Fear as rare cassava pest resurfaces

Dec 03, 2021

Cassava green mite, which devastated cassava gardens in the 1980s and 1990s, was effectively managed through biological means.

Fear as rare cassava pest resurfaces

Sam Wamani
Journalist @New Vision

Several acres of cassava in the areas of Kichwamba, Hima and Nyamwamba in Kasese district, were attacked and devastated by a strange pest during the dry spell in January.

Julius Rukara, the principal agriculture officer of Kasese district, says the affected cassava crops had symptoms ranging from leaf yellowing and twisting to stuntedness and eventual leaf fall.

“About 15% of the gardens were devastated by this strange pest,” he says.

Christopher Mbusa, a 40-year-old farmer in Hima parish, Kichwamba sub-county, says the pest devastated 15 of his 35-acre farm of NAROCASS-1 hybrid cassava.

“The pests attacked my cassava and as a result, the leaves began yellowing and dropping off. Also, the cassava plants became stunted. Much as I have not yet harvested it, I believe that the yield has been seriously compromised because of this pest,” he adds.

After consultation with the scientists at the National Crop Resources Research Institute (NaCRRI) in Namulonge, Wakiso district, the pest was confirmed to be the cassava green mite.

Besides Kasese, reports also indicate the pest devastated cassava farms in other areas including Teso, which is the leading cassava producing region in Uganda.

Authorities in Katakwi district confirmed cassava green mite devastating farms during the dry spell.

James Akoom, the Magoro sub-county agricultural officer, says in February, the pest was reported in about 50 cassava gardens in Kamenu village alone.

Akoom says the cassava green mites feed on the underside of young leaves and the green stems of cassava plants.

He says a severe attack by cassava green mite causes the terminal (or newest) leaves to wither and fall off. The shoot tip ends up looking like a candle stick.

How the pest spreads

Research reports from NaCRRI indicate that cassava green mite (Mononychellus tanajoa) was first introduced in Uganda in 1971 through the infected cassava cuttings that had been imported from South America.

“Since its first appearance in East Africa, the pest has spread to most cassava growing belts, where it causes estimated yield losses ranging between 30-80%,” a report by NaCCRI explains.

Samuel Opio Morris, a scientist working on cassava pests at NaCRRI, says the cassava green mite is a pest that attacks cassava by piercing and sucking juices from the leaves, most commonly from the underside.

“This then causes yellowing, twisting, death and leaf fall. The stems also show a ‘candle stick’ appearance after the loss of terminal shoots. This is directly linked to yield loss in cassava and no wonder, it is regarded as a serious threat to cassava production in Uganda,” Opio adds.

He says cassava green mite is mainly spread by human activity, whereby infested planting materials and contaminated media are transported over long distances.

He also says natural dispersal by wind and water may spread this pest.

He says the pest is prevalent in all the cassava growing areas of Uganda, though the magnitude of its damage is more in the hot and dry areas.

“High temperatures promote fast reproduction rates of cassava green mite. The availability of new foliage (leaves) on cassava plants and the presence of rainfall are the major factors that determine the population abundance of the pest in the field. The population increases on new leaf growth during the early dry season but declines when defoliation (leaf fall) and reduced leaf production follow the drought. Prolonged rainfall also leads to a decrease in population because the mites are washed off the plant,” says a report by NaCRRI.

Efforts against the pest

Dr Richard Molo, a senior crop entomologist and former head of the bio-control unit based at the National Agriculture Research Laboratories (NARL) in Kawanda, says cassava green mite, which seriously devastated cassava gardens in the 1980s and 1990s, was effectively managed through biological control options.

He, however, says the pest seems to be resurgent and as such requires concerted efforts to effectively contain it.

“Initial biological control efforts involved evaluation of over 11 mite species of Colombian and Brazilian origin in 1986-1993. However, none of these species became successfully established in Uganda. In the later years ofevaluation, one predatory mite species, typhlodramulus aripo, was introduced from                             Brazil. Through field releases, this type was identified as the most successful biological control agent and since then, it has been considered as the key predator of cassava green mite in the cassava belts of Africa and specifically in Uganda,” Molo says.

Similarly, Opio confirms that currently, under the root crops programme at NaCRRI, there are deliberate efforts to breed cassava varieties that confer resistance to the deadly cassava green mite.

He says in the next few years, cassava varieties with resistance to the pest will be developed and officially released to the farmers.

‘Uganda’s cassava production dropping’

Cassava is a major source of carbohydrates in tropical areas.

It is the second most widely grown and consumed food crop in Uganda after bananas and a staple in the diet for approximately 57% of Uganda’s population.

However, cassava pests and diseases have been reported as major constraints to the production of this crop. According to the 2003, 2008, 2010 and 2019 Uganda Bureau of Statistics reports, cassava is mainly grown in the eastern and northern parts of the country on 342,387 and 269,886 hectares respectively.

However, these reports indicate that annual cassava production steadily declined from 4,966,000 tonnes in 2000 to 3,017,000 tonnes in 2010.

Control tips

Samuel Opio Morris, a scientist working on cassava pests at NaCRRI, advises farmers to take practical steps to control cassava green mite in gardens:

Plant cassava early, especially on the onset of the rains. By the time the dry spell hits, the crop will have finished the process of tuber formation, and any subsequent pest attack will not significantly lead to yield loss.

Carefully select and only plant clean cassava planting materials that are free from the cassava green mite.

Spray the affected cassava plants bi-monthly with abamectin at a rate of 250ml per hectare.

This story is part of the CABI, SciDev.Net and Robert Bosch Stiftung Script science journalism project

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