Development and Integration of Biocontrol Products in Striga hermonthica (Del.) Benth Management Strategyا‬

Abstract

Sorghum (Sorghum bicolor L. Moench) is an important crop in Sudanese economy and diet. The area under the crop is increasing, but average yields are, invariably, low. The low yields are attributed, among other factors, to heavy infestations by the root parasitic weed Striga hermonthica. Research yielded several control measures. However, most of them are not adopted because of erratic performance or high cost. An integrated approach, in which biological control is deployed as a component is imperative. In the present study a series of laboratory experiments was undertaken at the College of Agricultural Studies, Sudan University of Science and Technology at Shambat in 2008-2012 to develop and integrate biological products in Striga management. Seven Fusarium spp. and /or variants isolated from diseased Striga plants, collected from under rainfed sorghum in Sinar State, were tested for efficacy on Striga. The amino acids L-leucine, L-therionine, L- tyrosine and L-tryptophan were tested for ability to suppress Striga germination and radicle extension with the intention of employing them to enhance fungal virulence. Nitrogen, mesquite powder and mesquite based compost, each alone and in a combination with Fusarium isolate 2, identified as F. brachygibbosum employing DNA analysis, were evaluated for their effects on Striga germination, radicle extension and pre- mature haustorium initiation. The results were further validated in a series of greenhouse experiments. Striga seed bank size and sorghum genotype, used in the greenhouse trials, were pre-determined employing the sorghum genotypes Abu Sabeen, Arfa Gdmak and Wad Baco. Fusarium isolate 2 (F. brachygibbosum) and the sorghum cultivar Arfa Gdmak were, accordingly, selected for the greenhouse experiments. The fungal isolates displayed xv differential effects on germination of Striga seeds in response to GR24 and on radicle extension of the resulting germilings. Conditioning in Fusarium isolates 2, 3, 4 and 6 cultures, for 10 days or more, reduced germination and radicle length by over 90%. Isolates 1, 5 and 7 were less suppressive. A brief exposure (5 days) to fungal toxins, in an alternating conditioning regime, reduced germination and radicle length significantly. Among the amino acids tested, L- tryptophan was the most suppressive to Striga germination and radicle extension. Air-dried mesquite leaves powder and aqueous extract promoted germination at low concentrations, but were inhibitory at high concentrations. Irrespective of concentration, mesquite powder induced pre-mature haustoria. Soil amendments with F. brachygibbosum, mesquite powder and mesquite based compost; each alone and in various combinations, reduced Striga germination and radicle length. Delayed applications were often more effective than early ones. In the greenhouse experiments Striga emergence was highest on Abu Sabeen, lowest on Arfa Gdmak and was positively correlated with the seed bank [r = 0.84-0.63 (P<0.001)]. Striga dry weight, was highly correlated with the seed bank on Abu Sabeen, and Wad Baco [r = 0.71 and 0.91 (P<0.001)], but not on Arfa Gdmak (r = 0.35, P<0.05). Striga reduced height and dry weight of the three sorghum genotypes. Sorghum height and dry weight were less associated with Striga seed bank (r = -0.49 to-0.35). F. brachygibbosum reduced Striga emergence and its effects was modulated by inoculum size and Striga seedbank. At the highest fungus incoulum (10mg/kg soil) Striga emergence was abolished at the lower seed bank and reduced to 28% at the higher seed bank size. F. brachygibbosum, irrespective of inoculum levels and Striga seedbank, resulted in sorghum height and dry weight comparable to the parasite free control. Nitrogen at 1 and 2N reduced Striga dry weight by 11 and 24%, respectively F. brachygibbosum, delayed and suppressed Striga emergence and growth considerably. F. brachygibbosum at 2 and 10g/kg soil reduced Striga dry weight xvi by 54 and 68%, respectively, while the combinations F. brachygibbosum at 2g and 10g/kg soil with nitrogen at 1 and 2N reduced Striga dry weight by 26 and 85% and 66 and 84%, respectively. Striga reduced sorghum height in a time dependent manner. Unchecked Striga infestation reduced sorghum height by 21, 27, 41 and 47% 6, 8, 10 and 12 weeks after sowing (WAS), respectively. Nitrogen and F. brachygibbosum, each alone, displayed little and moderate effects, respectively. However, their combinations were more effective. In presence of nitrogen at 2N and F. brachygibbosum at 10g/kg soil Striga reduced sorghum height by 0, 0, 8 and 12% 6, 8, 10 and 12 WAS, respectively. The trends in sorghum dry weight, despite variability, were similar. The, study, unequivocally, showed the importance of Striga seedbank as a key factor in determining response to treatments and that the fungus F. brachygibbosum effected good suppression of the parasite and ameliorated, to a large extent, the parasite devastating effects on sorghum. The study reported isolation and efficacy of F. brachygibbosum, for the first time, from diseased Striga plant and suggests the plausibility of deploying the fungus as a possible bioagent in an integrated Striga management strategy.