EFFECT OF CULTURAL TREATMENTS ON THE INCIDENCE OF STRIGA HERMONTHICA (DEL.) BENTH. AND YIELDS OF SORGHUM IN THE SUDAN: FIELD EXPERIMENTS 1957/8

Trap cropping, nitrogenous fertilizers, selective weed‐killer sprays (2:4‐D) and sorghum varieties affected the incidence of Striga hermonthica.S. hermonthica developed more on the variety Debekri than Feterita. Usually, applying increasing amounts of nitrogen (sulphate of ammonia or urea) progressively decreased the incidence of S. hermonthica on Feterita, Dwarf White Milo and Wad Fahl. Early applications were more effective than late applications. Nitrogen given to Debekri never significantly decreased the amount of S. hermonthica and often increased it. In an experiment on very infertile and heavily infested soil, nitrogen increased the amounts of S. hermonthica on all varieties but less on Feterita than Debekri. The weights of S. hermonthica on Feterita reached a maximum with 40 lb. N/acre but were still increasing after adding 80 lb. N to Debekri. Weed‐killer sprays affected S. hermonthica on Feterita and Debekri equally. They delayed the appearance of its aerial stems and usually decreased their subsequent growth. Spraying with 1.6 lb. 2:4‐D/acre was usually more effective than with 0.8 lb. and sometimes spraying 3 weeks after sowing was better than after 2 weeks. Trap cropping with Sudan grass ( Sorghum sudanense ) for 5 weeks, but not for 3, significantly decreased the incidence of S. hermonthica in succeeding sorghum crops and sometimes increased grain yields. Phosphorus and potassium affected neither the incidence of S. hermonthica nor sorghum yields. On lightly infested soil where S. hermonthica was not damaging, 80 lb. N/acre trebled grain yields (from 1140 to 3530 lb./acre). As infestations increased in severity, the absolute gains decreased but the relative effects were greater. On crops infected early and severely, yields increased by 820% from 180 to 1505 lb./acre. Nitrogen applied at sowing and after 3 weeks gave larger increases than later applications. Yield increases caused by 2:4‐D sprays are associated with the delayed emergence of aerial stems of S. hermonthica. Where infection developed rapidly and the mean levels of infection were at least 20% within 50 days of sowing, sprays increased both grain and straw yields; where infection developed slowly, only grain yields were increased. Although 1.6 lb. of 2:4‐D/acre decreased S. hermonthica more than 0.8 lb., they affected yields equally as did spraying 2 or 3 weeks after sowing. 0.8 lb. of 2:4‐D/acre and 80 lb. N/acre when applied separately to severely infected crops increased grain yields from 38 lb. in the untreated plots to only 163 and 588 lb./acre respectively, whereas applied together they gave 2170 lb./acre. In the same experiment nitrogen increased the yields of the less heavily infected Feterita more than those of Debekri. 2:4‐D affected both varieties similarly.