Striga Biocontrol on a Toothpick: A Readily Deployable and Inexpensive Method for Smallholder Farmers

Abstract

Striga hermonthica (witchweed) is a parasitic weed that attacks and significantly reduces the yields of maize, sorghum, millet, and sugarcane throughout sub-Saharan Africa. Low cost management methods such as hand weeding, short crop rotations, trap cropping, or conventional biocontrol have not been effective. Likewise, Striga-tolerant or herbicide-resistant maize cultivars are higher yielding, but are often beyond the economic means of sustenance farmers. The fungal pathogen, Fusarium oxysporum f.sp. strigae, has been the object of numerous studies to develop Striga biocontrol. Under experimental conditions this pathogen can reduce the incidence of Striga infestation but field use is not extensive, perhaps because it has not been sufficiently effective in restoring crop yield and reducing the soil Striga seed bank. Here we brought together Kenyan and US crop scientists with smallholder farmers to develop and validate an effective biocontrol strategy for management of Striga on smallholder farms. Key components of this research project were the following: (1) Development of a two-step method of fungal delivery, including laboratory coating of primary inoculum on toothpicks, followed by on-farm production of secondary field inoculum in boiled rice enabling delivery of vigorous, fresh inoculum directly to the seedbed; (2) Training of smallholder farmers (85% women), to produce the biocontrol agent and incorporate it into their maize plantings in Striga-infested soils and collect agronomic data. The field tests expanded from 30 smallholder farmers to a two-season, 500-farmer plot trial including paired plus and minus biocontrol plots with fertilizer and hybrid seed in both plots and; (3) Concerted selection of variants of the pathogen identified for enhanced virulence, as has been demonstrated in other host parasite systems were employed here on Striga via pathogen excretion of the amino acids L-leucine and L-tyrosine that are toxic to Striga but innocuous to maize. This overall strategy resulted in an average of >50% increased maize yield in the March to June rains season and >40% in the September to December rains season. Integration of this enhanced plant pathogen to Striga management in maize can significantly increase the maize yield of smallholder farmers in Kenya.

Recent research progress in combatting root parasitic weeds

“…In more than 500 farmers' fields in Kenya, incorporation of F. oxysporum f.sp. strigae inoculum into the soil resulted in an average of 40%-50% increased maize yield [101]. By contrast, no difference was observed in the plant growth parameters and yield between maize plants grown from seeds inoculated with F. oxysporum f.sp.…”

“…In addition, the absence of Fusarium toxin production by these variants and wildtype F. oxysporum f.sp. strigae was verified [101]. Plants of O. cernua infected by F. oxysporum Schltdl.…”

“…The incorporation of three variant strains of F. oxysporum f.sp. strigae, each of which excrete tyrosine, leucine and/or methionine, decreased S. hermonthica emergence and increased maize yield in S. hermonthica-infested fields [101]. In addition, the absence of Fusarium toxin production by these variants and wildtype F. oxysporum f.sp.…”

“…Incorporation of Fusarium oxysporum (isolate 34-FO) inoculum into sorghum planting hills decreased S. hermonthica emergence and increased sorghum biomass in a field in Burkina Faso [100]. Nzioki et al [101] developed a simple method using toothpicks for fungal delivery and trained smallholder farmers in Kenya to produce the biocontrol agent. In more than 500 farmers' fields in Kenya, incorporation of F. oxysporum f.sp.…”

Intractable weed problems need innovative solutions using all available technologies

“…This is still not sufficient; the biocontrol agent needs sustenance until it encounters the weed in the soil and can attack it. The one recently successful case had nearly double than average crop yield in 500 trials in Striga infested farmers' fields over two seasons (Nzioki et al 2016). Their solutions were to mutagenize their fungal pathogen to overproduce and excrete amino acids that are lethal to the parasite and without effect on maize, affording the needed hypervirulence.…”