Disease Guide

Identification and Control of Root Rot Diseases of Cassava

E. Moses, J.N. Asafu-Agyei and F. Ayueboteng

CSIR-Crops Research Institute, P.O. Box 3785, Kumasi, Ghana

Published online 2005 by the International Society for Plant Pathology (ISPP)

Sponsored by ISPP through the competitive Congress Challenge scheme of the Task Force on Global Food Security

Introduction

Cassava is cultivated in almost all the districts of Ghana and it is the number one staple food crop for majority of Ghanaians. Cassava is also fast becoming an important crop for industries because of its high starch content. Diseases and pests cause severe yield losses in all production districts where susceptible cultivars are grown. The major diseases of the crop in Ghana continue to be African Cassava Mosaic Disease (ACMD), Cassava Bacterial Blight (CBB) and Cassava Anthracnose (CAD).

Root rot diseases of cassava are gradually becoming important in the major cassava producing regions of the country with high yield losses due to rots being reported regularly by farmers and Agriculture Extension Agents. Complete crop failure due to root rots have been observed in farms in different regions of the country.

Rotten roots are not suitable for food or any processed product such as ‘gari’ or flour.

Severe yield losses due to root rots affect cassava supply to urban markets and weakens food security of rural communities. Root rots of cassava leave farmers poor.

This small guide introduces farmers and extension agents to root rot diseases of cassava and how they can be controlled using simple measures to increase production by rural poor farmers.

Causes of root rot diseases

Most farmers think root rot of cassava is a problem when the crop is cultivated in water logged soils or on land that becomes flooded at certain times of the year. It is true that water logged soils are not good for cassava production because they promote rots. There are however, other causes of root rot in cassava.

Several of the reported root rot outbreaks in recent years occurred in well drained soils with no history of flooding. These rots are often caused by microorganisms (fungi and bacteria). Lately, a large parasitic mushroom has been found to be causing severe root rot of cassava in some parts of Volta, Central, Ashanti and Eastern regions of the country. This parasitic mushroom shown in Figure 1 is capable of causing 100% yield loss in susceptible cultivars.

Common symptoms associated with cassava root rot diseases

• Leaves of plants affected by root rot diseases become brown and wilt (lose water) even in the wet season.

• Defoliation (loss of leaves) often follows the wilting stage.

• Storage roots of affected plants may be swollen and are often coloured when cut open.

• Rotten roots may be soft and give out offensive odour particularly in infections involving bacteria.

• Shoot or stem dieback may be observed.

• Stems may become weak at the base and lodge.

• Affected plants finally die.

Methods of spread of root rot diseases

• Root rot caused by some microorganisms require water to spread. Some reproductive or infective units of disease causing fungi have to swim to reach a new host.

• Plant debris left on fields after harvest are often contaminated with disease causing fungi and are good sources of spores for infecting new plants.

• Diseases are also transmitted through wounds by using contaminated farm tools such cutlasses and hoes.

Characteristics of the new parasitic mushroom (Polyporus sulphureus) root rot disease of cassava

The Polyporus root rot disease is a very devastating disease of cassava that has been found in Ashanti, Central, Eastern and Volta regions. In areas where the disease is found, all local cultivars have been identified as susceptible and 100% yield loss on fields have been observed or reported. Conditions favouring the growth and spread of this fungus in the four regions can be found also in Western, Greater Accra and Brong-Ahafo regions. Farmers in these regions must therefore, be on the look out for this parasite and report its presence to Agricultural Extension Agents of the Ministry of Food and Agriculture who operate in farming communities in the districts.

Hot spot areas for Polyporus root rot disease are in the Sabadu and Avemedra areas in the Kpando District of the Volta region and the Gomoa and Awutu-Bawjiase Districts of the Central region.

The bright yellow fruiting body of the parasitic mushroom can be found developing on other plants even in the absence of cassava plants (Figure 1). Often the fruiting bodies of the parasitic mushroom as shown in Figure 1 will be growing attached to the roots of a secondary host plant. The parasite has also been found growing on yam (another important staple food in Ghana). The Polyporus cassava root rot fungus has a wide host range and can grow on other plants for several years but prefers cassava when it is present. The first sign of the disease on cassava is the presence of its bright yellow or dark brown fruiting body growing on an infected plant (Figures 2 and 3).

Figure 1. Bright yellow fruiting bodies of the parasitic mushroom (Polyporus sulphureus) growing in the absence of cassava plants. The fruiting bodies are not growing unaided. Digging the soil around its base will show that the fruiting body is firmly growing attached to the main root of a host plant.

Figure 2. Bright yellowish brown fruiting body of Polyporus sulphureus (the root rot causing mushroom) growing on a young cassava plant.

Figure 3. The fruiting body of P. sulphureus develops rapidly and can reach a size of 30 cm or more in few weeks. The fruiting body darkens as it matures.

Figure 4. The fruiting body of the parasitic mushroom (P. sulphureus) growing on a woody host plant. The fungus can attack several tree species and kill them eventually.

Root rot at harvest

At harvest, yield losses due to the parasitic mushroom can be very high. Figures 5 and 6 show rotten roots which is all the harvest from fourteen month old affected plants. Yield losses of this nature are common in endemic areas of the disease. Farmers cultivating susceptible cultivars of this kind can experience 100% yield loss in disease hot spot areas.

Figure 5. An attacked fourteen month old plant with all storage roots rotten in a farm in an endemic area in the Volta region.

Figure 6. A harvest of rotten roots from a single plant attacked by the Polyporus root rot fungus. The bright yellow structure is the developing fruiting body of the parasitic mushroom.

Control of root rot diseases of cassava

Control of root rot diseases can be achieved through effective farm management practices. Cassava is the most cultivated food crop in the country and is planted in backyards even in the cities and as such the use of chemicals in the control of cassava diseases is not recommended. The measures discussed below when applied singly or in combination as a field situation may determine can help control root rot diseases of cassava.

Site and land selection

Select a site that is not prone to flooding or is not water logged at any part of the year.

Avoid farming very close to rivers and streams that are likely to overflow their banks in the rainy season. It is common to find farmers who have limited access to land cultivating cassava in valleys that are subjected to flooding in good rainy days.

Water logged soils are poorly aerated and cassava roots developing in such soils will suffer from rots not necessarily due to diseases. Roots of plants growing in water logged soils are not healthy and can easily be attacked by diseases. A sandy loamy soil that is well drained is a good soil type for cassava.

Some of the organisms or agents that cause root rot diseases of cassava such as certain types of fungi require some amount of water to allow them swim to effect new infections. This is one of the reasons why root rot diseases of cassava are common in poorly drained soil.

The soil should be fertile enough to give a good healthy crop. Avoid planting in soils that are poor in nutrients. Efforts must be made to improve soil fertility by the addition of inorganic or organic fertilizers whenever possible. A good example of organic fertilizer is poultry manure. What must be kept in mind is that a healthy plant is not easily attacked by diseases but when even attacked it is able to fight the disease better.

Do not cultivate cassava on land that has a history of root rots and other major diseases. This information can be obtained from neighbours farming close to the land. If the only available land has a history of rots, then good disease management practices must be maintained if good yields are expected.

In Polyporus endemic areas avoid cultivating cassava on lands showing a high incidence of the mushroom growing in the absence of cassava as shown in Figure 1 above.

Use disease resistant or tolerant varieties

Improved cassava varieties with resistance or tolerance to certain important diseases and pests (that are also high yielding) have been developed and released by a number of Agricultural Institutions. It is therefore important to report any disease situation that is affecting your yields to Agricultural Extension Agents who operate in your communities. Ask questions on the availability of resistant materials and how to obtain them. It is always advisable to start your farm with disease resistant varieties if they are available. This helps you among other things to obtain higher crop yields.

Use healthy planting materials

• Avoid using planting materials from fields with visible signs of root rot diseases even if the stems look healthy.

The structures of disease causing fungi that give rise to new attacks or infections called spores are microscopic and planting materials from farms with visible root rot attacks are likely to carry several of these spores to new farms.

� The Ministry of Food and Agriculture must institute and maintain policies that will check movement of cassava planting materials from defined endemic areas with observed and reported cases of devastating diseases into new districts. The current uncontrolled free movement of planting materials to and from any part of the country promotes spread of diseases.

Practise good farm sanitation

• Collect plant debris (stems and roots) with fungi together and burn to destroy particularly after harvest.

Rotten roots (Figures 5 and 6) and plant debris bearing fruiting bodies of disease causing fungi (Figure 7) must be destroyed by burning to reduce the spread of root rot diseases. Disease severity on farms can be reduced through destruction of debris that carry spores into the next planting season.

In Polyporus endemic areas it is good for a farmer to do regular field monitoring particularly after the first rains following the long dry season. This is the time the fruiting bodies of the parasitic mushroom starts to develop on susceptible cultivars (Figure 2). The young fruiting body can be hand picked as soon as they appear, kept in polythene bags and destroyed by burning. This practice does not allow the fruiting body to develop to produce spores that are the reproductive units of the parasite.

Regular field inspection to remove and destroy few plants showing disease symptoms such as patches of fungal growths, wilting and defoliation (that may be symptoms of root rot and other diseases) by burning reduce spread of diseases on farms. This practice called roguing can be an effective disease management strategy if regular inspections are well planned and followed.

Figure 7. Plant debris after harvest bearing the fruiting body of the root rot fungus. The leathery fruiting body after harvest can persist for over a year and are good sources of spores for new attacks in the following seasons crop.

Early harvesting

Some root rot causing organisms actively degrade storage roots when they are well developed and harvesting is delayed. Typical rot in cassava fields where harvesting of matured roots is delayed is shown in Figure 8. Early harvesting therefore, prevents or reduces the incidence of rots of this nature on farms.

Figure 8. Root rot of cassava from a plant harvested 24 months after planting.

Practise crop rotation

Continuous cropping of cassava on the same piece of land contributes to increase in incidence and severity of diseases including root rots. It is generally effective to rotate cassava with cereals to help reduce the levels of inoculum (spores etc.) on fields. It is advisable not to plant cassava continuously for three years on the same piece of land particularly in localities where high cassava disease pressures exist.

Practise land fallow

Fallowing of land after five or more years of continuous cassava cultivation for a period of three to five years is a good measure that can reduce incidence and severity of diseases. When practised properly, it can help eliminate a disease from a locality or reduce its incidence and severity significantly. The absence of host plants from a field deprive pathogens or disease causing organisms of their nutrient resulting in decline of pathogen populations. In localities where increases in human population is exerting strong pressures on available farmland, this practice will not be a popular option.

Clean farm equipment

Farm equipment such as cutlasses, hoes and ploughs used on fields with root rot history must be cleaned immediately after use before being used on a second farm. This reduces the spread of soil borne diseases from farm to farm.

Avoid planting cassava as the first crop after clearing woodlands or forests

Crops that are not susceptible to pathogens that infect cassava particularly cereals must be planted as the first crops just after clearing forests or woodlands. Attacks by fungal pathogens particularly parasitic mushrooms can be prevented through this practice. Woody stumps left in farms after clearing woodlands may serve as hosts or reservoirs to fungal and other pathogens that attack cassava. Stumps must be removed or destroyed.

Host range

Pathogenic organisms causing root rot diseases of cassava particularly fungi can attack a wide range of crops. Some root rot fungi of cassava can attack soybean and sunflower. The Polyporus root rot mushroom can attack a number of crops including yam and citrus.

Care should therefore, be taken in the selection of crops for rotation or intercropping with cassava as a measure to control root rot diseases.

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