planting material  (this portion of the stem dehydrates quickly and is easily attacked by agents of diseases.

• Early planting of cassava at the beginning of the rains is better than late planting (early planting produce vigorous growing plants that can withstand diseases better in the dry season).

• It is good to destroy plant debris with diebacks and fungal growths and other diseases through controlled burning particularly after harvest.

This factsheet was developed and prepared by E. Moses as a literature for transferring simple disease control methods to literate cassava farmers.

Address of author:  Crops Research Institute, P.O. Box 3785, Kumasi, Ghana 

e-mail: criggdp@gh.com