Biological control of parasitic plants is a promising field. Fusarium oxysporum f. sp. strigae has shown great potential for biocontrol of striga and shows clear host specificity (crops sorghum, pearl millet, maize, rice, fonio, cotton, groundnut, cowpea and okra) was unaffected by this pathogen. However, while wild type strains of F. oxysporum have been reported to reduce the incidence of Striga infestation, their use in the field has not yet been significant.
Specific strains of F. oxysporum are selected for enhanced virulence, which strengthens its killing power towards striga. Based on amino acid sensitivity studies, we select variants of the wild type strain that overproduces tyrosine and leucine. In addition to these two amino acids, we also select for variants that overproduce methionine because methionine converts to ethylene by soil microbes. Ethylene has been shown to germinate Striga, which makes the weed easier to kill. The three selected strains (Leu2a, Z6a, and Z5a) produce excess leucine, methionine, and tyrosine as shown by GC-Mass Spectroscopy analysis of CATSUP broth grown cultures of F. oxysporum. The product is locally sourced from Kenya, host specific to only harm Striga, and shows no signs of toxins based on tox/ecotox tests at Virginia Tech and the University of Nairobi.
Each amino acid overproducing strain of F. oxysporum are cultured on PDA on which are placed sterile wooden toothpicks. As the fungus grows, it embeds into the wood grain of the toothpick. Once fully dried, the trio of toothpicks are stored together in sealed, sterile drinking straws and transported to the farmers. We are currently working on developing the most cost-efficient process to produce and deliver this primary inoculum to the village level.
Kenyan smallholder farmers, who are familiar with cooking rice, were given the fungal-grown toothpicks to inoculate cooked and cooled pearled rice to provide fresh on-farm inoculum of the fungi. The cooked rice and toothpick are placed into an alcohol-swabbed sterile, plastic-lidded container at room temperature. Pearled rice is an ideal home to support abundant mycelial growth if kept unopened and shaken twice daily for a three-day period. It was determined that three days of incubation at room temperature was optimal to prevent external contamination and avoid depletion of carbohydrates. In an effort to make the product as affordable as possible for farmers, we are evaluating substrates other than rice (eg. gritted maize cob).
Armed with a Grand Challenges Exploration grant from the Bill and Melinda Gates Foundation, we conducted trials on 500 Striga infested farms in western Kenya. With data collected from two growing seasons, most (99.6%) of the farmers had equal or greater yield in their Foxy T14 plots relative to yield in their comparable Farmer-practice plots without Foxy T14 (see the table - everything over the 1:1 line shows an improvement over the control). The average maize yield in the long season was increased by 56.5% in Foxy T14 plots relative to the Farmer-practice plots (P < 0.0001, pair-wise t-test). Approximately one third of the farmers doubled their yield in the long-season Foxy T14 plots. Overall, yield in the short season was reduced by drought but we still observed an average increase yield of 42% (P< 0.0001).
In addition to seeing yield increases, Striga infestation was reduced in 80% of long season Foxy T14 plots and in 92% of the short season Foxy T14 plots (Fig. 4). Maize yield increases were correlated with inherent yield capacity of fields and with above-ground Striga density.
Typical of maize smallholder farmers in Kenya, 85% of the farmers were women.
On the 500 farms tested, the FOXY T14 technology led to a yield increase of 56.5% in the long season and 42% in the short season.