Identification of simple sequence repeat markers for sweetpotato weevil resistance

The development of sweetpotato [Ipomoea batatas (L.) Lam] germplasm with resistance to sweetpotato weevil (SPW) requires an understanding of the biochemical and genetic mechanisms of resistance to optimize crop resistance. The African sweetpotato landrace, ‘New Kawogo’, was reported to be moderately resistant to two species of SPW, Cylas puncticollis and Cylas brunneus. Resistance has been associated with the presence of hydroxycinnamic acids esters (HCAs), but the underlying genetic basis remains unknown. To determine the genetic basis of this resistance, a bi-parental sweetpotato population from a cross between the moderately resistant, white-fleshed ‘New Kawogo’ and the highly susceptible, orange-fleshed North American variety ‘Beauregard’ was evaluated for SPW resistance and genotyped with simple sequence repeat (SSR) markers to identify weevil resistance loci. SPW resistance was measured on the basis of field storage root SPW damage severity and total HCA ester concentrations. Moderate broad sense heritability (H2 = 0.49) was observed for weevil resistance in the population. Mean genotype SPW severity scores ranged from 1.0 to 9.0 and 25 progeny exhibited transgressive segregation for SPW resistance. Mean genotype total HCA ester concentrations were significantly different (P < 0.0001). A weak but significant correlation (r = 0.103, P = 0.015) was observed between total HCA ester concentration and SPW severity. A total of five and seven SSR markers were associated with field SPW severity and total HCA ester concentration, respectively. Markers IBS11, IbE5 and IbJ544b showed significant association with both field and HCA-based resistance, representing potential markers for the development of SPW resistant sweetpotato cultivars

Effects of hydroxycinnamic acid esters on sweetpotato weevil feeding and oviposition and interactions with Bacillus thuringiensis proteins

Sweetpotato weevil (SPW) pest management is challenging because the pest target is sub-terranean, so the application of pesticides is impractical and usually ineffective. Host plant resistance and the genetic transformation of sweetpotatoes to produce entomotoxic Bt proteins offer potential for environmentally benign pest control. Resistance can be conferred by naturally occurring hydroxycinnamic acids which protect against oviposition by adults, but these compounds are restricted to the root surface so do not protect against the cortex bound larvae where the greatest damage occurs. Resistance could be enhanced if combined with expression of Bt proteins in transformed plants but interactions between hydroxycinnamic acids and Bt proteins remain unknown. Here the bioactivity of Cry7Aa1 protein and hydroxycinnamic acid esters was evaluated individually and in combination against SPW larvae and mortality determined. Low and high concentrations of hydroxycinnamic acid esters alone caused significantly higher mortality of both weevil species in all experiments compared to the control. SPW larval mortality was greater when tested as a combination of hydroxycinnamic acid esters and Bt protein but this effect was additive not synergistic. Although we report no evidence of antagonistic interactions the antifeedant effects of the plant compounds conferring host plant resistance could have reduced consumption of the Bt protein in our assays leading to a lower efficacy when combined. Further work is required to determine if the toxic effects of Bt proteins function alongside host plant resistance in sweetpotato under field conditions

Biological control of cassava green mite in Tanzania

The cassava green mite Mononychellus tanajoa (Bondar) (Acari: Tetranychidae) is one of the most important pests of cassava, a main staple food crop in Tanzania. The International Institute of Tropical Agriculture (ETA) in collaboration with the Tanzania National Biological Control P r o m (NBCP) launched in 1998 the biological control campaign against cassava green mite in Tanzania with the release of the neotropicd phytoseiid predator Z)phlodromallus aripo DeLeon (Acari: Phytoseiidae), which had proven to be an efficient biological control agent of the cassava green mite in West Mca. The campaign consisted of new introductions and redistributions of I: aripo from infested cassava shoot tips, follow-up surveys to determine establishment, spread, and impact, as well as extension and farmer training on pest and natural enemy recognition and means of enhancing predator efficacy. ~phlodromallusa ripo was h t fo und in March 1998 in the Tanga region, most likely invading h m the southern Kenya coast where it was released in 1996. Subsequently, the Tanzania NBCP in close collaboration with IITA has carried out numerous introductions and redistributions of the predator. Surveys conducted in the following 6 years revealed success M establishment, persistence and spread in five agro-ecological zones including the Lalce (Mars and Kagera regions, except parts of Mwanza region), Western (Kigorna region except Shinyanga region), Southern Highlands (Mbeya and Iringa region), Eastem (Tanga and Coast regions) and Southern (Lindi and Mtwara regions) zones. Up to 2005, there was still no T. arip in parts of 3 regions: Mwanza, Shinyanga and Ruvuma. Cassava green mite mean densities have declined to low levels (less than 20 actives per leaf) in all regions where 'I: a r b has been present. In on-fm impact assessment trial, 'I: aripo was capable of reducing population density of cassava green mite by 64.3% and increasing total and marketable cassava root weights by 61.2% and 71.7%, respectively. There was also a significant increase in total number of roots (25.4%), number of marketable roots (45.78%), stem weights (47.39%), and leaf weight (40.7%) where T, aripo was not eliminated. This report present evidence of the impact of biological control on cassava green mite populations and cassava yield in Tam&, and recommends the use of exotic isolates of the fungal pathogen Neozygites tanajoae, which has been established in West Africa, as a complementary alternative approach in controlling further the cassava green mite in the remaining spots of high infestations

Distribution, incidence and severity of cassava diseases and pests in Mozambique

Two countrywide surveys were conducted in April-May and May-June 2004 throughout the cassava growing belt of Mozambique including several provinces: Gaza, Inhambane, Sofala, Manica, Zambezia, Nampula and Cabo Delgado. Both surveys were planned to assess the distribution, incidence and damage severity of diseases and pests that affect cassava production in Mozambique. Using the methodology developed by the International Institute of Tropical Agriculture (IITA), 202 and 175 cassava fields between 4-8 months old were sampled for pests and diseases in April-May 2003 and May-June 2004, respectively. The results of both surveys showed in the case of diseases that cassava mosaic disease (CMD) was found to be the most widespread disease in Mozambique although the distribution, incidence, and severity varied among provinces, among fields and within fields. The most severe damage was recorded in Gaza, Sofala and Nampula where the average scores were above 3 on a 1-5 damage scale but only in a limited number of sample fields. In other provinces, the disease was either absent or damage symptoms were slight to moderate (2-3). The DNA analysis of the leaf samples coIlected in 2004 throughout the country revealed the presence of several strains of CMD virus including, the African Cassava Mosaic Virus (ACMV), the Eastern African Cassava Mosaic Virus (EACMV), and a combination of both African and EastAfrican Cassava Mosaic Virus (ACMV+EACMV) in 80.5% and 7.6% of the field samples respectiveIy. The devastating East African Cassava Mosaic Virus - Ugandan Strain (EACMVUG2) was absent from all sampled fields. The presence of cassava brown streak disease (CBSD) was confirmed as a serious threat particularly in two of the three Northern provinces of Zambezia and Nampula and at lesser extent in Cabo Delgado. However, the severity was the highest in Zambezia and in the district of Nakala in Nampula province. The other known common tropical diseases of cassava such as the cassava bacterial blight (CBB) and cassava anthracnose disease (CAD) were of minor importance. The cassava green mite (CGM), Mononychellus tanajoa Bondar remained a problem in the southern provinces (Gaza, Inhambane) where the exotic predatory mites Typhlodromalus aripo De Leon was not yet established.. CGM infestations were followed by whitefly (WF) infestations in a rank-order hierarchy of infestation rates. Although generally known as vectors of the cassava mosaic disease (CMD), the high whitefly population densities recorded in some locations (>lo0 adults per plant, e.g. in Zambezia province), was considered a direct pest causing considerable leaf damage and covering plants with sooty molds. The incidence and damage severity of other common pests of cassava (i.e. cassava mealybug, termites, and grasshoppers) appeared insignificant

Resistance to the weevils Cylas puncticollis and Cylas brunneus conferred by sweetpotato root surface compounds

Seven resistant varieties of sweetpotato were compared with three susceptible varieties in field trials and laboratory bioassays and showed that resistance was an active process rather than an escape mechanism, as field resistant varieties also had reduced root damage and oviposition compared with susceptible varieties in the laboratory. Liquid chromatography–mass spectrometry (LC–MS) of root surface and epidermal extracts showed significant variation in the concentration of hexadecyl, heptadecyl, octadecyl, and quinic acid esters of caffeic and coumaric acid, with higher concentrations correlated with resistance. All compounds were synthesized to enable their positive identification. Octadecyl coumarate and octadecyl caffeate applied to the surface of susceptible varieties in laboratory bioassays reduced feeding and oviposition, as observed on roots of resistant varieties, and therefore are implicated in weevil resistance. Segregating populations from breeding programs can use these compounds to identify trait loci for resistance and enable the development of resistant varieties