African Cassava Mosaic Disease Research Articles

Genetic Structure and Diversity Study of Cassava (<i>Manihot esculenta</i>) Germplasm for African Cassava Mosaic Disease and Fresh Storage Root Yield

A better understanding of population structure and genetic diversity among cassava germplasm for African cassava mosaic disease and fresh root yield traits is useful for cassava improvement programme. Phenotype-based selection for these traits is cumbersome due to phenotypic plasticity and difficulty in screening of phenotypic-induced variations. This study assessed quantitative trait loci (QTL) regions associated with African cassava mosaic disease (ACMD) and fresh storage root yield (FSRY) in 131 cassava (Manihot esculenta) genotypes using a genome-wide association study (GWAS). The single nucleotide polymorphism (SNP) loci and associated candidate genes, when validated, would be a valuable resource for marker-assisted selection in the breeding process for development of new cassava genotypes with improved resistance to ACMD and desirable high root yield. Population structure analysis using 12,500 SNPs differentiated the 131 genotypes into five distinct sub-groups (K = 5). Marker-trait association (MTA) analysis using the generalized linear model identified two QTL regions significant for ACMD and three for FSRY. This study demonstrated that DArTseq markers are useful genomic resources for genome-wide association studies of ACMD and FSRY traits in cassava for the acceleration of varietal development and release.

Genetic diversity of provitamin-A cassava (Manihot esculenta Crantz) in Sierra Leone

Understanding the genetic diversity among accessions and germplasm is an important requirement for crop development as it allows for the selection of diverse parental combinations for enhancing genetic gain in varietal selection, advancement and release. The study aimed to characterize 183 provitamin A cassava (Manihot esculenta Crantz) accessions and five Sierra Leonean varieties using morphological traits, total carotenoid content and SNP markers to develop a collection for conservation and further use in the cassava breeding program. Both morphological parameters and 5634 SNP markers were used to assess the diversity among the provitamin-A cassava accessions and varieties. Significant differences were observed among the accessions for most of the traits measured. The first five PCs together accounted for 70.44% of the total phenotypic variation based on yield and yield components among the 183 provitamin-A cassava accessions and five Sierra Leonean varieties. The present study showed that provitamin-A cassava accessions in Sierra Leone have moderate to high diversity based on morphological and molecular assessment studies. The similarity index among the 187 and 185 cassava accessions grouped them into 6 and 9 distinct clusters based on morphological and molecular analyses, respectively. A significant positive, but low correlation (r = 0.104; p < 0.034), was observed between the two dendrograms. The results obtained will serve as a guide and basis of germplasm management and improvement for total carotenoid content, yield and African cassava mosaic disease resistance in Sierra Leone.

Field Evaluation of Some Cassava Cultivars against the African Cassava Mosaic Disease in the Humid Forests of Cameroon

There is a considerable deficit in the annual production of cassava in Cameroon of about 31 million tons, and this has been mainly due to constraints related to pest attacks and most especially diseases like the African Cassava Mosaic Disease (ACMD). This study was therefore undertaken on three sites in the locality of Bityili (South Region of Cameroon), to determine amongst improved and local cassava cultivars those that provide resistance to the development of the ACMD. The severity and incidence of this disease was evaluated and its impact on cassava yield. In each site, cassava was grown in a randomized complete block design. The local cultivars (Ekobele and Ngon kribi) showed higher severity (76-100%) and incidence of ACMD compared to the improved cultivars (TMS 92/0326 and TMS 96/1414) of 0-25% and 0-10%, respectively. Strong inverse correlations were observed between ACMD severity and yield performance, measured in terms of number of tubers/plant and weight of fresh tubers. The improved cultivars, TMS 92/0326 and TMS 96/1414, could, therefore, be recommended for large-scale planting in a bid to promote cassava production in the South Region of Cameroon.

Effect of African cassava mosaic disease on morphological characteristics of cassava, Manihot esculenta (Crantz)

Field trials were conducted with 10 improved clones of cassava to study the effect of ACMD on their morphological characteristics. The clones used were: 97 / 4763, 97/2205, 91/02324, 98/0505, M98/0068, 96/1089A, 96/1632, M98/0040, 99/2123 and 97/0162. The parameters measured were plant height, leaf area, number of branches and petiole length. The research revealed differences within the clones for the parameters studied, these differences seemed to be mainly genetic, as they were not positively correlated to the disease incidence and severity. Based on the findings in this research the following recommendations were made. On the basis of their branching pattern, highly branched clones 97 / 2205, 97/4763 and 98 /0505 are recommended for selection. The heavy branches bear a lot of leaves or produce a thick canopy which helps to reduce the need for weeding, and also reduce the effect of loss of leaves that occur due to disease incidence. However, for industrial / mechanized agriculture, clones that are less branched should be selected as they are easier to harvest mechanically. Though there is disadvantage with their small leaf areas; the crops would have to be highly resistant to fungal attack which tends to cause leaf drop that will reduce leaf material for food / starch production (examples of less branching clones are, M98 / 0068, 96/1632,91/02324).Key Word: African Cassava Mosaic Disease (ACMD), Cassava Mosaic Virus.(CMV)

Incidence, Severity and Gravity of Cassava Mosaic Disease in Savannah Agro-Ecological Region of DR-Congo: Analysis of Agro-Environmental Factors

African Cassava mosaic disease (ACMD) is the most severe and widespread disease caused by viruses limiting production of the crop in sub-Saharan Africa. The objective of the present study was to evaluate CMD incidence, severity, and gravity under different agro-environmental conditions. A total of 222 fields were surveyed in 23 different locations. All the farmers grow only local cassava varieties without applications of fertilizers. Overall, mean CMD incidences for all sites surveyed were 58.2% and 51.7%, in 2009 and 2010, respectively. Disease severity ranged from 2.4 to 3.1 on a scale of 1 to 5. Mean disease gravity varied from 29.7% to 62%, in 2010, and 2009, respectively. Detailed analysis of agronomic and environmental revealed no significant association between cassava stand locations, age, land topography and the development of CMD. Likewise intercropping practices and field topping did not affect the development of CMD in all the fields surveyed. There were significant differences in the number of white flies (Bemisia tabaci) per plant in 2009 and 2010, but no significant correlations between the number of B. tabaci per plant and CMD incidence, severity, and gravity was found. In most fields, CMD appears to originate mostly from unhealthy cassava cuttings used for planting.

What do complex adaptive systems look like and what are the implications for innovation policy?

AbstractThis paper explores the use of complex adaptive systems theory in development policy analysis using a case study drawn from recent events in Uganda. It documents the changes that took place in the farming system in Soroti district during an outbreak of African cassava mosaic virus disease (ACMVD) and the subsequent decline in cassava production—the main staple food in the area. Resultant adaptation impacts are analysed across cropping, biological, economic and social systems each of which operate as an interlinked sub‐systems. The policy implications of this story suggest a policy agenda that recognises adaptation capacity as the life blood of complex adaptive systems. Since these types of system are found in all realms of human activity, it follows that strengthening this capacity is a key developmental priority that requires linking together new configurations of actors and resources to tackle an ever changing set of contexts. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Genotype X environment effects on cassava response to the african cassava mosaic disease in derived guinea savannah of Nigeria

Genotype X environment effects on cassava response to the african cassava mosaic disease in derived guinea savannah of Nigeria

Sustainable production of root and tuber crops (potato, sweet potato, indigenous potato, cassava) in southern Africa

Africa, including South Africa, is faced with a problem of increasing rural poverty that leads to increasing urbanisation, joblessness, crime, food insecurity and malnutrition. Root and tuber crops such as sweet potato and potato, as well as cassava and indigenous potato are important crops for food security. The latter are also important due to their tolerance to marginal conditions. Potato and sweet potato are of great economic value in South Africa, with well-organised marketing chains and, for potato, a large processing industry. There is one cassava starch extraction factory in operation in South Africa. A number of diseases are of importance in potato in South Africa: early blight, late blight, bacterial wilt, scab and virus. Insect pests such as tuber moth and leaf miner are also constraints. In sweet potato the occurrence of viruses and weevils, as well as the availability of healthy planting material are the most important limiting factors in production. African Cassava Mosaic Disease (CMD) caused by a virus, is a problem in growing cassava. Plant biotechnology applications offer a number of sustainable solutions. Basic applications such as in vitro genebanking where large numbers of accessions can be maintained in a small space, meristem cultures to produce virus-free plants and mass propagation of popular cultivars in order to make planting material available for sustainable production. More advanced biotechnology applications that may be of value are molecular marker technology and genetic engineering. The latter can play a role in overcoming virus and potato tuber moth in potato, in resistance to CMD in cassava and possibly in sweet potato to incorporate virus and weevil resistance.

Effect of Detopping on Disease Incidence and Symptom Severity of African Cassava Mosaic Virus Disease (ACMD) on Some Newly Developed Cassava Cultivars from Landraces Introgression

Twenty-five cassava genotypes were exposed to natural infection by African cassava mosaic disease (ACMD) in plots at the International Institute of Tropical Agriculture (IITA), Ibadan (forest-savanna transition zone), Nigeria. The effect of removing the shoot tips (detopping) of cassava plants on disease incidence and symptom severity was assessed fortnightly for 14 weeks, starting 8 weeks after planting (WAP). There were highly significant differences (P < 0.01) in disease incidence (DI) and symptom severity (ISS) among clones throughout the period of observation, indicating different levels of resistance to infection. Detopping produced a significant effect on disease incidence in clones 91/02322, 91/02324, 91/02327 and 92/0427. It also had a significant effect on symptom severity of clones 92/0342, M94/0177 and TMS 4(2) 1425. The interaction between the treatment (TRT) and clone was highly significant (P < 0.01) for DI and ISS throughout the period of observation, an indication that there are differential responses of the clones to detopping. Cassava genotypes M94/0121 and Isu were observed to be highly resistant and highly susceptible respectively to the disease, while plants of genotypes 82/00058 and 91/02322 showed moderate susceptibility. None of the genotypes was immune to the disease. There was also a highly significant and positive correlation between DI and ISS in both detopped and undetopped plants. A conclusion from this study is that removal of shoot tips from moderately resistant cassava clones for consumption should be discouraged as it increases the severity of ACMD infection in the regenerating shoots of these clones.

The Relative Resistance of Cassava Cultivars to African Cassava Mosaic Disease (ACMD) as Determined by Two Methods: Rank-Sum and the Area Under the Disease Progress Curve

Twenty-five newly bred improved cassava cultivars, twenty-three improved from the International Institute of Tropical Agriculture and two local cultivars were evaluated for their relative resistance to African cassava mosaic begomovirus disease (ACMD) at Ibadan, in an area of high disease pressure representative of the forest/savanna transition zone of Nigeria. These cultivars were exposed to natural infection by the viruliferous whiteflies ( Bemisia tabaci Gennadius) and the disease incidence (DI) and index of symptom severity (ISS) were assessed for all clones. Results for the Rank-sum (i.e., sum of ranks for DI and ISS for each cultivar) and the area under the disease progress curve (AUDPC) were used to assess the relative resistance of the cassava clones. Those that showed low Rank-sum and AUDPC values were rated 'moderately resistant (MR)', 'resistant ( R )', and 'highly resistant (HR)' to ACMD depending on their respective values and deviation from the mean distribution curve. Clones M94/0121 and 94/0239 were rated HR under the two methods. Clone M94/0583 was rated HR under the AUDPC with a deviation from the mean distribution curve of m 2.00 while it was rated R under the Rank-sum method with a deviation from the mean distribution of m 1.99. Also plants of clones ISU and TMS 30572 were rated highly susceptible (HS) under both methods. Clone TME-1 was intermediate between Moderately resistant (MR) and Moderately susceptible (MS) under the AUDPC method with a deviation from the mean distribution of 0.00 but observed to be MS under the Rank-sum method with a deviation of + 0.2. The two methods of evaluation gave similar results as revealed by Spearman rank correlation ( r equals; 0.99, P <0.01). However, the AUDPC method is less cumbersome compared to the Rank-sum method. None of the clones was observed to be immune to the disease.

Response of cassava genotypes to four biotic constraints in three agro-ecologies of Nigeria

(Received 13 February, 2001; accepted 14 November, 2001) Abstract Eight improved cassava (Manihot esculenta Crantz) genotypes and one local variety were grown over three years in three agro-ecological zones of Nigeria, to study their response to natural infestations of African cassava mosaic disease (ACMD), cassava bacterial blight (CBB), cassava anthracnose disease (CAD) and cassava green mite (CGM). Additionally, genotypes with stable resistance using the Additive Main Effects and Multiplicative Interaction (AMMI) statistical model was identified. Environments, genotypes and genotype x environment (G x E) interactions were highly significant (P < 0.0001) for the pests, implying that the G x E effects were sufficiently high to mask differences among genotypes. The local variety, TME1, was more tolerant (30% severity) to cassava green mite across sites, compared to other clones, while clone 30001 exhibited the highest susceptibility to the pest (54%). Genotypes (30001, TME1 and 30573) gave lowest severity scores for ACMD. Clones used in the study, especially U/41044, 4(2) 1425 and 63397, showed tolerance to CBB and CAD with severity ratings as low as 2.0. Across test sites, CGM had the highest severity scores (3.12) averaged over 3 years in Ilorin (in the Southern Guinea savanna ecology). The lowest severity scores of this disease were observed in Ubiaja and Onne. The severity of ACMD and CBB was highest in Ibadan and Ubiaja and lowest in Onne and Owerri. Cassava anthracnose disease was more severe in Ibadan (score of 2.5) and least severe in Mokwa (1.12). For CGM the highest severity was recorded in the Southern Guinea savanna (score 2.88) and lowest in the humid forest zone. Correlation coefficients calculated between pairs of biotic constraints revealed that cassava bacterial blight was negatively correlated (r=-0.78) with the green mite but positively correlated with anthracnose disease (r=0.94). The AMMI model selected AMM11 for CGM and CBB, and AMM14 and AMM13 for ACMD and CAD, respectively, as the best predictive models since these models had the smallest actual root mean square prediction differences (0.60598 and 0.37646 for CGM and CBB, respectively and 0.37297; and 0.40929 for ACMD and CAD, respectively. Genotypes 30572, 63397, 50395 and 4(2)1425 with low ACMD scores and low IPCA1 scores were the most stable to environmental changes as they combined resistance with stability of response. Such genotypes would be most appropriate in breeding programmes. Key Words: African cassava, anthracnose, bacterial blight, green mite, mosaic disease Résumé Un essai a été mené sur huit clones améliorés de génotypes de manioc et une variété locale, dans trois zones agro-écologiques du Nigéria, pour étudier leur comportement à la mosaïque, bactériose, anthracnose et acarien vert du manioc. L'étude s'était étalée sur une période de plus de trois ans. Additionnellement, les génotypes à résistance stable étaient identifiés en utilisant la méthode statistique des effets principaux additifs et intéractions multiplicatives. Les effets de l'environnement, du génotype et l'intéraction génotype et l'environnement (GXE) étaient significativement élévées (P

The effect of controlling Mononychellus tanajoa (Acari:Tetranychidae) the cassava green spider mite using Typhlodromalus aripo (Acari: phytoseiidae) on the severity of cassava diseases in Transition forest, Nigeria

An experiment to control the cassava green spider mite (CGM) Mononychellus tanajoa Bondar, an exotic pest of cassava, using an exotic predator Typhlodromalus aripo in cassava fields in the transition forest in Nigeria revealed a significant effect on the severity of cassava diseases. High population densities of T. aripo/cassava leaf in cassava fields were associated with low disease severity of cassava bacterial blight, cassava anthracnose disease and African cassava mosaic disease. Also, the population densities of the T. aripo did not have a significant effect on the severities of cercospora leaf blight, brown leaf spot and white leaf spot. Controlling the CGM using T. aripo therefore reduced the severity of 3 most important diseases of cassava in the transitional forest zone.

A method for determining tolerance of cassava genotypes to African cassava mosaic disease in the screenhouse

An investigation was conducted into the primary incidence and severity of the African cassava mosaic disease (ACMD) in four exotic cassava cultivars TMS 4(2) 1425, TMS 30572, TMS 50395 and TMS 91934 at Fumesua. TMS 4(2) 1425, TMS 50395 and TMS 30572 showed tolerance to ACMD by expressing low primary incidence and severity, while TMS 91934 and ‘Ankra' were susceptible to ACMD by showing a relatively high primary incidence and severity. IT was also observed that temperature variations had an effect on virus expression of cassava seedlings in the screenhouse. The methodology developed for this screenhouse study could be a useful tool in determining tolerance of cassava genotypes to ACMD in Ghana. Une méthode pour la détermination de la tolérance de génotype de manioc à la maladie mosaïque de manioc africain (MMMA). Résumé Une enquête de la fréquence primaire et la sévérité de la maladie mosaïque de manioc africain (MMMA) en quatre variétés de manioc exotique TMS 4(2) 1425, TMS 30572, TMS 50395 et TMS 91934 s'est déroulée à Fumesua. TMS 4(2) 1425, TMS 50395 et TMS 30572 montraient la tolérance à MMMA par la manifestation de basse fréquence primaire et la sévérité alors que TMS 91934 et ‘Ankra' étaient susceptibles à MMMA par la manifestation d'une fréquence primaire et une sévérité relativement élevées. Il était également observé que les variations de températures avaient un effet sur la manifestation de virus de semis de manioc dans la cage de Faraday. La méthodologie developpée pour cet etude en cage de Faraday pourrait être un outil utile pour la détermination de la tolérance de génotypes de manioc à MMMA au Ghana. Ghana Jnl agric. Sci. Vol.33(1) 2000: 29-32

Bemisia tabaci (Hemiptera: Aleyrodidae) trap catches in a cassava field in Côte d'Ivoire in relation to environmental factors and the distribution of African cassava mosaic disease

AbstractAfrican cassava mosaic virus (ACMV), vectored by the whitefly, Bermisia tabaci (Gennadius), is considered the most damaging pathogen of any African crop. Information about vector movement is important for understanding the epidemiology of this disease and the experiments reported here were designed to examine B.tabaci flight activity both in and around a cassava crop in relation to time of day, crop growth stage, wind direction and speed, and to the resulting pattern of infected plants within the field at harvest. At wind speeds of &lt;0.4 ms−1 adult B. tabaci approached the yellow traps by flying upwind. At greater wind speeds, significantly fewer B. tabaci adults approached the traps from downwind, thus reversing the directionally of the catch. When the direction of the prevailing south-west wind reversed, so did the directionality of the catch. Bemisia tabaci adults were flight active throughout the day and the greatest percentage were caught above the canopy between 06.00–08.00 h, when wind speeds were lowest. Trap height and position significantly affected catch with the greatest numbers caught on the lowest traps. More than three times as many B. tabaci adults were caught on traps situated downwind from the field compared to those upwind. Suggesting that the field was acting as a source of whiteflies. In both years, Africa cassava mosaic disease (ACMD) incidence was highest and lowest, respectively, on the edges and in the middle of the trials, with the highest incidence occuring on the edges facing the prevailing wind direction. These results are discussed in relation to the epidemiology of ACMD and to potential cultural control methods such as the use of ACMD-resistant guard rows to protect a mainly susceptible crop.

Changes in the incidence of African cassava mosaic virus disease and the abundance of its whitefly vector along south–north transects in Uganda

Abstract: Surveys of the incidence of African cassava mosaic disease (ACMD) and its whitefly vector, Bemisia tabaci, were made at three‐month intervals in 1992 and 1993 along two south–north transects. One was in central and the other in eastern Uganda. ACMD incidence was high within the northern localities of each transect and low towards the south. Incidence increased over the period at localities on the ACMD epidemic ‘front’ which was the interface between high incidence areas to the north and low incidence areas to the south. Combining data for both transects, the relationship between ‘infective potential’ (the product of whitefly number and the incidence of ACMD attributed to the use of infected cuttings, ‘x’) and the increase in proportion of infected plants caused by whiteflies (‘y’, as transformed to account for multiple infection) was described by the straight line regression: y = 47.8x + 9.2 (F ratio = 63.0, r2 = 0.78). This emphasises the importance for control of reducing ACMD incidence by phytosanitation and/or use of resistant varieties since reduction of whitefly numbers is not easily achieved. Possible mechanisms for the expansion of the ACMD epidemic and implications of the study findings for control of ACMD are discussed.

Incidence and severity of African cassava mosaic disease (ACMD) and cassava bacterial blight (CBB) on some local and exotic cassava varieties in different ecological zones of Ghana

Seven exotic varieties of cassava, namely TMS 30001, TMS 91934, TMS 4(2)1425, TMS 30555, TMS 50395, TMS 63397, and TMS 30572 developed in IITA, Ibadan, Nigeria, and three local varieties, namely 'Ankra', 'Bosomensia' and 'Katawia' were evaluated for African cassava mosaic disease (ACMD) and cassava bacterial blight (CBB) resistance in five ecological zones of Ghana. The trials were conducted in experimental fields of Crops Research Institute at Fumesua (Forest zone) in 1988, 1989, 1990; Aiyinase (High Rain Forest zone), Pokuase and Ohawu (Coastal Savanna zone), and Ejura (Forest-Savanna Transition zone) in 1989 and 1990; Techiman (Forest-Savanna Transition zone); and Nyankpala and Damongo (Guinea Savanna zone) in 1990. ACMD and CBB damage at 3 and 6 months after planting were scored on a 1-5 scale based on the severity of the diseases. The exotic varieties were significantly more tolerant to ACMD across the 3 years at Fumesua as well as in 1989 and 1990 across all locations. For CBB in 1990, the exotic varieties were significantly more tolerant than the local varieties in the Coastal Savanna zone where CBB incidence and severity was relatively high. Ghana Jnl agric. Sci. Vol.31(1) 1998: 35-43

An Epidemilogical Model Incorporating Vector Population Dynamics Applied to African Cassava Mosaic Virus Disease

An epidemiological model of African cassava mosaic virus disease (ACMD) was developed in which the dynamics within a locality, of healthy and infected cassava, and of infective and non-infective whitefly vectors, were specified. Cassava is propagated routinely from stem cuttings. If the exclusive use of virus-free cuttings was assumed then either a high rate of virus transmission by the vector or a large population of vectors could lead to persistent cycles in disease incidence. Such cycling could seriously impair cassava production. When frequency-dependent use of infected cuttings was admitted, however, persistent cycles of infection were no longer possible. Instead, three states could occur: disease eliminated (E), both healthy and diseased cassava persisted (P), or infection became total (T). The outcome depended on the particular parameter values, but some parameter combinations gave state P or T, depending on the initial conditions. In some instances, low initial incidence of ACMD posed a greater risk of progressing to state T than did higher levels of infection. In general, disease incidence exhibited a discontinuous, threshold response to changes in the proportion of infected cuttings used. An increase in their use could appear to have little impact on incidence, although the system was moving closer to a point of sudden increase in infection. Simulation of the removal (roguing) of diseased plants had little influence on disease incidence, but had an important and largely cryptic effect by helping to move the system away from the threshold of collapse of the healthy cassava population. Intensification of cassava production (represented in the model as an increase in the maximum possible abundance of cassava) had the opposite effect, moving the system closer to this threshold. Assuming a degree of disease 'reversion', whereby a proportion of the cuttings derived from infected plants are uninfected, had a stabilizing influence on the model, making even damped cycles of disease very unlikely. Total infection did not now occur except at very low reversion rates. The response of disease incidence to the increased use of infected cuttings was no longer discontinuous but remained nonlinear and there was a region of rapid change corresponding to the threshold seen in the absence of reversion.

Cassava virus diseases and their control with special reference to southern Tanzania

Cassava is a major smallholder crop in much of Africa where it is attacked by two main virus diseases. African cassava mosaic disease (ACMD) occurs almost everywhere that the crop is grown causing severe losses in some countries. Cassava brown streak disease (CBSD) is of more restricted distribution being prevalent mainly on the east African coast and shores of Lake Malawi. Although both diseases have been known for many decades and much is known about ACMD, the aetiology and epidemiology of CBSD remain poorly understood. Control measures for African cassava mosaic virus (ACMV) have been recommended and in some cases implemented in a number of countries. Resistant varieties have been developed and national research programmes and international agencies are supporting phytosanitation programmes, based mainly on the distribution of ACMV-free planting material. It may be possible to use the same control measures against CBSD but the lack of basic information on the disease and difficulties of disease diagnosis are obstacles to the design of control strategies. ACMD is found in most areas where CBSD occurs and control measures must con sider the disease complex. This paper reviews the current knowledge about the two diseases in the context of possible integrated control

Analysis and Mapping of the Spatial Spread of African Cassava Mosaic Virus Using Geostatistics and the Kriging Technique

Lecoustre, R., Fargette, D., Fauquet, C., and de Reffye, P. 1989. Analysis and mapping of the spatial spread of African cassava mosaic virus using geostatistics and the kriging technique. Phytopathology 79: 913-920. Theories of regionalized variables and kriging were used to assess the incidence along the wind-exposed southwest field borders, disease spatial pattern of African cassava mosaic virus (ACMV). A linearlike gradients, and other less obvious features. Up to 60% of the total variance semivariogram without a range characterizes the ACMV distribution and was reconstructed from a 7% sample. Kriging was successfully applied indicates a strongly spatially dependent structure with limited random to characterize the spatial pattern of spread in cassava fields differing variation. Oriented semivariograms reveal a strong anisotropy in relation in planting date, size, arrangement, orientation, and method of sampling. to the prevailing wind direction. Further features of the semivariogram This technique was also efficient when the pattern of spread was and comparisons of semivariograms between fields and between surveys heterogenous, although more intensive surveys were then required. provide additional information and support various hypotheses on the Practical applications of geostatistics and kriging in epidemiology are pattern of spread. From a sample of limited size, kriging reproduced discussed. the main characteristics of the spatial pattern of spread, including higher Spatial patterns of disease level can provide important clues and pedology (21). to the ecology of disease (e.g., direction and distance of spread, African cassava mosaic disease is caused by a whitefly-borne importance and proximity of the sources of virus and vectors, geminivirus (2). The spatial patterns of spread of this disease vector mobility) (20). Patterns must be considered in designing have been studied intensively in the Ivory Coast (5,6) and are sampling methods and sound control measures (8,20). Hence, mainly characterized by gradients oriented in the direction of efficient methods of analysis and interpretation of spatial pattern the prevailing southwest wind. In this article, the theory of are needed to provide the greatest possible information in relation regionalized variables is used to assess the spatial patterns of to the time and effort involved. Past studies of spatial patterns the spread of African cassava mosaic virus (ACMV) in various have relied mostly on methods based on the examination of the cassava fields that differ in total area, subplot size, planting dates, mean and variance or on the frequency distribution of observed and orientation. We also describe the application of kriging to

Relations Between Biomass Accumulation and Infection by Vesicular‐Arbuscular Mycorrhizal Fungi in Cassava as Affected by Planting Date

AbstractRoots of 12, 3‐month‐old, field‐propagated clones of cassava (Manihot esculenta Crantz) were more heavily mycorrhizal in the dry (Jan.–March and Oct.–Dec.) (39–83 %) than in the wet season (April–Sept.) (20–71%). Mean dry weights (biomass) of roots and shoots in the wet season (3.8–7.9 and 9.7–19.1 g/plant) were higher (P = 0.01) than in the dry season (2.1–5.9 and 5.4–12.8 g/plant, respectively). Clones with symptoms of the African cassava mosaic disease (ACMD) were less mycorrhizal, (20–69%) than mosaic symptom‐free clones (51–83%). Higher colonization of roots of the clones by indigenous fungal symbionts and lower biomass accumulation in the dry season are attributed mainly to soil moisture and other effects, while reduced infections in cassava with ACMD symptoms may be due to ACMD‐induced reductions in carbohydrate levels.