No abstract

Crossability within lpomoea section Batatas is complex because of genetic, cytogenetic and physiological interactions . This complexity is responsible for the fact that knowledge about phylogenetic relationships within this section remains preliminary . Between 1988 and 1991, studies of planting, pollination and evaluation of progenies were carried out at the facilities of CIP in La Molina and San Ramon, Peru . Self-compatibility was found in nine diploid species . Self-incompatibility was found in diploid I. trifida, tetraploid I. tiliacea and both tetraploid and hexaploid I, batatas. Sexual compatibility is thought to be related to a multiallelic sporophytic incompatibility system which is expressed in the stigmatic papillae . Altogether, 4,162 cross pollinations were made between 11 species and 76 interspecific combinations of 110 possible combinations in a diallel 11x11 design . From these 76 interspecific combinations only 38 survived, and in these crossability (= proportion between number of fruits harvested/pollinations made) ranged from 0 .01 to 1 .00 at La Molina. At San Ramon, 11 out of 17 cross combinations were successful, and in these crossability ranged from 0 .01 to 0 .71 . Crossability appeared to be influenced by latitude which is related to flowering synchrony . The early death of seedlings is hypothesized to be related to a genomic imbalance between embryo and endosperm . Maternal effects were found in crosses between diploids and tetraploids which produced triploid and diploid offspring, and in crosses between hexaploids and diploids which gave rise to hexaploid offspring . This study shows that IL trifida and I . x leucantha may act as "bridge species" for gene flow from wild Ipomoea species to the genepool of the sweetpotato .

F1 hybrid true potato seed progenies with multiple sources of specific resistance to Pseudomonas solanacearum and adaptation were evaluated under tropical greenhouse conditions for resistance to a race 1 isolate of P. solanacearum . Results indicated that genes for adaptation are involved in conferring resistance to bacterial wilt . The effect of a particular resistant parent clone on the resistance level of its progeny depended on the resistance, adaptation or the combination of both characteristics of its mate . A heat tolerant parent gave a higher frequency of resistant offspring in combination with an ascertained source of resistance . Combining ability was an apparent feature of resistance to bacterial wilt and resistance was highest in the majority of the crosses that had a wide genetic background for both resistance and adaptation . The possible genetic nature of the resistance to bacterial wilt is discussed.

Host-pathogen-environment interaction in the potato -P. solanacearum pathogenic system was investigated by testing eight true potato seed progenies and four clones for resistance to three potato isolates of P . solanacearum under two temperature regimes . Highly significant interactions of all types : progeny x isolate, isolate x temperature, progeny x temperature, and progeny x isolate x temperature were observed . Changes in pathogenicity of isolates with temperature were the major source of the total variation . Host-pathogen interaction effect was small as compared to the main (progeny, isolate) effects . Aspects of strain specificity and host-pathogen interaction are discussed .

SummaryWe examine the changing character of supply and demand for potatoes over time and space with an eye towards implications for agricultural research. Diversification in consumption and specialization in production are our organizing themes. The foundation for comparative analysis is a map of the global distribution of potato growing area. We highlight the adverse impact of price risk on specialization and discuss several sources of uncertainty that condition the size but not the direction of major trends.