The effect of soil temperature, moisture and nitrogen on Striga asiatica (L.) Kuntze seed germination, viability and emergence on sorghum (Sorghum bicolor L. Moench) roots under field conditions

Experiments were conducted in a Striga-sick field to study the effect of soil tempcriiture, moisture and nitrogen on Striga parasitism on sorghum. Striga seeds contained in nylon bags and buried at 2 em in the soil, were exposed to different tcmpcraturc and nioisturc treatments. Clear polythenc, hay rnulch and bare soil treatments were used to vary soil temperature. These treatments gave mean maximum temperatures of 60°, 48" and 37°C. respectively at 2-cm soil depth. Irrigation levels of 0 , 30 and 00 mm wcre applied to change soil moisture. Strigi~ seed germination, viability and emcrgcnce were studied. After 34 days of preconditioning, the exhumed Striga sccds from polytlienc-covered plots (solarized plots) did not germinate or retain viability when thcse seeds wcrc exposed to sorghum root exudate. However, seeds sin~ilarly buried under hay mulch or barc soil, with mean maximum soil temperatures of 48" and 37"C, respectively, had similar germination and viability percentages. Of these 75% germinated and 85% of them were viable, regardless of the temperature treatment. Although seeds stored at high tcmperaturc and humidity (solarization) wcre killed, more Striga plants cmcrged under the polythenc treatment compared to hay mulch and barc soil treatments. The observed Striga plants in the polythene mulch treatment were, therefore, assumed to have come from deeper layers where solarization was not effective. Irrigation treatments did not have significant effects on Striga seed germination and viability, but a slightly higher numbcr of plants cmerged at 60-mm irrigation level than at 30-mm and 0-mm. Striga emergence, on the other hand, was directly related to the rate of N application. Nitrogen rates of 0, 25, 50 and 100 kg h a ' resulted in the emergence of 11, 34, 38 and 40 Striga plants per plot, respectively. Despite the high infestation at high N levels, sorghum plahts did not show a loss of vigor. Nitrogen application, therefore, does not reduce Striga incidence, but seems to neutralize the harmful effects of Striga without reducing the extent of parasitism