RESPONSE OF GENETICALLY IMPROVED POTATO CLONES TO THE CYST NEMATODE, GLOBODERA ROSTOCHIENSIS, AND RESPONSE OF A RESISTANT CLONE TO THE NEMATODE IN MICROPLOTS
The response of 15 genetically improved potato clones, resistant to Phytophthora infestans, to a Venezuelan population of G. rostochiensis pathotype Ro2, was evaluated in pots outdoors. Potato plants were grown in pots containing soil infested with 20 eggs of nematode/cm3 soil. The response of the potato clones to the nematode was evaluated on the basis of numbers of females on the roots and multiplication rates recorded 70 and 120 days after inoculation, respectively. The clones 393465-38 and 392634-21 were resistant because of the low numbers of females and multiplication rates of the nematode of 0.02 and 0.08, respectively. The clones 393558-44, 392636-9 and 393073-15 were considered moderately resistant as the multiplication rates of the nematode were 5.06, 6.53 and 4.83, respectively. The remaining clones were susceptible. In a separate experiment, the relationship between a geometric series of ten initial densities of the nematode, ranging from 0 to 256 eggs/cm3 soil, and growth and yield of the potato clone 393465-38 was investigated in 30 dm3 microplots. Fitting the Seinhorst model, y = m + (1 - m) z[supP][supiû[supT], to average yield, the tolerance limit of the clone 393469-38 to the nematode was estimated 0.5 eggs/cm3 soil. Yield reductions of 20% and maximum of 66% occurred in soil infested with 8 and = 64 eggs/cm3 soil, respectively. At the end of the experiment, the final populations of G. rostochiensis were lower than those at the beginning, confirming the resistance of this clone to the nematode. However, the plant damage observed at high nematode densities suggests an intolerant response of clone 393465-38 to the nematode.
