CASSAVA BROWN STREAK DISEASE (CBSD)

By Penjani Nzima

IN AFRICA, cassava is the main staple food for over 500 million people who cultivate it for its roots and leaves.

While in Zambia Cassava is the second most important cash and staple crop after maize as it provides and cares for approximately 30%of the population.

Biotic factors affecting cassava productivity in Zambia have assumed greater importance over both abiotic and socioeconomic factors, which is largely attributed to the arrival of the invasive Cassava Brown Streak Disease (CBSD).

CBSD is not yet widespread in Zambia, but occurs severely in most of the historical cassava growing areas of Luapula, Northern, North-Western and Western provinces of the country.

Since its first detection in Zambia, CBSD has continued to spread rapidly to the more southerly parts of the country and is exacerbated by many identified production constraints that span biotic, abiotic and socioeconomic factors.

Recently, cassava farmers in Northern province of this country, precisely Chiengi and Kaputa districts respectively had their Crops ravaged by one of the deadly diseases that affect cassava which is Cassava Brown Streak Disease (CBSD).

But the Food and Agricultural Organization (FAO) of the United Nations has come to the aid of the affected farmers in the two districts by providing disease free cassava cuttings.

CASSAVA BROWN STREAK DISEASE (CBSD)

ORIGIN

Although it is well established that CBSD originated from the coastal regions of East Africa close to the mid-20th century, the exact origin of CBSD in Zambia is uncertain. Zambia is a landlocked country with eight neighboring countries.

There is speculation that CBSD spread into Zambia from the DRC, owing to the current localized distribution of the disease in the northern regions and the porous border between the two.

It is caused by two types of virus, Cassava Brown Streak Virus (CBSV) and Ugandan Cassava Brown Streak Virus (UCBSV), transmitted by the vector Bemisia tabaci Gennadius, which is also responsible for transmitting many of the viruses responsible for Cassava Mosaic Disease (CMD).

The typical symptoms of CBSD in cassava that have been observed in the affected regions of Zambia include feathery chlorotic veins; chlorotic blotches and mottling on the leaves; brown streaks on the stems; and hard rots on roots caused by necrosis (Mulenga et al. 2018).

These symptoms do not always occur simultaneously, which exacerbates the difficulty of identifying CBSD in the field. This is particularly the case where both leaf and stem symptoms are absent, and only root symptoms are present. Leaf symptoms of CBSD predominantly occur on the 9 lower leaves of the plant, which makes it difficult for farmers to identify them promptly before leaf senescence (Legg etal. 2011)

POTENTIAL IMPACTS OF CBSD ON TRADE

Zambia’sexports of fresh cassava roots and starch are low compared to the volume ofcassava production. CBSD is spreading across districts in Zambia, underscoringthe need to reinforce sanitary and phytosanitary measures.

                                                                                                                                                                                              Stringent cross-border sanitary and phytosanitary measures will most likely impede the growth of Zambia’s cassava export market if deliberate remedial actions are not taken.

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