EFFECT OF TEMPERATURE AND CASSAVA GENOTYPE ON THE DEVELOPMENT, FECUNDITY AND REPRODUCTION OF Bemisia tabaci SSA1

The Bemisia tabaci complex is currently recognised as key agricultural pests that cause economic damage globally. Temperature is the most important driver of changes in behaviour, abundance and distribution of insect pests, including the whitefly (Bemisia tabaci). The objective of this study was to evaluate the development, fecundity and reproduction of B. tabaci SSA1 on cassava genotypes under a range of temperatures. A laboratory study was conducted using three cassava genotypes (Alado alado, NAROCASS 1 and NASE 14) at five constant temperatures (16, 20, 24, 28 and 32 \ub0C). The parameters assessed included development duration, survival, fecundity and population parameters for B. tabaci SSA1. Temperature had significant effects (P<0.001) on development time, survival and fecundity of B. tabaci; while cassava genotype had no effect (P>0.05). An inverse relationship was observed between development time and temperature for all stages across all cassava genotypes. The total life cycle was 63.8 days at 16 \ub0C and 17.9 days at 32 \ub0C on NAROCASS 1. Survival for each stage throughout the entire life cycle increased with temperature and was highest at 32 \ub0C, although this was not significantly different from that at 28 \ub0C. Fecundity increased with temperature and was highest at 32 \ub0C on all cassava genotypes. For all cassava genotypes, the intrinsic rate of increase (rm), finite rate of increase (\u3bb) and net reproductive rate (Ro) increased with temperature, while mean generation time (T) reduced following a similar pattern. At 32 \ub0C, rm, Ro, \u3bb and T were 0.2, 48.7, 1.2 and 22.6 days, respectively; compared to 0.01, 1.9, 1.0 and 71.2 days at 16 \ub0C on Alado alado. Therefore, the ideal development temperature for B. tabaci SSA1 is 32 \ub0C. Thus, there is a risk of accelerated future expansion of B. tabaci SSA1 populations globally, with global warming and climate variability.Le complexe Bemisia tabaci est actuellement reconnu comme un ravageur agricole cl\ue9 causant des dommages \ue9conomiques \ue0 l\u2019\ue9chelle mondiale. La temp\ue9rature est le facteur le plus important des changements de comportement, d\u2019abondance et de r\ue9partition des insectes ravageurs, y compris l\u2019aleurode (Bemisia tabaci). L\u2019objectif de cette \ue9tude \ue9tait d\u2019\ue9valuer le d\ue9veloppement, la f\ue9condit\ue9 et la reproduction de B. tabaci SSA1 sur des g\ue9notypes de manioc sous une gamme de temp\ue9ratures. Une \ue9tude en laboratoire a \ue9t\ue9 men\ue9e en utilisant trois g\ue9notypes de manioc (Alado alado, NAROCASS 1 et NASE 14) \ue0 cinq temp\ue9ratures constantes (16, 20, 24, 28 et 32 \ub0C). Les param\ue8tres \ue9valu\ue9s comprenaient la dur\ue9e du d\ue9veloppement, la survie, la f\ue9condit\ue9 et les param\ue8tres de population pour B. tabaci SSA1. La temp\ue9rature a eu des effets significatifs (P<0,001) sur le temps de d\ue9veloppement, la survie et la f\ue9condit\ue9 de B. tabaci, tandis que le g\ue9notype du manioc n\u2019a eu aucun effet (p>0,05). Une relation inverse a \ue9t\ue9 observ\ue9e entre le temps de d\ue9veloppement et la temp\ue9rature pour tous les stades dans tous les g\ue9notypes de manioc. Le cycle de vie total \ue9tait de 63,8 jours \ue0 16 \ub0C et de 17,9 jours \ue0 32 \ub0C sur NAROCASS 1. La survie pour chaque \ue9tape tout au long du cycle de vie entier augmentait avec la temp\ue9rature et \ue9tait maximale \ue0 32 \ub0C. Cependant, la survie \ue0 28 \ub0C n\u2019\ue9tait pas significativement diff\ue9rente de celle observ\ue9e \ue0 32 \ub0C. La f\ue9condit\ue9 augmentait avec la temp\ue9rature et \ue9tait maximale \ue0 32 \ub0C sur tous les g\ue9notypes de manioc. Pour tous les g\ue9notypes de manioc, le taux d\u2019accroissement intrins\ue8que (rm), le taux d\u2019accroissement fini (\u3bb) et le taux net de reproduction (Ro) ont augment\ue9 avec la temp\ue9rature, tandis que le temps de g\ue9n\ue9ration moyen (T) a diminu\ue9 selon un sch\ue9ma similaire. A 32 \ub0C, rm, Ro, \u3bb et T \ue9taient respectivement de 0,2, 48,7, 1,2 et 22,6 jours ; contre 0,01, 1,9, 1,0 et 71,2 jours \ue0 16 \ub0C sur Alado alado. Par cons\ue9quent, d\u2019apr\ue8s cette \ue9tude, la temp\ue9rature de d\ue9veloppement id\ue9ale pour B. tabaci SSA1 est de 32 \ub0C. Ainsi, il existe un risque d\u2019expansion future acc\ue9l\ue9r\ue9e des populations de B. tabaci SSA1 \ue0 l\u2019\ue9chelle mondiale, avec le r\ue9chauffement climatique et la variabilit\ue9 climatique

CASSAVA BROWN STREAK DISEASE EFFECTS ON LEAF METABOLITES AND PIGMENT ACCUMULATION

Cassava Brown Streak Disease (CBSD) is a threat to productivity and product quality in East Africa. The objective of this study was to understand the effect of CBSD on the primary photosynthetic apparatus of cassava ( Manihot esculenta Crantz). Three cassava varieties with varying levels of reaction to infection by CBSD were chosen and subjected to field disease pressure. Disease progression and the resulting effects on leaf morphology, metabolite and pigment accumulation were assessed over a period of five months, beginning from 3 months after planting. Slight increments in leaf dry matter were observed up to 4 MAP, and there after a drop was registered. A significant (P<0.05) reduction in photosynthetic pigments occured with a fall in the Chla:Chlb and Chla:Carotenoid ratio, indicative of specific reductions in chlorophyll a (-80%) compared to Chlorophyll b (-41 to -62%) and Carotenoid (-11 to -18%). Total reducing sugar and starch content also dropped significantly (-30 and -60%, respectively), much as NASE 14 maintained a relatively higher amount of carbohydrates. Leaf protein levels were significantly reduced at a rate of 0.07 ug g-1 leaf per month in diseased treatments. Significant reductions in primary metabolites show altered leaf photosynthetic and growth metabolism, resulting into a compromised plant system that cannot perform optimally.La Maladie Stri\ue9e Brune du Manioc (CBSD) constitue une menace pour la productivit\ue9 et la qualit\ue9 des produits du manioc en Afrique de l\u2019Est. L\u2019objectif de cette \ue9tude \ue9tait de comprendre l\u2019effet de CBSD sur l\u2019appareil photosynth\ue9tique primaire du manioc ( Manihot esculenta Crantz). Trois vari\ue9t\ue9s de manioc poss\ue9dant differents niveaux de r\ue9sistance \ue0 CBSD ont \ue9t\ue9 soumises \ue0 la pression dans les conditions de plein champs. La progression de la maladie et son effet sur la morphologie foliaire, les metabolites et accumulations de pigments ont \ue9t\ue9 \ue9valu\ue9s sur une p\ue9riode de cinq mois, \ue0 compter du troisi\ue8me mois apr\ue8s semis. Il a \ue9t\ue9 observ\ue9e une l\ue9g\ue8re augmentation de mati\ue8re s\ue8che foliaire jusqu\u2019\ue0 4 MAP, suivie d\u2019une chute. Une r\ue9duction significative (P<0,05) des pigments photosynth\ue9tiques a \ue9t\ue9 observ\ue9e avec une chute dans le Chla:Chlb et Chla:ratio de Carotenoide, indiquant des r\ue9ductions sp\ue9cifique sen chlorophyllea (-80%) en comparaison avec la Chlorophylleb (-41 \ue0 -62%) et les Carotenoides (-11 \ue0 -18%). La r\ue9duction du sucre et d\u2019amidona connu aussi une diminution significative (-30 et -60%, respectivement), de m\ueame NASE 14 a maintenuun equantit\ue9 relativement\ue9 lev\ue9e d\u2019hydrates de carbone. Les niveaux de proteines foliaires \ue9taient significativement r\ue9duits \ue0 0,07 ug g-1 de feuille malade par mois. Des r\ue9ductions significatives des metabolites primaires sont \ue0 l\u2019origne de l\u2019alt\ue9ration du m\ue9tabolisme photosynth\ue9tique et de croissance, resultant \ue0 cycle v\ue9g\ue9tatif compromis qui ne peut permettre un d\ue9veloppement optimal de la plante

Cassava whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), in sub-Saharan African farming landscapes: a review of the factors determining abundance

Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is a pest species complex that causes widespread damage to cassava, a staple food crop for millions of smallholder households in Sub-Saharan Africa. Species in the complex cause direct feeding damage to cassava and are the vectors of multiple plant viruses. Whilst significant work has gone into developing virus-resistant cassava cultivars, there has been little research effort aimed at understanding the ecology of these insect vectors. In this review we critically assess the knowledge base relating to factors that may lead to high population densities of Sub-Saharan African (SSA) Bemisia tabaci species in cassava production landscapes of East Africa. We focus first on empirical studies that have examined biotic or abiotic factors that may lead to high populations. We then identify knowledge gaps that need to be filled to deliver long-term sustainable solutions to manage both the vectors and the viruses that they transmit. We found that whilst many hypotheses have been put forward to explain the increases in abundance witnessed since the early 1990s, there are little available published data and these tend to have been collected in a piecemeal manner. The most critical knowledge gaps identified were: (i) understanding how cassava cultivars and alternative host plants impact B. tabaci population dynamics and its natural enemies; (ii) the impact of natural enemies in terms of reducing the frequency of outbreaks and (iii) the use and management of insecticides to delay or avoid the development of resistance. In addition, there are several fundamental methodologies that need to be developed and deployed in East Africa to address some of the more challenging knowledge gaps

Re-emergence of Cassava Brown Streak Disease in Uganda

During November 2004, veinal chlorosis on mature cassava leaves, typical of cassava brown streak disease (CBSD), was observed at Mukono in central Uganda. Five out of 11 cultivars at the site showed CBSD symptoms (incidence range 4 to 64%). In a survey of farmers' fields, CBSD was observed in Wakiso and Mukono districts. Incidence of cassava mosaic disease was also recorded and averaged 60% for landraces (range 16.7 to 100%) and 20% for resistant varieties (range 0 to 65%). Leaf samples of plants with CBSD symptoms produced an amplicon of 222 bp using reverse transcription-polymerase chain reaction with primers that amplify a fragment of the coat protein (CP) gene of Cassava brown streak virus. Sequence comparisons based on the amplified CP gene fragment indicated that the isolates have 77 to 82.9% nucleotide and 43.9 to 56.8% amino acid identity with those from Mozambique and Tanzania. There was 95.9 to 99.5% nucleotide and 85.1 to 90.5% amino acid identity among the Ugandan isolates. These results confirm the re-emergence of CBSD in Uganda after it was first observed in the 1930s in cassava introduced from Tanzania and controlled by eradication. Prior to this report, CBSD was known to be restricted to the coastal lowlands of East Africa

Pests and Diseases on Cowpea in Uganda: Experiences From a Diagnostic Survey

A diagnostic survey (DS) using a questionnaire covering 525 farm households was conducted in 1993/94 to determine the status of cowpea (Vigna unguiculata L. Walp) and its production constraints in Uganda. Subsequent on-farm assessments were made during the two rainy seasons of 1994 to verify and quantify the survey information. Additionally, on-station trials were conducted to address issues arising from the information. Insect pests, low plant population densities, poor weed management and labour bottlenecks are the most important constraints to cowpea production in Uganda. Insect pest damage, particularly by bruchids, is most important. Research to develop integrated pest management (IPM) strategies based on the finding of this study are in progress.Une enquete diagnostique (ED) utilisant un questionnaire couvrant 525 m\ue9nages a \ue9t\ue9 conduite en 1993/94 pour d\ue9terminer la situation du ni\ue9b\ue9 (Vigna unguiculata L. Walp) et ses contraintes de production en Uganda. Par suite des \ue9valuations en milieu r\ue9el ont \ue9t\ue9 faite pendant deux saisons pluvieuses de 1994 pour v\ue9rifier et quantifier les r\ue9sultats de l\u2019enqu\ueate. En plus, des essais en station ont \ue9t\ue9 conduits pour addresser les probl\ue8mes indiqu\ue9s par l\u2019enqu\ueate. Les pestes d\u2019insectes, les faibles densit\ue9s des populations des plantes, la pauvre gestion du sarclage et le goulot de la main d\u2019oeuvre sont les plus importantes constraintes de production du ni\ue9b\ue9 en Uganda. Demage de pestes d\u2019insectes, particulicr\ue8ment par les bruches, est le plus important. La recherche pour d\ue9velopper des strat\ue9gies de la gestion int\ue9gr\ue9e des pestes (IPM) bas\ue9es sur les r\ue9sultats de cette \ue9tude est en cours

Stability of resistance to cassava brown streak disease in major agroecologies of Uganda

Cassava brown streak disease (CBSD) is the most devastating disease of cassava in southern, eastern and cntral Africa, and can cause up to 100% yield loss. Limited progress has been made in breeding for host plant resistance due to limited knowledge on the resistance variability to the disease. Reaction of promising cassava genotypes to CBSD in multi-environments are also unknown. Therefore, this study intended to: (1) Identify additional sources of resistance to CBSD; (2) Determine the stability of resistance to CBSD, and (3) mega-environments for screening resistance to CBSD. Field evaluation of 19 genotypes was conducted in RCBD with three replications at three agro-ecologies of Uganda for two cropping cycles. Additive Main Effects and Multiplicative Interaction (AMMI) and (GGE) biplot models were used to analyze genotype-environment interactions. Based on mean field reaction, the six best genotypes identified for resistance to CBSD were: TZ/06/140, TMS30572, TZ /06/130, N3/66/1, N3/58/1 with N3/104/3 and N3/66/1 being the most stable. While N3/66/1, N3/58/1 and N3/104/3, Mzungu and Kigoma Red were reported to be putative new sources of resistance to CBSD in Uganda. Genotypes (G), Environments (E), and GxE interactions were all significant, with no genotype exhibiting complete resistance. The significant result for GxE interaction to CBSD indicates the need for multi-environment screening and is suggestive of quantitative nature of CBSD resistance