AI IDRIS¹, NE AHMED² and AGT BABIKER²

¹Rahad Agric. Scheme, Khartoum, Sudan; ²Agricultural Research Corporation , Medani, Sudan

Background and objectives
Striga hermonthica represents the major dominant and economically important noxious parasitic weed of sorghum in Sudan. Yield losses are high and complete crop failure is very common under heavy infestation. Available control of Striga has always to deal with its special biology, close linkage and damage to host plants. This necessitates exploration of pathogens that inhibit germination of its seeds and/or affect the parasite's pre-penetration, or attack Striga seedlings before emergance. Recent observations of sporadic natural striga death caused by Fusarium solani prompted further investigations into evaluation of the potential of this fungus for biological control of Striga.

Results and discussion
F. solani inhibited Striga seed germination by 100% at inoculum density 3x10⁶ spores/ml and when diluted 10- and 100-fold. However, 1000- and 10,000-fold had no effect on seed germination. Moreover, no variability in effectiveness resulted from the use of fungal suspension or filtrate. It is obvious that the fungus activity on Striga seed germination was not directly related to the number of spores in the suspension or its parasitic or saprophytic growth in the seeds. This was confirmed by a tetrazolium test which clearly indicated that the fungus was not able to attack the seed and did not damage or break down the cell wall of Striga seeds. However, germination was irreversible on rinsing ungerminated seeds with either water or alcohol.

The results clearly indicated that extracellular metabolites secreted by F. solani woud be at least partly responsible for the observed inhibition of Striga germination. The inhibitory effect of F. solani to Striga seeds was not affected by the stimulants ACC (200 p.p.m.), OR24 (1 p.p.m.) and ethephon (1 p.p.m.). OR24 alone and ACC or ethephon did not trigger Striga seed germination in presence of F. solani filtrate. Addition of the fungus at 0, 2 and 4 h after OR24 application greatly inhibited Striga seed germination (3%). However, delay of application between 6 and 12 h after addition of stimulant increased germination to 33-88%. Failure of F. solani filtrate, when applied 6 h after GR24 to inhibit germination, indicated that germination had already started.

The inability of ACC and GR24 to stimulate Striga seed germination in presence of F. solani filtrate suggests that the filtrate inhibits either ethylene biosynthesis or ethylene action [1]. Ability of the filtrate to inhibit germination in the presence of ethephon suggests that the fungus filtrate is not an ethylene biosynthesis inhibitor, but more likely it inhibits ethylene action. However, an effect of fungal filtrate may be due to alteration on seed structure leading to changes in structure of the stimulant or ethylene binding sites [2]. The fungus satisfied most requirements of a successful biocontrol agent, and inhibition of Striga seeds before germination will save the crop completely from Striga damage.

References
1. Babiker AGT, Hamdoun AM, 1983. Weed Research 23, 125-131.
2. Ciotola M, Watson AK, Hallet SO, 1995. Weed Research 35, 303-309.