Potato cyst nematode

Potato cyst nematodes (PCN), Globodera pallida and G. rostochiensis, are a serious soil pest of potatoes in the Andean region (where they are native), in temperate regions, and in a few other highland locations. Although PCN reduce yield by about 30%, farmers usually overlook their occurrence because of their minute size and the lack of specific aboveground symptoms. Research on this pest focuses on two aspects: breeding and evaluation of resistant clones, and field management trials (mostly rotations).

Research

Although there are different races of G. pallida, the most common are P4A, P5A, and P6A, whose interactions with the potato plant vary to such an extent that a plant resistant to one race may be susceptible to another. For this reason, the three main races were used in resistance evaluation tests with the greenhouse tests and laboratory tests. Further testing took place under natural nematode infestations in the field.

From crosses made in 1992, 400 genotypes were tested for the first time in pots. Results indicated that 36 percent were resistant to P4A, 27 percent to P5A, 9 percent to P6A, and 3 percent were simultaneously resistant to the three races. When 314 previously selected clones were subjected to the petri dish test, 96 percent to 98 percent of the clones were resistant to individual races, and 71 percent were resistant to the three races.

Field testing involved 77 genotypes that were evaluated for a second year. Of these, 45 inhibited the development of female nematodes. From these, 19 were selected for agronomic characteristics, including yields higher than those of cultivar María Huanca, a resistant cultivar released in 1989. Seven other genotypes were selected as tolerant.

Another group of clones (85) was field tested; 73 percent confirmed their resistance and 10 were selected for high yield. Seven were considered tolerant.

Studies in Bolivia confirmed the presence of both G. rostochiensis and G. pallida, which caused yield losses of up to 58 percent in infested areas. Previously selected native potato cultivars confirmed their resistance to mixed populations of Globodera. Selected clones from CIP breeding programs showed resistance, good yield, and good quality.

Potato rosary nematode

The potato rosary nematode (PRN), also called the false root-knot nematode (FRKN), Naccobus aberrans, is a severe pest in the Andean plateaus of Peru and Bolivia, where crop losses vary from 11 percent to 62 percent. The presence of many cultivated and noncultivated plant hosts limits alternatives for rotations and reduces efficiency. Therefore, management of this nematode mainly focuses on the use of resistant cultivars and soil amendments with organic matter, and the potential use of "trap plants."

Research

Resistant genotypes with high levels of resistance and high yield potential were selected after testing under field conditions for more than three years. Root nodulation in these clones was minimal. In Bolivia, native cultivars and a specific clone confirmed their resistance in Cochabamba field tests. Tests in Chuquisaca, however, have shown the occurrence of a more aggressive population of this nematode, which is able to overcome available resistance.

Soil amendments with sheep and cow manure reduced nodules to 5 to 18, whereas the control plots had an average of 40 percent nodulation. Meanwhile, yields increased 28 to 45 percent. In Bolivia, similar results were obtained with chicken manure. These results have been consistent during the past two to three years.

The third-year evaluation of crop rotations (of a five-year program with 25 different crop sequences) showed that the most economical sequences, so far, are potato-horsebean (broad bean)-potato and potato-barley-potato. In Bolivia, the use of Vicia faba and Lupinus mutabilis as rotation crops or green manure and early plowing (one month before planting) reduced the nematode population and increased potato yields.

Finally, because nematode-infested seed tubers are the main source of spreading the pest, we conducted trials to disinfect tubers. Thermotherapy (placing in water at 50 °C for 15 minutes) killed 93 to 100 percent of the nematodes. Immersion of tubers (10 minutes) in pesticide dilutions was also effective.