Every farmer wants high yields from their crop to get more income. But, do farmers know the best planting time if they are to reduce losses and maximize returns? Do they also know what happens when they replant stems from the previous cassava crop year after year?
Answers to the questions above can be found in a paper that was published in the scientific journal Plant Disease which shows “Masika” is the best planting season for cassava farmers and seed producers in coastal Tanzania. Masika is the long rainy season which occurs from March to June. Although some farmers plant cassava in Masika, it’s more common for them to plant during the short “Vuli” rainy season, which runs from October to December. The new research shows that there is a much higher degree of infection by cassava brown streak disease (CBSD) in Vuli than in Masika. This is why yields are lower in Vuli and higher in Masika.
According to the researchers led by Rudolph Shirima, an IITA plant virologist, “CBSD-causing viruses are transmitted by insects called whiteflies. The viruses causing CBSD are generally referred to as cassava brown streak ipomoviruses. During the Vuli season, cassava plants are infested by higher numbers of whiteflies leading to more virus transmission and higher CBSD incidence, whereas in Masika there are much fewer whiteflies which also means that there is much less CBSD infection. CBSD causes rotting of the tuberous roots, which results in a reduction of marketable yield. So, farmers can maximize their yields by planting in Masika. Conversely, for researchers working to breed resistant varieties, it’s very important to also plant in Vuli as this is when the plants will be challenged most strongly by CBSD.
Le Plant Disease study also presents the first evidence for cassava degeneration caused by the cassava brown streak ipomoviruses. Degeneration refers to the increase in CBSD incidence and reduction in marketable root yield over time that results from the repeated planting of stem cuttings sourced from the previous crop. Most smallholder farmers reuse stem cuttings from previous crops for planting in the new season. Recycling planting material in this way can lead to “degeneration” as virus infection increases from one season to the next. In order to study this effect, the researchers compared seven cassava varieties (Chereko, KBH2002_135, Kipusa, Kizimbani, Mkuranga1, Kiroba, and Kikombe) under conditions of high CBSD pressure in Bagamoyo, coastal Tanzania from 2013 to 2017. The experiment was run in both the Masika and Vuli seasons. At the end of the 4-year study period, the most important result was that CBSD infection increased and yields decreased in the susceptible varieties, while there was little change in the performance of the resistant varieties. This confirmed that there is degeneration in cassava in coastal Tanzania resulting from CBSD infection. However, planting season was shown to have a big effect on this phenomenon, as degeneration was more rapid and severe when cassava was planted in the Vuli season compared with Masika plantings. In other words, if a farmer plants cassava in the short rainy season using recycled stems or new ones, the crop will be more severely affected by CBSD and yields will be lower. These results have important practical implications for farmers in coastal Tanzania and elsewhere, and an important next step for the research team will be to develop communication messages so that farmers are made aware of the right season to plant.
The research was carried out by researchers from the International Institute of Tropical Agriculture (IITA), Tanzania in collaboration with the University of Dar es Salaam and Tanzania Agricultural Research Institute.
The open-access Plante Disease article can be found at:
https://apsjournals.apsnet.org/doi/10.1094/PDIS-05-18-0750-RE
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