|Leaves of the cassava plant|
|A cassava tuber (waxed)|
Cassava undergoes post-harvest physiological deterioration (PPD) once the tubers are separated f
Frozen cassava leaves in a Los Angeles market
Cassava undergoes post-harvest physiological deterioration (PPD) once the tubers are separated from the main plant. The tubers, when damaged, normally respond with a healing mechanism. However, the same mechanism, which involves coumaric acids, starts about 15 minutes after damage, and fails to switch off in harvested tubers. It continues until the entire tuber is oxidized and blackened within two to three days after harvest, rendering it unpalatable and useless. PPD is related to the accumulation of reactive oxygen species (ROS) initiated by cyanide release durin
Cassava undergoes post-harvest physiological deterioration (PPD) once the tubers are separated from the main plant. The tubers, when damaged, normally respond with a healing mechanism. However, the same mechanism, which involves coumaric acids, starts about 15 minutes after damage, and fails to switch off in harvested tubers. It continues until the entire tuber is oxidized and blackened within two to three days after harvest, rendering it unpalatable and useless. PPD is related to the accumulation of reactive oxygen species (ROS) initiated by cyanide release during mechanical harvesting. Cassava shelf life may be increased up to three weeks by overexpressing a cyanide insensitive alternative oxidase, which suppressed ROS by 10-fold. PPD is one of the main obstacles preventing farmers from exporting cassavas abroad and generating income. Fresh cassava can be preserved like potato, using thiabendazole or bleach as a fungicide, then wrapping in plastic, coating in wax or freezing.
While alternative methods fo
While alternative methods for PPD control have been proposed, such as preventing ROS effects by use of plastic bags during storage and transport, coating the roots with wax, or freezing roots, such strategies have proved to be economically or technically impractical, leading to breeding of cassava varieties more tolerant to PPD and with improved durability after harvest. Plant breeding has resulted in different strategies for cassava tolerance to PPD. One was induced by mutagenic levels of gamma rays, which putatively silenced one of the genes involved in PPD genesis, while another was a group of high-carotene clones in which the antioxidant properties of carotenoids are postulated to protect the roots from PPD.
A major cause of losses during cassava storage is infestation by insects. A wide range of species that feed directly on dried cassava chips have been reported as a major factor in spoiling stored cassava, with losses between 19% and 30% of the harvested produce. In Africa, a previous issue was the cassava mealybug (Phenacoccus manihoti) and cassava green mite (Mononychellus tanajoa). These pests can cause up to 80 percent crop loss, which is extremely detrimental to the production of subsistence farmers. These pests were rampant in the 1970s and 1980s but were brought under control following the establishment of the "Biological Control Centre for Africa" of the International Institute of Tropical Agriculture (IITA) under the leadership of Hans Rudolf Herren. The Centre investigated biological control for cassava pests; two South American natural enemies Apoanagyrus lopezi (a parasitoid wasp) and Typhlodromalus aripo (a predatory mite) were found to effectively control the cassava mealybug and the cassava green mite, respectively.
The African cassava mosaic virus causes the leaves of the cassava plant to wither, limiting the growth of the root. An outbreak of the virus in Africa in the 1920s led to a major famine.The African cassava mosaic virus causes the leaves of the cassava plant to wither, limiting the growth of the root. An outbreak of the virus in Africa in the 1920s led to a major famine. The virus is spread by the whitefly and by the transplanting of diseased plants into new fields. Sometime in the late-1980s, a mutation occurred in Uganda that made the virus even more harmful, causing the complete loss of leaves. This mutated virus spread at a rate of 80 kilometres (50 miles) per year, and as of 2005 was found throughout Uganda, Rwanda, Burundi, the Democratic Republic of the Congo and the Republic of the Congo.
A wide range of plant parasitic nematodes have been reported associated with cassava worldwide. These include Pratylenchus brachyurus, Rotylenchulus reniformis, Helicotylenchus spp., Scutellonema spp. and Meloidogyne spp., of which Meloidogyne incognita and Meloidogyne javanica are the most widely reported and economically important. Meloidogyne spp. feeding produces physically damaging galls with eggs inside them. Galls later merge as the females grow and enlarge, and they interfere with water and nutrient supply. Cassava roots become tough with age and restrict the movement of the juveniles and the egg release. It is therefore possible that extensive galling can be observed even at low densities following infection. Other pest and diseases can gain entry through the physical damage caused by gall formation, leading to rots. They have not been shown to cause direct damage to the enlarged storage roots, but plants can have reduced height if there was loss of enlarged root weight.
Research on nematode pests of cassava is still in the early stages; results on the response of cassava is, therefore, not consistent, ranging from negligible to seriously damaging. Since nematodes have such a seemingly erratic distribution in cassava agricultural fields, it is not easy to clearly define the level of direct damage attributed to nematodes and thereafter quantify the success of a chosen management method.
The use of nematicides has been found to result in lower numbers of galls per feeder root compared to a control, coupled with a lower number of rots in the storage roots. The organophosphorus nematicide femaniphos, when used, did not affect crop growth and yield parameter variables measured at harvest. Nematicide use in cassava is neither practical nor sustainable; the use of tolerant and resistant cultivars is the most practical and sustainable management method.