Cassava brown streak disease is a devastating disease that is hampering cassava yield in most African countries especially in East Africa. In some cases, the disease exposes farmers to 100 percent loss.
CBSD causes rotting of the storage roots rendering the spoilt roots inedible. The CBSD is caused by two viruses namely the Cassava Brown Streak Virus and the Ugandan Cassava Brown Streak Disease. A virus cannot be seen by naked eyes and only survives inside a living organism. Currently the CBSD is present in Uganda, Kenya, Tanzania, Malawi, Mozambique and Rwanda.
Symptoms of CBSD can be observed on leaves, stems and cassava tuberous roots. While some varieties can show symptoms on leaves, stems and roots, others may only show symptoms on leaves and not on roots. Some varieties do not show symptoms on leaves but on roots only. Symptoms are most visible in old cassava plants, especially on the lower mature leaves. It’s important to note that most young leaves at the tip of the plant do not show.
CBSD symptoms and therefore whenever old leaves drop off, the plants may appear healthy when in fact, they are diseased. The affected plants develop yellow lines or patches along the small veins within normal green leaf colour which is commonly known as chlorosis. In mature cassava plants, the yellowing on the lower leaves becomes larger until most of the leaf is yellow and only the small veins remain green. Leaves showing CBSD symptoms retain their normal shape and are not deformed as is the case with Cassava Mosaic disease.
The stems of the affected plants develop dark brown streaks or lesions that appear as scratch like wounds in some varieties. These streaks can be most clearly seen on upper, young green portions of the stem. In very susceptible varieties, the shoot tips may dry and fall off and this condition may progress to affect the whole cassava popularly known as ‘stem die-back’. Eventually, the whole plant may dry and die.
On the cassava roots, the CBSD manifests itself through turning it brown, yellowish and hard. The brown and hard rot consisting of dead tissues becomes evident in the root. In very susceptible varieties, CBSD causes deep constrictions, twisting and reduction in size of the storage roots. The outer skin of the storage roots may develop deep cracks on the surface.CBSD is spread mainly when cuttings taken from a diseased plant are used as planting material. The new plants that sprout will have CBSD. In some cases, the plant may be carrying CBSV or UCBSD without showing symptoms mainly when it was recently infected or when the plant os tolerant. In this case the virus will be spread to other plants that are not resistant by insects.
The viruses are spread from one plant to another by small white insects called whiteflies just the way malaria parasite is carried by mosquitoes. Whiteflies feed on the sap in cassava leaves, just like mosquitoes feed on human blood. The virus lives in the sap of the plant.. While sucking the sap of an infected plant, the virus is carried along with the sap into the body of the whitefly. If the same whitefly later feeds on a healthy plant, it will transmit the virus to the healthy plant.
CBSD causes both quantity and quality loss of cassava storage roots. In some susceptible varieties, the losses can be up to 100 percent and researchers advise farmers against consuming roots of affected plants. In order to curb the devastating losses that are caused by CBSD, farmers are advised to always get clean planting materials (stem cuttings) from CBSD free plants.
This can also be done by sourcing from certified multiplication farmers as well as gardens without CBSD history. In addition, those multiplying cassava planting materials should be trained in proper identification and management of CBSD and regularly work closely with plant inspectors form national research institutes.