The growth of four summer season crops, namely Cyamopsis tetragonoloba, Sorghum vulgare, Pennisetum americanum and Zea mays, in fields with or without residues of the preceding sunflower crop was poor. Crop density, weight of seed or grain and total yield were significantly lower in sunflower fields than in the control fields (i.e. those without previous sunflower crops). Growth in terms of plant height and biomass was drastically reduced after 60 days. The effect was more pronounced in the fields where sunflower residues were allowed to decompose than in those where residues were completely removed. The soil collected from sunflower fields (both with and without residues) was found to be rich in phenolics, which in a laboratory bioassay were found to be phytotoxic. The reduced growth and yield of crops can be attributed to the release of phytotoxic phenolics from decomposing sunflower residues.

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 .

Inheritance of bacterial wilt resistance in tetraploid potato was investigated in segregating progenies of parent clones with resistance derived from different specific sources and different types of adaptation. A race 1 and a race 3 isolate of Pseudomonas solanacearum were used to test the resistance under warm temperatures. Results obtained indicated partial dominance of resistance. Significant general and specific combining abiHties showed that both additive and non-additive gene ac'tions are important in conditioning resistance expression. There was evidence that epistasis is an important component of the non-additive gene action in the inheritance of resistance. Other aspects of the resistance and implications for breeding are discussed.

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