Fisiología de la yuca (Manihot esculenta Crantz)
Available from: Glenn Hyman
- "Cassava (Manihot esculenta Crantz, Family Euphorbiaceae) is one of the most important staple crops for farmers in the tropics due to its high calorie content, low production cost and ability to adapt to different soil types and climatic conditions (Cock 1982; FAO and IFAD 2000; Mejía de Tafur 2002). Recent estimates suggest that as many as 500 million to 1 billion people consume cassava, making it the third most important crop in the tropics after rice and maize (Dutt et al. 2005; Breu 2005; FAO 2008). "
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ABSTRACT: Insect pests and plant diseases reduce cassava yields substantially, posing a threat to food security throughout the developing
world. While agricultural scientists have recognized these threats, few assessments of the geographic distribution of cassava
pests and diseases have been made at the global scale. The goal of this study is to make such an evaluation for four key biotic
constraints to cassava production in developing countries: whiteflies, cassava green mites, cassava mosaic disease and cassava
brown streak disease. Occurrence records were obtained from laboratory and biodiversity databases and from the scientific
literature. These records were then used in ecological niche models to predict the potential distribution of cassava pests
and diseases. The distribution maps were cross validated by holding back 20% of the occurrence records. Potential distribution
maps were developed by combining the results of the best ecological niche models. Hotspots for potential cassava pest and
disease outbreaks include the Mato Grosso in Brazil, northern South America, the African rift valley, the southern tip of
India and much of Southeast Asia, where all four biotic constraints show high potential suitability. Our work highlights how
potential geographical shifts in infestation hotspots for several cassava biotic constraints will require intensified monitoring,
evaluation and research to prevent yield losses and ensure food security.
KeywordsCassava–Whitefly–Green mite–Cassava mosaic disease–Cassava brown streak disease–Geographic distribution–Ecological modeling
Available from: Mabrouk A. El-Sharkawy
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ABSTRACT: The review sums up research conducted at CIAT within a multidiscipline effort revolving around a strategy for developing improved technologies to increase and sustain cassava productivity, as well as conserving natural resources in the various eco-edaphic zones where the crop is grown, with emphasis on stressful environments. Field research has elucidated several physiological plant mechanisms underlying potentially high productivity under favourable hot-humid environments in the tropics. Most notable is cassava inherent high capacity to assimilate carbon in near optimum environments that correlates with both biological productivity and root yield across a wide range of germplasm grown in diverse environments. Cassava leaves possess elevated activities of the C4 phosphoenolpyruvate carboxylase (PEPC) that also correlate with leaf net photosynthetic rate (P
N) in field-grown plants, indicating the importance of selection for high P
N. Under certain conditions such leaves exhibit an interesting photosynthetic C3-C4 intermediate behaviour which may have important implications in future selection efforts. In addition to leaf P
N, yield is correlated with seasonal mean leaf area index (i.e. leaf area duration, LAD). Under prolonged water shortages in seasonally dry and semiarid zones, the crop, once established, tolerates stress and produces reasonably well compared to other food crops (e.g. in semiarid environments with less than 700 mm of annual rain, improved cultivars can yield over 3 t ha−1 oven-dried storage roots). The underlying mechanisms for such tolerance include stomatal sensitivity to atmospheric and edaphic water deficits, coupled with deep rooting capacities that prevent severe leaf dehydration, i.e. stress avoidance mechanisms, and reduced leaf canopy with reasonable photosynthesis over the leaf life span. Another stress-mitigating plant trait is the capacity to recover from stress, once water is available, by forming new leaves with even higher P
N, compared to those in nonstressed crops. Under extended stress, reductions are larger in shoot biomass than in storage root, resulting in higher harvest indices. Cassava conserves water by slowly depleting available water from deep soil layers, leading to higher seasonal crop water-use and nutrient-use efficiencies. In dry environments LAD and resistance to pests and diseases are critical for sustainable yields. In semiarid zones the crop survives but requires a second wet cycle to achieve high yields and high dry matter contents in storage roots. Selection and breeding for early bulking and for medium/short-stemmed cultivars is advantageous under semiarid conditions. When grown in cooler zones such as in tropical high altitudes and in low-land sub-tropics, leaf P
N is greatly reduced and growth is slower. Thus, the crop requires longer period for a reasonable productivity. There is a need to select and breed for more cold-tolerant genotypes. Selection of parental materials for tolerance to water stress and infertile soils has resulted in breeding improved germplasm adapted to both favourable and stressful environments.
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