Ecogeography and breeding potential of the wild Peruvian tuber-bearing species ofSolanum

Abstract

About 225 wild tuber-bearing species of Solanum occur on the American continent. Seventy-five percent of these are distributed in South America, with the largest concentration (approximately 100 species) occurring in Peru. Located on the western side of South America, Peru has five main regions: the Pacific Ocean Territory, the Coast, the Sierra, the Ceja de Selva, and the Selva. Wild potatoes are found only along the Coast, the Sierra, and the Ceja de Selva. The Coast (Fig. 1), a vast desert territory 2500 km long and 150 km wide, varies in elevation from sea level to 1000 m. The central and southern part of this region is usually covered with low clouds from April or May until October or November. When these clouds touch the mountains to the east, they produce a very fine condensation or mist called garua. This mist gives rise to a special ephemeral plant formation called the loma, which is notably lacking in species diversity. Within this coastal fog zone, the precipitation is extremely low, ranging from 4 to 80 mm of rainfall per year. Temperatures range from 13 to 23?C, and the humidity varies from 65 to 95 percent. During the winter season, some tuber-bearing species of Solanum occur in the lomas formation, including S. immite, S. medians, S. mochiquense, S. chancayense, S. neoweberbaueri and S. wittmackii. Not only are these species remarkable for their drought tolerance, but some are also quite resistant to viruses, fungi and bacteria. Solanum immite, for example, the first tuber-bearing species described for Peru by Dunal in 1852, is resistant to the spindle tuber viroid (PSTVd) and to the fungal wilt caused by a Verticillium species. Solanum medians, a coastal mountain species, has perhaps the widest distribution in the loma formations of the Peruvian coast. It is found from the Lomas de Manchay (350 m asl.) near La Molina, Lima (12?05' S and 76056' W) all along the southern Peruvian coast to the Lomas de Ilo (400 m asl.) and the Lomas de Sama (550 m asl.) in the departments of Moquegua and Tacna (or approximately 17?48' S and 70030' W). At Manchay-La Molina, the average annual precipitation is only 18 mm, while at Sama Grande it is 40 mm. The monthly median temperature for both places is 24?C and the minimum is nearly 13.5?C. Solanum medians represents a valuable source of genes for potato breeding programs. It is resistant to the attack of several potato viruses, including potato virus X (PVX), leaf roll (PLRV), potato viruses S (PVS) and M (PVM) and the potato spindle tuber viroid (PSTVd). It is also resistant to the tuber canker fungus, Synchytrium endobioticum; to the potato ring rot disease caused by the bacterium, Corynebacterium; the cyst nematode Globodera rostochiensis; and the tuber moth, Phthorimaea operculella. The most northern loma formation along the Peruvian coast is found at Cerro Campana (400 m asl.) in the vicinity of Trujillo (08005' S and 79?06' W). Here, thrives Solanum mochiquense, a species resistant to the potato black leg bacterium, Erwinia carotovora; the fungal organism, Synchytrium endobioticum; potato viruses PVS, and PVM; and, the nematode Meloidogyne. The Sierra divides the Coast from the Selva