Gwanda — Sithandile Ncube, a member of a farmers' cooperative in southern Zimbabwe, combs through her plot of rape, turning over each leaf and checking it for the yellowing or shrivelling that will reduce its value at the local market.

Customers want their leafy vegetables to be firm, smooth and healthy-looking and Ncube, who relies on the income from her market garden, is particularly worried about the red spider mite - like other farmers in Singukwe, a community in Gwanda district, more than 200 km (125 miles) south of the city of Bulawayo.

"We have had a dry season because of a prolonged hot spell, and after the little rain that fell, the red spider mite just multiplied even though we had sprayed to control it," laments Ncube, a member of the Injabulo Garden cooperative of market gardeners and crop farmers.

"We have implemented the principles of integrated pest management and crop rotation to keep the pest at bay but without success."

Her experience bears out the findings of a plant virology symposium held in February in Tanzania, which heard that climate change and extreme weather in Africa are making pests and plant diseases more virulent.

Scientists at the conference sought a plan to contain the spread of plant virus diseases that - driven by the warming climate - are affecting key staple crops.

Nteranya Sanginga, director general of the International Institute of Tropical Agriculture (IITA), said plant viruses - including the deadly cassava brown streak, banana bunchy top disease, rice yellow mottle and maize streak virus - are spreading rapidly, frustrating efforts to boost the food security and livelihoods of millions of people.

"Poor smallholder farmers - the majority of the population and of the food growers - with limited resources are bearing the brunt of these virus diseases.

They are least able to invest in inputs such as pesticides and herbicides and improved disease-resistant varieties," he said. "We need science-based solutions to these challenges."

CONTINENT-WIDE EFFORT

The IITA is leading a continent-wide effort to use scientific research and innovation to fight plant viruses, focusing on cassava, maize, yam, banana and plantain, soybean and cowpea. It has also helped control diseases affecting cacao, rice and vegetable crops.

Its work includes the use of resistant cultivars and clean planting materials, and the development of diagnostic tools for monitoring disease spread and establishing disease distribution maps. The tools have been effective in controlling cassava mosaic disease (CMD) and maize streak disease (MSD), both endemic in Africa.

More than 60 percent of sub-Saharan Africa's population is employed in agriculture, making it a linchpin of the region's economy and underlining the importance of tackling plant diseases.

In Zimbabwe's Matabeleland South province, a largely arid region, most of this year's maize crop failed because of the drought, and the red spider mite feared by Ncube and her neighbours flourished.

In a good month, each member of Ncube's cooperative can make at least $15 from the sale of vegetables and maize grown on the plots they have been allocated.

But this year, they have lost half their potential income from leafy vegetables and all their income from tomatoes because of disease and pests.

Another farmer, Sethukile Khumalo, a mother of three, harvested some tomatoes last season but abandoned the crop when she could not control the pests.

"The garden is our livelihood, we have never gone hungry because of the vegetables we grow here," Khumalo told Thomson Reuters Foundation.

"The weather has changed and we have to adopt better practices like mulching and weeding to keep down pests and diseases. It is not easy but we have no other option because I have been able to provide food and pay school fees for my children through this."

Community leader John Ncube said the lack of scientific expertise is a problem.

"We have not had a resident extension officer for the last six months since the one who served our community was transferred," Ncube said. "The changing weather has presented new challenges for us such as the increase in pests and diseases, but we have no one well informed on how to deal with this problem."

MORE INVESTMENT NEEDED

The red spider mite (tetranychus spp) is a common sap-sucking pest which attacks vegetables, legumes and ornamental plants, causing mottled leaves.

The Matabeleland North provincial agronomist, Davison Masendeke, is helping farmers to adopt integrated pest management practices to keep pests and disease at bay.

"Weather changes have an impact on the prevalence of pests and diseases," Masendeke said. "Pest numbers tend to be low in cool conditions and multiply in hot conditions and farmers have to be more vigilant then."

Scientists told Thomson Reuters Foundation they needed African governments to increase investment in agricultural research, modern laboratories and the training of plant virologists, while farmers should adopt new techniques, clean planting materials and improved crop varieties.

Lava Kumar, a virologist with the IITA, said diseases and pests are threatening crop production, quality and international trade.

Climate change alters the interaction between the host plant and the virus, and viruses can wipe out 80 percent of a crop, Kumar said, while warm conditions also favour high levels of insect activity.

Managing viral diseases requires unique skills and facilities not found in many countries in sub-Saharan Africa, Kumar said.

Finding solutions requires innovation, and more investment is required in both technical capacity and human resources to tackle plant viruses. The lack of concerted disease-control programmes in Africa does not help, he added.

According to the Forum for Agricultural Research in Africa (FARA), the continent needs $4.4 billion a year for agricultural research and development, against a current budget of only $2.2 billion a year, of which $500 million is provided by regional bodies like the New Partnership for Africa's Development (NEPAD), the African Union and FARA.

Busani Bafana is a freelance contributor for the Thomson Reuters Foundation, based in Zimbabwe and covering climate change issues.