Two naturally-occurring polyacetylene derivatives are found in Tagetes minuta L. and Bidens bipinnata L. which, when released into the soil, may affect crop growth. They are alphaterthienyl in roots of T. minuta and phenylheptatriyne in leaves of B. bipinnata. Soil samples were collected from a lot where pure stands of the two weeds were found to be growing. Similar soil was also sampled for control purposes, where the two weeds did not occur. The effect of aqueous extracts, taken from the weed-infested soils, on seed germination of seven horticultural crop species was determined in the laboratory. Nine crop plant species were also grown in pots containing the sampled soil. The species were: carrot, cucumber, lettuce, maize, onion, radish, squash, sunflower and tomato. Seed germination was delayed by aqueous extracts from the weedinfested soils. Where soil was infested with Tagetes during the previous season, dry mass of top growth and plant height of all the test plants were greatly reduced. Soil from an earlier Tagetes infestation had less of an effect on growth, with carrot and maize growing normally and growth of cucumber stimulated. Soil from a Bidens infestation significantly affected the eight plant species tested, although not to such a marked degree as soil from a recent Tagetes infestation.

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Survival of Ralstonia solanacearum biovar 2 (race 3) was monitored in an artificially infested field over a five year period. Four cultural practices (maize monoculture, potato monoculture, bare fallow and weed-fallow) were applied. Ralstonia populations in the soil were quantified by planting potatoes in sub-plots of all treatments after a three-year and five-year cropping sequence. In the potato monoculture plots 88% and 96% of the plants wilted after three and five year cycles respectively. The pathogen's ability to survive without the presence of a host was demonstrated by the high wilt index recorded in the fallow plots (58% and 42%). After five years of maize monoculture 40% of potato plants wilted. Less wilting occurred in weed-fallowed plots (31% and 27%), even though some host species were present. Survival of this organism in the soil of all treatments exceeded the twoyear period generally believed to be sufficient for eradication.Oorlewing van Ralstonia solanacearum bioVar2. in grond onder verskillende verbouingstelsels Grondoorlewing van die organisme wat bakteriese verwelksiekte by aartappels veroorsaak, Ralstonia solanacearum biovar 2 (ras 3) is ondersoek in 'n veldproef. Vier verbouingstelsels (mielies in monokultuur, aartappels in monokultuur, kaal-braak en ouland met onkruid hergroei) is toegepas. Na drie en vyf jaar onderskeidelik is subpersele op al die persele met aartappels beplant. Die persentasie plante met sigbare verwelksimptome in die aartappelpersele was 88% na drie jaar en 96% na vyf jaar onderskeidelik. Die patogeen se vermoe om te oorleef in die afwesigheid van 'n gasheer blyk uit die hoe persentasie verwelking van die toetsplante op die kaal-braak persele, naamlik 58% na drie jaar en 42% na vyf jaar. Na vyf jaar van mielie monoku!-tuur het 40% van die toetsplante steeds verwelk. Minder verwelking het voorgekom op die persele waar onkruid toegelaat is (31 % en 27% onderskeidelik) ten spyte van die feit dat sommige van die onkruidspesies bekende gashere is. Oorlewing van Ralstonia in die grond van al die behandelings vir minstens vyf jaar dui daarop dat die aanname in die praktyk dat 'n twee jaar wisselbou-periode voldoende is vir die vernietiging van die organisme, foutief is.

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