Ecologically sustainable plant protection

Objective
Evaluate, develop and integrate relevant pest and disease management systems that will reduce pre- and post-harvest losses

Background information and methodology
Historically, cassava had few serious pests and diseases in Africa. However, the situation changed as cultivation intensified and exotic pests were introduced. Although it is now widely accepted that cassava in Africa is attacked by a number of serious pests, few in-depth studies of the ecological and economic importance of any of these have been done. The major cassava pests in Africa include relatively few phytophagous arthropods, plant pathogens and weeds compared to the pest complex found in the neotropics. Together, these species could reduce cassava production by as much as 50%. The most severe pests are the exotic species accidentally introduced into areas where the local germplasm is susceptible to attack, where effective natural enemies/antagonists are absent, and where a tradition of practices to cope with the introduced pests had not had time to evolve. In addition, pest problems are being created where intensification of cassava production erodes the environmental stability inherent in balanced agro-ecosystems. The major pests of concern are cassava green mite (CGM), variegated grasshopper during outbreak episodes, root mealybug in the rain forest eco-zones, cassava mosaic disease (CMD), cassava bacterial blight (CBB), cassava anthracnose disease (CAD) and root rots in the humid lowlands. Root knot nematodes have been reported to cause yield losses in Uganda and Madagascar. The role of termites and certain weed species particular to specific eco-zones have been reported as constraints but have not received adequate attention.

The appearance of cassava mealybug (CM) and CGM as introduced pests in the 1970s in Africa had devastating effects in farmers' fields. In particular, CM attack was so severe that it threatened the future of cassava in Africa. Massive efforts spanning several continents and involving numerous international and national research institutes under the leadership of IITA led to the development of a successful continent-wide biological control program. Natural enemies of CM were identified in South America, and the parasite Apoanogyrus lopezi has been released in many countries in Africa. Biological control, along with improved varieties and cultural practices, provide a cost-effective, sustainable, and environmentally friendly technology for the control of CM without using insecticides. The wide spread establishment and documented impact of exotic predatory mite species offers good hope for biological control of the CGM as well.

Cassava mosaic disease:
CMD continues to be prevalent in all the main cassava-growing areas in the ECA sub-region and is generally regarded as the most important disease causing between 20 and 90% crop losses based on the cultivar, viral strain and environmental factors. It is becoming increasingly apparent that the status of CMD in East Africa is deteriorating. Changes in this status are more apparent in Uganda, DR Congo and Kenya. Plants infected through white fly transmission dominate affected areas. The lower leaves of the infected plants look apparently healthy while the leaves above the point of first infection show severe symptom expression, drastic reduction in leaf size with marked distortion. The plants harbor numerous adult white fly populations on the young shoots and large nymph populations on the lower surface of the apparently healthy lower leaves. Lack of alternative propagation stock in the disease-infected areas leaves farmers no choice but to use material from the previous harvest of infected plants. Environmental factors favoring the development and fecundity of the white fly (vector) enhance the spread of the disease. The mosaic virus spread is therefore highly linked with its white fly vector.

Despite numerous research done on white flies in the past decades, our knowledge on the tropical white fly pests and vectors is inadequate. Current estimates put losses to CMD in Uganda at US dollar 60 million annually, and on a more practical level, food shortages resulting from this problem led to localized famine in 1993 and 1997. A number of countries (Kenya, Burundi, and Madagascar) have made significant progress in selecting resistant/tolerant clones, which are being evaluated within their different ecological zones. To date, several studies/surveys have been conducted to increase the knowledge base to better understand the virus and its vector.

Cassava Anthracnose Disease (CAD):
Cassava anthracnose disease is an important disease of both stems and leaves of cassava. The disease causes cankers of the stems, twigs and fruit. On the young stem a patch of greenish tissue develop into lesions which turns brown and disrupt translocation of water, minerals and photosynthates within the plant. In older stems the lesions develop into deep cankers which cause stems to be brittle and easily break under strong winds. The physical damage caused on the stems does not only permit entry of secondary pathogens but result in stems of inferior quality which does not establish well as a consequence, and yields are reduced. Leaf spotting and tip die back seriously affects photosynthetic efficiency of the leaves. No attempts, however, have been made to correlate disease incidence and severity with yield loss. Preliminary screening results indicate that use of tolerant and high yielding varieties can reduce crop losses due to CAD.

Cassava Bacterial Blight (CBB):
Cassava bacterial blight is a serious disease of stem and leaf found in most areas where cassava is grown. In a susceptible variety CBB causes leaf blighting, wilting and defoliation. In the stem and roots, the systemic nature of discoloration of the vascular system can be detected resulting into wilting. Under severe attacks rapid defoliation occurs leaving a bare stem, commonly referred to as "candlestick" which severely affects photosynthesis, leading to yield loss of between 20-100% depending on the bacterial strains, varieties of cassava being grown and environmental conditions. In addition, stems of inferior quality are produced.

Early work indicated positive correlation between CMD and CBB such that selection for resistance for both diseases could be accomplished concurrently. However, based on the current understanding, these associations do not seem to hold hence the need to review the situation. There exists a wide scope for genetic improvement for resistance to CBB while exploiting other integrated approaches in disease management.

Cassava mealybug:
CM has spread to all of the cassava growing areas in the region with reported yield losses of up to 80%. Owing to the stunted growth of infested plants, the mealybug restricts availability of planting materials for subsequent seasons in addition to severe losses of root yield. In collaboration with IITA's Biological Control Program, NARS within the African cassava belt have introduced a number of exotic predators in an attempt to control these pests. Current results show that Apoanagyrus lopezi, a predator of CM, is now established in most infested areas with good control in about 90% of the areas. In East Africa, CM populations have been kept at low levels wherever A. lopezi is established. However, sporadic cases of mealybug outbreak continue to be reported in many release areas of Kenya, Burundi, Rwanda and Uganda, making it necessary to develop the capacity to respond to the new outbreaks with release of A. lopezi and ability to understand the causes of the frequent outbreaks. The initiation in Uganda of an A. lopezi rearing unit will boost the parasitoid populations in the region during the dry season

Cassava green/red mite:
The cassava green mite (CGM) was accidentally introduced into Africa in the early 1970 and has spread in most areas where cassava is grown. It is mainly destructive during dry spells, causing severe defoliation, and stunted growth and yield reduction of up to 40%. However, natural enemies introduced from South America have been released, for their control. Successful establishment of bio-control schemes against CGM has been reported in Kenya and Uganda. N. ideaus has established for 27 months in Katumani but not in the humid coastal zone. Typhlodromalus limonicus and T. aripo have been imported and released in Kenya and Uganda. Preliminary results show that T. aripo has a high potential in controlling the green mite. Selective establishment of these phytoseiids in cassava agro-ecological areas has prompted a survey of distribution of indigenous phytoseiids and their alternative hosts. E. fustis was found in surveyed sites in Uganda and western Kenya. Its wide distribution suggests that it may be the dominant species, which can tolerate variations in temperatures. However being a generalist feeder makes it a less likely candidate. Because CGM is still absent in Madagascar, the only mite of economic importance is the red spider mite. It is observed to be equally destructive as green mite especially under dry weather conditions. Crop loss due to this pest as well as its spread has not been evaluated.

Reports of Neozygitis cf. floridana causing epizootics among population of Mononychellus tanajoa aroused interests in the use of the fungal pathogen as a biological control agent. The pathogenecity of this fungus to the active developmental stages of M. tanajoa was tested. The result indicated the effectiveness of the fungus in reducing mites at some development stages, however differences in time of death among and between sexes needed further investigations. In an effort to control CM and CGM an integrated pest management strategy is being adopted with additional components of weed control, planting dates, plant population, intercropping, resistance screening and use of quality planting material.

Root knot nematodes:
The root knot nematodes (Meloidogyne incognita and M. javanica) are probably the most important of all the plant parasitic nematodes found in association with cassava. They cause physiological disorders which vary from root growth suppression, disruption of water and mineral absorption and translocation, induction of necrotic nutritional deficiencies, temporal wilting during dry and hot stresses, reduced yields and quality of produce. Generally, physical damage caused by nematodes is used as entry points for secondary pathogens including rot-causing organisms. Severity of symptoms is related to the number of juveniles established within the root tissue as influenced by environmental factors such as drought and high temperatures. Use of tolerant and high yielding varieties, manipulations of planting dates, intercropping and soil fertility improvement has shown potential for control of these pests.

Based on the understanding of the local farming systems in various agro-ecological zones, appropriate cultural practices, which will contribute to reduce pest populations, would be identified and evaluated. The most relevant components of beneficial insect populations, better crop varieties and cropping systems will be integrated and evaluated for effectiveness as pest management packages for disease, pest and nematode control.

Cassava root scale:
Cassava root scale is an increasingly important problem on cassava in Central Africa. In the D.R. Congo, it was first reported in the eastern province but has now spread to Bas-Congo in the west, central and northern provinces, respectively. Recent reports mention total yield loss and in some cases arrested production. A report from Cameroon in the mid 1980s noted that 60% of the 450 villages surveyed in the central forest region had problems with tuberization related to this pest. Reports from the IITA Humid Forest Station benchmark sites in Cameroon identify the pest as number one or number two problem of cassava in most villages. Little is known about its natural hosts or indigenous natural enemies and the extent and severity of the problems is not known. There is need to understand its biology, ecology and crop and farming systems interactions in order to be able to formulate control strategies.

Larger grain borer:
Although cassava is easy to grow, the storage roots are difficult to keep after harvest because it deteriorates within a few days. To extend the shelf life, cassava farmers process storage roots into various products such as flour and dry chips. In storage, a whole host of insects, fungi and other microorganisms invade the products. A range of fungi is known to infect cassava chips during processing and handling in the field or storage. Their presence may lead to formation of mycotoxins, making the chip quality very low to meet trade and health standards. The larger grain borer (LGB) Prostephanus truncates (Horn) is the most important pest of stored maize. It is also currently the most serious pest of dried cassava in storage. Weight losses as high as 70% after four months of storage has been reported elsewhere. In Kenya the LGB was first observed in 1983 at Taveta region which is located in Southwestern Kenya. It was accidentally introduced into Kenya from neighboring infested areas in Tanzania. Currently the pest has spread to the eastern province, rift valley province, coast province, central province and most recently to the neighboring district of Busia (Uganda). Its spread is likely to continue because of the unchecked movement of maize grain in the region. In order to develop management strategies that have high chances of adoption by farmers, it is essential to have a clear understanding of the ecology and epidemiology of this potentially dangerous stored product pest.

Variegated and elegant grasshoppers (Zonocerus variegatus and Z. elegans): These are pests of a wide range of crops including cassava. The nymphs and adults cause the major damages. The leaves and green stems are all consumed leaving only white-wooded stumps. The emergence of the nymphs coincides with the beginning of the dry season and damage is usually severe because of slow recovery.

Dut to the seasonality and sporadic nature of outbreak, distribution and severity in relation to crop loss is not well documented. The control of freshly hatched nymphs using insecticide or poisonous bait is recommended. There is however need to understand the ecology and crop farming systems interactions, indigenous natural enemies in order to be able to formulate environmentally friendly control strategies.

In realisation that pests and diseases still take their toll on cassava production (in store or fields), plant protection research needed to address these issues can broadly be grouped into characterisation and adaptive/strategic activities. The major characterisation themes include yield loss due to the CGM, and root scale, and plant diseases like ACMD, CAD, and root rots in specific eco-zones, resistance screening, and soil nutrient trials. These investigations should provide a quantitative basis for deciding whether or not to develop specific pest intervention technologies.

Adaptive/strategic research themes for cassava plant protection and production should include the classical biological control of LGB and CGM, and the identification and integration of sustainable control methods for ACMD, CBB, CAD, the root mealybug, weeds, termites, nematodes and vertebrates and host plant resistance. The development of packages of integrated control methods for root rots will have a direct impact on the quantity and quality of marketable tuberous roots. Methods to protect cuttings from infection by root and stem rot pathogens would be part of the development of protocols for producing and managing clean and vigorous cuttings. Integrated participatory on-farm trials, where appropriate, and the elucidation of indigenous knowledge systems should also be pursued.

Sub themes
¨ Ecology and epidemiology of major diseases and pests
¨ Yield loss assessment
¨ Control of major cassava pests
¨ Assessment of the importance and distribution of understudied pests

Expected output
¨ Effects of soil and climatic factors on the red mite population dynamics documented
¨ The distribution of root scale in the region established.
¨ The life cycle and population dynamics of root scale established
¨ Yield losses due to red mite, root scale and pest complexes in different AEZ documented.
¨ Factors inducing CM resurgence identified.
¨ Bio-insecticidal effect of Manipuera against cassava mealybug evaluated
¨ Effectiveness of T. aripo in biological control of CGM documented
¨ Low cost crop protection control options tested and verified
¨ Economic importance of stem scale and nematodes documented

Indicators for monitoring impact
· Number of scientific publications and reports on red mite and root scale
· Proportion of yield loss due to major pests and diseases known
· Number of strategies to minimize yield loss formulated and evaluated
· Number of sustainable plant protection strategies/options developed and evaluated for the control pest complexes in major ecologies
· Reduction in levels of resurgence of cassava mealybug

Network thrusts and accomplishment

Cassava Mosaic Diseases:
Presently, the mosaic epidemic in Uganda seem to be associated with a new strain of virus, whose genome consist of parts from ACMV and EACMV in a hybrid form. Rigorous screening of promising new cassava through germplasm development, grafting and participatory farmer evaluation has been initiated in collaboration with ESARC and the Uganda national program. Mechanisms for mosaic virus infection were investigated in local and improved cassava genotypes across four locations. Improved varieties sustained very low infections and harbored low whitefly population. Variety resistance to mosaic was found to be due to components that delayed infection, reduced the rate of infection, restricted symptom expression, and to resistance to the vector. A survey of cassava in western Kenya has shown that the virus epidemic has spread some 30-40 km along the border between Mt. Elgon and Lake Victoria. Strategies involving multiplication and distribution of resistant clones to combat the spread was initiated.

A monoclonal-based diagnostics for the East African cassava mosaic virus (EACMV) and the African cassava mosaic virus (ACMV) were assessed under the conditions encountered in NARS laboratories and at locations from eastern to western Kenya and along several sampling transects in Uganda. Only EACMV was detected in eastern Kenya while the two viruses were detected and differentiated in western Kenya. The use of the MAb BGMV detected the occurrence of other strains of EACMV or another geminivirus in the Kilifi district. In Uganda, only ACMV was detected. Six samples collected from the south eastern region showed possible strains of ACMV. Severe strains or multiple infection might explain the higher disease pressure which has led to degeneration of Ebwanateraka.

The African Cassava Mosaic Virus-tolerant varieties provide one key for stabilizing cassava production among the poorest of Eastern and Central Africa's farmers. In north-east and eastern Uganda alone, an estimated 50% of the cassava crop is lost to ACMV annually. The disease, which is spread by white flies and diseased planting materials, is said to be moving at a rate of about 20 km a year and western Kenya is now quickly testing and multiplying ACMV tolerant varieties to prepare for the onslaught. The rapid identification and multiplication of ACMV tolerant varieties, resulting from combined efforts of IITA, national programs and the EARRNET, have helped to stave off some of the massive losses in Zaire, Rwanda, Kenya and Uganda. It is primarily due to the network that tolerant varieties were quickly developed. The ACMV problem was identified in 1988, with the first release already delivered by 1994. The normal breeding process takes at least 8-10 years. Because IITA has some tolerant germplasm varieties (which were themselves built on former East African research), the Uganda national program was able to jump most of the early germplasm development and screening stages.

As of 1996, Uganda has released 3 ACMV tolerant varieties and has 4 more at the end of the pipeline; Kenya is screening the same ACMV-tolerant varieties in advanced yield trials. The network also facilitated the movement of tolerant planting material across national boundaries at a speed not before possible. Here, the networks show their utility for both production increase and disaster prevention.

Cassava mealybug (CM):
Surveys conducted to determine levels of infestation, damage and fauna associated with CM shows an infestation range of 40 - 50% with CM population of 14-50 individuals per tip. Several local insect predator species were found to be associated with CM. Shipments of Apoanagyrus diversicornis (Encyrtidae) Hypersaspis notata, Diomus sp. (Coccinelidae) and Allotropa sp. (Platygastridae) were released. Apoanagyrus lopezi has been found to be spreading with CM.

Cassava Green Mite (CGM):
Survey of distribution of indigenous phytoseiids and their alternative host plants were carried out in three ecological zones. Wide range of phytoseiids were found to vary from one ecology to another. Cassava green mite average densities fluctuated between 50 and 70 in the rainy season to 100 and 133 in the dry season. Typhlodromalus aripo dispersal from release sites can cover 100 km in 20 months. A study of survival and establishment of T. aripo on different cassava suggests that there are differences among varieties on the number of T. aripo they can support, Variety Bao seemed to sustain constant populations when compared to other varieties.

Pathogenicity of Neozygite cf floridana to the active development stages of M. tanajoa was tested at 280C, 12L:12D photoperiod and 90-100% RH. The percent mortality and time to death after infection were significantly affected by the mite developmental stage. Mortality among larvae protonymphs was significantly lower than among adult females and males. Between 76 and 93% of the infected mites died during the scotophase. The test demonstrates the pathogenecity of the fungus.

Root knot nematodes:
Incidence of root-knot nematodes on cassava was investigated at 88 sites confirming Meloidogyne as the cause of galling. Use of one node cuttings to screen germplasm for root-knot nematode resistance has been developed.