Stolen harvests in Bolivia

Wilge Velasco is an agronomist who works in the isolated community of Tumuyu B in the municipality of Pocona, Bolivia, as part of a project with ATICA (Agua Terra Campesina meaning Water, Earth, Peasant). Pests and diseases are a constant threat to crop production in the area and Wilge is helping farmers find out which ones are most serious and how to control them.

credit: Eric Boa, CABI

'Rhizoctoniosis', named after a common fungus which attacks many crops, is evident in potato crops across the area. Tubers appear to bulge out of the earth, but these purple and greasy looking structures are not the normal red and cream colour of true potatoes. And they are not tubers either, but swollen and highly abnormal aerial shoots.

It is commonly believed by those involved in potato production in Bolivia (including Wilge) that this is a fungal disease. But Phil Jones of CABI's Global Plant Clinic* and Rothamsted Research has confirmed a phytoplasma as the most likely cause. This is a completely different type of organism to a fungus. Phytoplasmas have a unique biology and ecology that require different control measures.

"In Quechua, farmers call it phurmu, meaning overflow," says Wilge. Other locals call it qherqe, meaning greasy or oily; it is an apt name given the symptomatic sheen on the potato skin. Local dirigente Justo Rojas, who is in his early 30s, explains that it is a new disease, one they had not known as children. They do not do anything special when they notice the disease, he says, as it does not lower production. CABI believes that more research is needed to examine the true impact of the disease which, under its mistaken name of rhizoctoniosis, occurs from Cochabamba to the valleys of Santa Cruz.

This was not the only example of a mistaken diagnosis that CABI came across. ATICA agronomist Bernardino Soliz explained that the most serious problem described by the people of Tumuyu B for their main crop, potato, was low soil fertility. This yielded poorly despite applying large amounts of chemical fertilizer. PROLADE, an NGO based in Cochabamba, won the contract to help solve Tumuyu's soil problems, and employed Wilge to help. But the first time he helped farmers harvest the mishka (early season potatoes) in November 2002, he saw the typical galls of root knot nematodes (Nacobbus aberrans).

Credit: Eric Boa, CABI

At first the local farmers did not believe that their problems were due to nematodes and not low soil fertility. Looking at the tiny black cysts, they argued that they were the seeds of certain common weeds. So Wilge took the villagers to the PROINPA experimental station at Toralapa nearby, where they saw the tiny nematodes for themselves and learnt that each cyst contained over 50 eggs. The farmers were impressed and, more importantly, were convinced that nematodes were the real problem.

Wilge also found that soil fertility was not especially low, and that over-use of chemical fertilisers could itself have been causing problems. He began doing simple field trials with broad beans as green manure and found that late blight (Phytophthora infestans) was a common fungal disease in potatoes. He explained to the people that if they alternated systemic and contact fungicides they could control the disease better with fewer applications.

The pest and disease problems did not stop there. According to community leader Soylo Yarhui, weevils were also causing huge losses in maize crops. "When we were children, we saved our own maize and planted it again. Now we have to sell it all and buy new maize for seed. Within a month these bugs ruin the maize until it's only fit to feed to the pigs." Local people refer to the weevils as jak'u, which in Quechua means "flour" after the inedible flour the weevils produce from the once healthy maize crop.

Although the maize weevil is a new problem, farmers are well aware of its existence. There are several kinds, some with snouts, some without, explains another local farmer Erminio Sutiya; "They are similar, but different to the kinds of weevils found in wheat." Wilge confirms the accuracy of this local knowledge: the ones with snouts are true weevils in the family known as Curculionidae, while the others are beetles from other families with similar ecologies. And there are many species, attacking different grains.

Other disease problems in Tumuya are more difficult to identify. The potatoes appear to have viruses and the broad beans have yellowing symptoms that also suggest a virus. The damage appears slight, but overall production can be severely reduced, particular in potato. And there are other pests and diseases that need attention: peaches have sun-damage, rust fungus, leaf curl and serious wood-boring beetles and the apples are so loaded with epiphytes (probably Eryngium paniculatum) that they hardly yield fruit. Whatever the soil conditions in Tumuya, pests and diseases are stealing much of the harvest. Fortunately, the community has a full-time agronomist on its side to help diagnose problems reliably and identify the best methods of control.

Article submitted by Jeffery Bentley and Eric Boa, CABI

* CABI Bioscience's Global Plant Clinic provides a free diagnostic and advisory service on disease problems on all tropical crops with the backing of DFID. It works with all pathogens, crop plants as well as trees. Email plant.clinic@cabi.org See also: www.globalplantclinic.org