| Potatoes have become one of the most valuable staple food crops for humankind, even in the tropical warm climates under conditions of minimum agricultural inputs. There, late blight (LB), bacterial wilt (BW), root-knot nematodes (RK), and major potato viruses (PVX, PVY, PLV) are the most important diseases.;Out of the 39 remaining clones which tested negative for PVX, PLRV, and PVY, there were selected six resistant to RK, 26 resistant to BW, and 18 resistant to LB. For multiple disease resistance, only one clone showed a high degree of combined resistance to LB, BW, and RK. Six clones were resistant to both LB and BW. Another five clones which were resistant to RK and BW were also partially resistant to LB. In 1989, those 39 clones were evaluated in the field at two locations nearby Ithaca, New York, and in four highland sites in Rwanda, Central Africa. The Ithaca field evaluation resulted in 12 clones resistant or moderately resistant to LB. All virus diseased clones were discarded. Almost all the test clones proved to be susceptible to blight about 60 days after planting during the heavy rainy season in Rwanda. In terms of LB resistance, the Rwanda field results were positively correlated with the Ithaca greenhouse results but were not statistically significant. Due to severe blight attack immediately after potato emergence, it was not possible to record any reliable observations about bacterial wilt and root-knot nematodes.;In the fall of 1986, three potato populations were intercrossed at Ithaca, N.Y., to generate one potato population segregating for good horticultural traits and disease resistance. In the summer of 1987, nine thousand five hundred seedlings were screened for resistance to PVX, PVY and LB, and were reduced to 1591 clones. In 1988, these clones were grown in the field, then evaluated again in the greenhouse and laboratory for resistance to LB, BW, RK, PVX, and PVY. |
