One-hundred and fourteen virus species are transmitted by whiteflies (family Aleyrodidae). Bemisia tabaci transmits 111 of these species while Trialeurodes vaporariorum and T. abutilonia transmit three species each. B. tabaci and T. vaporariorum are present in the European–Mediterranean region, though the former is restricted in its distribution. Of the whitefly-transmitted virus species, 90% belong to the Begomovirus genus, 6% to the Crinivirus genus and the remaining 4% are in the Closterovirus, Ipomovirus or Carlavirus genera. Other named, whitefly-transmitted viruses that have not yet been ranked as species are also documented. The names, abbreviations and synonyms of the whitefly-transmitted viruses are presented in tabulated form together with details of their whitefly vectors, natural hosts and distribution. Entries are also annotated with references. Whitefly-transmitted viruses affecting plants in the European–Mediterranean region have been highlighted in the text.

/pdf/plant-viruses-transmitted-by-whiteflies-1pfqpoyvjo.pdf

Plant viruses transmitted by whiteflies

Virus diseases that have emerged in the past two decades limit the production of important vegetable crops in tropical, subtropical, and temperate regions worldwide, and many of the causal viruses are transmitted by whiteflies (order Hemiptera, family Aleyrodidae). Most of these whitefly-transmitted viruses are begomoviruses (family Geminiviridae), although whiteflies are also vectors of criniviruses, ipomoviruses, torradoviruses, and some carlaviruses. Factors driving the emergence and establishment of whitefly-transmitted diseases include genetic changes in the virus through mutation and recombination, changes in the vector populations coupled with polyphagy of the main vector, Bemisia tabaci, and long distance traffic of plant material or vector insects due to trade of vegetables and ornamental plants. The role of humans in increasing the emergence of virus diseases is obvious, and the effect that climate change may have in the future is unclear.

Emerging Virus Diseases Transmitted by Whiteflies

Cassava or manioc (Manihot esculenta Crantz), a perennial shrub of the New World, currently is the sixth world food crop for more than 500 million people in tropical and sub-tropical Africa, Asia and Latin America. It is cultivated mainly by resource-limited small farmers for its starchy roots, which are used as human food either fresh when low in cyanogens or in many processed forms and products, mostly starch, flour, and for animal feed. Because of its inherent tolerance to stressful environments, where other food crops would fail, it is often considered a food-security source against famine, requiring minimal care. Under optimal environmental conditions, it compares favorably in production of energy with most other major staple food crops due to its high yield potential. Recent research at the Centro Internacional de Agricultura Tropical (CIAT) in Colombia has demonstrated the ability of cassava to assimilate carbon at very high rates under high levels of humidity, temperature and solar radiation, which correlates with productivity across all environments whether dry or humid. When grown on very poor soils under prolonged drought for more than 6 months, the crop reduce both its leaf canopy and transpiration water loss, but its attached leaves remain photosynthetically active, though at greatly reduced rates. The main physiological mechanism underlying such a remarkable tolerance to drought was rapid stomatal closure under both atmospheric and edaphic water stress, protecting the leaf against dehydration while the plant depletes available soil water slowly during long dry periods. This drought tolerance mechanism leads to high crop water use efficiency values. Although the cassava fine root system is sparse, compared to other crops, it can penetrate below 2 m soil, thus enabling the crop to exploit deep water if available. Leaves of cassava and wild Manihot possess elevated activities of the C4 enzyme PEP carboxylase but lack the leaf Kranz anatomy typical of C4 species, pointing to the need for further research on cultivated and wild Manihot to further improve its photosynthetic potential and yield, particularly under stressful environments. Moreover, a wide range in values of Km (CO2) for the C3 photosynthetic enzyme Rubisco was found among cassava cultivars indicating the possibility of selection for higher affinity to CO2, and consequently higher leaf photosynthesis. Several plant traits that may be of value in crop breeding and improvement have been identified, such as an extensive fine root system, long leaf life, strong root sink and high leaf photosynthesis. Selection of parental materials for tolerance to drought and infertile soils under representative field conditions have resulted in developing improved cultivars that have high yields in favorable environments while producing reasonable and stable yields under stress.

Cassava biology and physiology

Although cassava is a major food crop, its scientific breeding began only recently compared with other crops. Significant progress has been achieved, particularly in Asia where cassava is used mainly for industrial processes and no major biotic constraints affect its productivity. Cassava breeding faces several limitations that need to be addressed. The heterozygous nature of the crop and parental lines used to generate new segregating progenies makes it difficult to identify parents with good breeding values. Breeding so far has been mainly based on a mass phenotypic recurrent selection. There is very little knowledge on the inheritance of traits of agronomic relevance. Several approaches have been taken to overcome the constraints in the current methodologies for the genetic improvement of cassava. Evaluations at early stages of selection allow for estimates of general combining ability effect or breeding values of parental lines. Inbreeding by sequential self-pollination facilitates the identification of useful recessive traits, either already present in the Manihot gene pool or induced by mutagenesis.

Cassava breeding: opportunities and challenges.

Many of the key problems in food systems are linked specifically to industrial agriculture: uniform crop monocultures relying on chemical fertilizers and pesticides, industrial feedlots (the infamous "CAFOs") using preventive antibiotics. This form of agriculture is generating negative outcomes on multiple fronts: widespread degradation of land, water and ecosystems; high GHG emissions; biodiversity losses; persistent hunger and micro-nutrient deficiencies alongside the rapid rise of obesity and diet-related diseases; and livelihood stresses for farmers around the world.

From uniformity to diversity: a paradigm shift from industrial agriculture to diversified agroecological systems

Origin, distribution and economic importance botany, crop physiology and agronomy genetics and crop improvement crop protection crop utilization.

Cassava: Biology, Production and Utilization

Two whitefly species, Bemisia afer (Priesner & Hosny) and B. tabaci (Gennadius) were used in transmission experiments with Cassava brown streak virus (CBSV) (Ipomovirus; Potyviridae). Adults of whiteflies were given access to CBSV by containing them in clip cages on CBSV-infected cassava plants. Whiteflies were then transferred, together or separately, to CBSD-susceptible cassava plants of var. 'Albert' in a controlled environment. In glasshouse experiments, whiteflies were caged with CBSV-infected and virus-free cassava plants. Transmission of CBSV was sporadic and occurred in three of seven experiments when inoculated by B. afer and B. tabaci or B. tabaci alone, but not by B. afer alone. However, rate of transmission was low (maximum 22%) even when using high whitefly numbers of up to 120 per target plant. Successful transmission was confirmed by the detection of CBSV by reverse transcription-polymerase chain reaction. Spread of cassava brown streak disease (CBSD) in the field in Tanzania coincided with increases in whitefly numbers; further supporting the evidence that B. tabaci is a vector of CBSV. The results of this study establish for the first time that B. tabaci is the vector of CBSV, similar to other ipomoviruses.

Transmission of Cassava brown streak virus by Bemisia tabaci (Gennadius)

Cassava mosaic disease (CMD) is caused by whiteflyborne viruses of the genus Begomovirus (family Geminiviridae). The disease has long been regarded as the most important of those affecting cassava in sub-Saharan Africa, and has been the subject of much research, especially since the onset of the current very damaging pandemic in eastern and central Africa. This review considers the main features of CMD and the various possible means of control. The main emphasis to date has been on the development and deployment of virus-resistant varieties. These are widely adopted in countries where CMD has caused serious problems, and provided a powerful incentive for farmers to abandon some of the most susceptible of their traditional varieties. Only limited use has been made of phytosanitation involving CMD-free planting material and the removal (roguing) of diseased plants. Cultural methods of control using varietal mixtures, intercrops or other cropping practices have also been neglected, and there is a need for much additional research before they can be deployed effectively. Nevertheless, the severe losses now being caused by CMD in many parts of sub-Saharan Africa could be greatly decreased through the application of existing knowledge.

https://bsppjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3059.2005.01282.x

Strategies for controlling cassava mosaic virus disease in Africa

The origins of cassava (Manihot esculenta Crantz subspecies esculenta) have long been obscure. The three important questions to answer concern the botanical origin, (i.e. the wild species from which cassava descends), the geographical origin, (i.e. the area where the progenitor evolved in the geological past) and the agricultural origin (i.e. the area of initial cultivation of the wild ancestor by Amerindians). The core of the above argument rests on the assumption that if there is a living wild ancestor, its discovery would be likely to indicate the taxonomy of cassava. In turn, this would indicate the ancestry of the crop (the evolution of the ancestor and its phylogenetic relationships with related species) and the cradle of domestication. Current knowledge on the three topics shows that studies on the botanical origin of cassava have progressed far and stand on firm ground, those on the geographical origin have progressed and conjecture on the area where cultivation began has recently experienced a renewed surge following the appearance of novel ideas. Studies on the taxonomy of the genus Manihot in Brazil led unexpectedly to progress in matters with a bearing on the origins and phylogeny of cassava. Accumulated empirical knowledge derived from field experience culminated with the formulation of a classification in which the Brazilian species were arranged in groups. The model of classification proposed was thought to mirror some degree of phyletic kinship between the species (Fig. 1.1). A number of the prospective clades proposed in Fig. 1.1 (e.g. the Manihot nana group; the Manihot salicifolia group, etc.) match remarkably closely with former taxonomic classifications advanced for the same groups in earlier classic treatments including that of Rogers and Appan (1973). Group VI in particular, the M. esculenta group, has been tested extensively by the scientific community through genetic studies and phylogenetic investigations, which in turn influenced cladistic and taxonomic classifications. This group is particularly highlighted in the present review. No attempt has been made in Fig. 1.1 to suggest that the sequence of groups presented represents evolution towards greater complexity, although there is a tendency in the model to progress from herbaceous to tree species.

/pdf/the-origins-and-taxonomy-of-cassava-2ks6jqs2w9.pdf

The Origins and Taxonomy of Cassava

Cassava contributes significantly to the nutrition and livelihood of up to 500 million people and thousands of processors and traders around the world. Besides serving as the primary staple food of millions of people in the tropics and subtropics, it can also be used as a carbohydrate source in animal feed. Cassava is used as a raw material in the manufacture of processed food, animal feed and industrial products. Wider utilization of cassava products can be a catalyst for rural industrial development and raise the incomes for producers, processors and traders. It can also contribute to the food security status of its producing and consuming households (Plucknett et al., 1998).

/pdf/cassava-utilization-in-food-feed-and-industry-48ro2ydfpc.pdf

J. M. Thresh

Papers

Cassava: Biology, Production and Utilization

Transmission of Cassava brown streak virus by Bemisia tabaci (Gennadius)

Strategies for controlling cassava mosaic virus disease in Africa

The Origins and Taxonomy of Cassava

Cassava utilization in food, feed and industry.