Late blight – new developments
At Pokeramanda, in the high mountains of eastern Papua New Guinea, villagers had uprooted the dying potato plants and laid them out at the roadside – like the victims of an unknown plague. “They just died,” So Wan Kusit said, bewildered. “I don’t know why.”
 |
| Fields of the PNG government seed system, which eventually made dissemination of resistant cultivars feasible. (CIP’s Greg Forbes on the right) |
But Dr. Sergi Bang, research leader of Papua New Guinea’s National Agricultural Research Institute (NARI) knew. He had travelled to the remote village with a deep sense of foreboding, and a brief inspection was enough to confirm his worst fears: late blight. Hitherto, Papua New Guinea had been free of the disease. Now, in a matter of weeks it was sweeping across the country, closing down a young but flourishing commercial potato-growing industry (already worth $11 million per year) and destroying crops that were to have fed the family of So Wan Kusit and many thousands like her.
Arrival from Irian Jaya
Dr. Bang could guess what had happened. Late blight, caused by the fungus-like Phytophthora infestans, had probably drifted into Papua New Guinea on winds from across the border with Irian Jaya, where thousands of refugees from elsewhere in Indonesia were planting infected seed potatoes. He told the villagers to bury the uprooted plants and advised them not to cultivate potatoes again for a couple of years “to allow the late blight fungus to die.”
In a country slightly larger than California, but with less than 500 miles of paved roads and only 1.9% of its land surface suitable for growing crops, the potato had been a prized crop. Especially among the 85 percent of a 5.4 million population that depends entirely on subsistence farming for food and livelihood. The potato was particularly well-suited to the country’s highlands, where altitude tempered the heat of an equatorial sun and year-round rainfall watered fertile soils. In such cool and humid conditions farmers could grow potatoes throughout the year. But once late blight arrived it spread very quickly. By May 2003 the devastation was complete. While subsistence farmers struggled to survive on alternative crops, and urban consumers bought imported potatoes at three or four times the price for local produce, the government appealed for help.
 |
| Market scene in Papua New Guinea |
Global Initiative on Late Blight
As part of an Australian response to the catastrophe, Australian and CIP scientists, including Greg Forbes, plant pathologist and co-ordinator of the Global Initiative on Late Blight (GILB) convened by CIP, visited the island and formulated an extended research and development project to help Papua New Guinea deal with the problem. “Initially, late blight-resistant cultivars from CIP will be introduced and evaluated to select a variety that will replace the susceptible variety currently grown in Papua New Guinea,” says Forbes. “To this end, 10 resistant CIP clones are being multiplied under quarantine conditions for shipment in June 2005,” he adds. “After further multiplication in Papua New Guinea, they will be assessed in the field for adaptation and late blight resistance.”
Potato late blight is a devastating disease. Resource-poor farmers in the developing world alone spend more than US$750 million per year on fungicides to control late blight while still losing an additional US$2.5 billion in yield; to which must be added the millions spent on controlling the disease in the developed world. The pathogen’s taxonomy and biology are very well understood, but a fully effective response remains elusive. Breeders have succeeded in producing varieties that will tolerate various degrees of infection but none is completely immune. The problem is that P. infestans is highly adaptable. “Over the past few years we’ve been finding new forms of P. infestans that have never been seen before,” says Forbes. “The pathogen is evolving faster than the control measures used to combat it. New approaches are urgently needed.”
 |
| Gathering data from test fields in Chacllabamba on late blight resistance |
Fuller understanding of pathogen populations
Among the new approaches being developed, CIP scientists are collaborating with colleagues from national programs world-wide to track movements and changes in pathogen populations in key locations. DNA fingerprinting techniques and other state-of-the-art methods are used to detect variation among pathogen strains. These investigations are giving scientists a fuller understanding of how the pathogen populations evolve, so that they can begin to build models predicting future directions and thus stay one step ahead of the disease.
Meanwhile CIP’s plant breeders have been working towards developing varieties with a resistance to late blight that lasts. So far, durability has been elusive. The point is that although a degree of resistance occurs naturally in some varieties, and breeders have been able to intensify that resistance, it has eventually broken down. “That’s because the resistance was due to either a major gene on the one hand, or to minor genes on the other,” Juan Landeo, of CIP’s breeding program explains. “In the case of major gene resistance, it’s as though there was one big padlock with one big key keeping the pathogen out. It worked very well for a while, but eventually the pathogen found a way through. And that’s it: no more resistance!”
Landeo speaks from personal experience. He is the man who bred Canchan, one of Peru’s most popular and widely grown varieties, which was renowned for its resistance to late blight. But not any more. In recent years Canchan’s ability to withstand the disease has broken down. Landeo shrugs. “Only one padlock,” he says. “A strong one, but not durable – or low levels of resistance due to minor genes built in.”
 |
| Local farmers in traditional dress gathering the potato harvest in Chacllabamba |
Recurrent selection for new varieties
With this in mind Landeo began a breeding program in the early 1990s that was designed to develop varieties which would be protected by many small padlocks (minor genes), and be free of major genes (single big ones). The technique, known as recurrent selection, was applied to two main genetic sources of resistance (breeding populations), with the aim of upgrading gene frequencies for partial resistance to late blight (due to minor genes and raising higher levels that would eventually lead to the selection of new varieties. “In effect, each partially resistant variety has numerous small padlocks,” Landeo explains. “And I wanted to incorporate as many as possible into breeding populations, from which varieties would emerge with a string of padlocks to keep the pathogen out. The plant wouldn’t be totally immune to infection, but if there were enough padlocks it would survive long enough to produce tubers.”
Working with thousands of ‘pure blood’ native Andean potato plants, Landeo found that although they all died when exposed to normal levels of late blight infection, specimens from 60 clones survived at lower levels. Crossing these clones and repeating the selection and crossing procedures through five cycles over a period of 13 years, he found 150 clones that survived to maturity when exposed to severe levels of late blight. “What”s more, they preserve the color, taste, texture and culinary qualities of the original varieties,” says Landeo.
Landeo had an opportunity to test his clones when the Quechua-speaking farmers of Chacllabamba, a remote village in the high Andes, appealed to CIP (via their local government agricultural advisory service) for help. Their crops of native potatoes had been almost totally destroyed by an enemy previously unknown to them: late blight.
In Chacllabamba, at more than 4,000 meters above sea level, harsh climate conditions had kept Phytophthora infestans at bay. But the increased rain and temperatures associated with global climate change had allowed late blight to creep up the mountainside, taking hold in areas where it was not previously a threat. “When we got news of their plight I selected 20 clones from among the late blight-resistant breeding populations we were working with,” says Landeo. “And sent 100 tubers of each up to Chacllabamba.”
New clones perform well
In May 2004, when Chacllabamba’s potato fields were due for harvesting, a team including scientists from CIP returned to see how the new clones had fared and hear what the villagers thought of them, in terms of both their agronomic and culinary characteristics. Appearance, taste and cooking qualities are very important to the people for whom native potatoes are the staple food. “They grow potatoes and eat potatoes at every meal – often potatoes are the meal – so of course they have strong opinions about what constitutes a good potato,” says Landeo.
The new clones had produced better harvests than the traditional highland varieties, and seven of them scored high marks in the villagers’ overall estimation. They had not succumbed to late blight or frost, they produced good harvests, their tubers were well-formed, the right color, they cooked well, had the right texture and tasted very good. “The program has been a success,” says Landeo. “We have bred highland-type potatoes that resist late blight while retaining the qualities that make them acceptable to consumers.” It is his hope that these new potatoes (and some more tuberosum-like clones also developed in the program) will eventually be available to people in Africa and Asia – and there can be little doubt that villagers in the highlands of Papua New Guinea would have a use for them too.
|
