June 1998, Volume 19 No. 2

• Editorial
• General News
• Biorational
• Internet Round-Up
• New Books
• Conference Reports
• Proceedings
• Training News

General News

Update on the Cassava Green Mite Biological Control Campaign in Africa

The continuing success story for exotic phytoseiid predators released in Africa against cassava green mite (Mononychellus tanajoa; CGM) is now being translated into financial gains for farmers.

Typhlodromalus aripo, originating from north-eastern Brazil and first released against CGM in Africa in Benin in 1993 [see BNI 18(1)], continued its rapid spread in 1997. The predator is now confirmed to be established in 12 countries and is suspected in two more. It has spread over the entire cassava production areas of Benin, Togo, Ghana and western Nigeria - an area estimated at ca 500,000 km². Significant spread has also occurred in Guinea-Conakry, Sierra Leone, Cote d·Ivoire, eastern Nigeria, Cameroon, Uganda and Kenya. The predator has been shown to spread at a rate of some 12 km/year in the first season after introduction, and up to 200 km/year in the second. CGM populations drop significantly where the predator is established, by an average of 50% to below 20 per leaf compared with previous years. Typhlodromalus aripo has also now been shown to have a significant impact on production, and a 35% increase in root yield was recorded in farmers· fields in Nigeria and Benin following its establishment. This translates into the equivalent of US$60 added value per hectare per season for the farmers in Benin - ca US$3,000,000 a year for the entire country. If this is extrapolated over the cassava zone in West Africa, additional production worth some US$60,000,000 can be expected.

Other practical factors have also been important in the successful use of T. aripo in the CGM biocontrol programme in Africa: it is ideal for national programmes to work with because it can be easily multiplied in field multiplication plots, distributed to new fields through phytoseiid-infested shoot tips, and tracked as it spreads by monitoring the incidence and distribution of phytoseiid-infested cassava shoot tips in the surrounding fields.

What remains to be done? The impact of T. aripo needs to be measured in a number of ecozones in West Africa. Implementation has only just begun in East and southern Africa, and is yet to start in most of Central Africa. Other natural enemies may still have a role to play. Another exotic phytoseiid Typhlodromalus manihoti, first released in 1989, is established and spreading (it is currently spread over 4300 km²) in the humid and transition forest ecozones in four countries. Since it is not limited by the presence of suitable shoot tips, it may be able to establish on cultivars unsuitable for T. aripo. Pathogenic fungi are being considered for similar reasons. Exploration will continue for natural enemies adapted to the subtropical highlands near the southern limit of the cassava belt, and to semi-arid conditions common in East and parts of West Africa. Substantial control of the cassava green mite can now be expected in much of Africa during the next five years.

By: J. S. Yaninek, IITA Cotonou, Benin
Contact: IITA, BP 08-0932, Cotonou, Benin
E-mail: IITA-Benin@cgnet.com
Fax: + 229350 556

Fungal Pathogens for Striga Control

A total of 48 million hectares of grain cultivating areas in Africa is potentially endangered by parasitic weeds of the genus Striga. The most extensive damage is caused in the savanna areas in western, central, eastern and southern Africa. In the Sahel it is one of the greatest obstacle to food production of subsistence farmers, with Burkina Faso, Cameroon, Ethiopia, Ghana, Mali, Nigeria and Sudan as the worst affected countries. The weed spreads rapidly in areas of low soil fertility and decreasing plant diversity in crop rotations, conditions often experienced by poor farmers in dryland zones. Of its several species, S. hermonthica and S. asiatica (which attack cereals including the important dryland crops, pearl millet and sorghum) and S. gesnerioides (which affects cowpeas) are the most serious.

The weed is extremely difficult to control. After successful germination, a germ tube which protrudes from the seed attaches itself to the root of a host plant. Much of the impairment of host growth and productivity is complete before the weed emerges above ground, some four to six weeks after germination. The reproduction rate of a Striga plant varies between 30,000 and 50,000, but it may produce 100,000 or more minute (0.2mm) seeds with a viability of up to ten or more years. Chemical control is partly effective but out of the question for resource-poor farmers. Breeding for host-plant resistance has met with limited success. Labour intensive and tedious manual weeding is only moderately successful at best, and is ineffective with severe infestations. Fertilizers bring about some improvement, but poverty is an impediment to their use. Farmers are often forced to abandon their fields as infestations become so severe that attempts to grow crops become futile. The best hope for successful control lies in an integrated approach, and recent advances in research on pathogens is one aspect showing promise.

In the early 1990s, looking in what is believed to be the area of origin of Striga hermonthica, scientists with the University of Hohenheim (Germany), identified the soil borne fungus, Fusarium nygamai, from the Blue Nile region which caused an often-fatal wilt disease in the parasitic weed1. When isolated and grown in culture, the fungus was shown to be highly effective in killing all developmental stages of Striga. But since this fungus is also able to produce, under certain growing conditions, the mycotoxin fumonisin B1, which is hazardous to mammals including humans, the fungus could not be considered for biological control.

Meanwhile in 1991, an independent survey was being conducted in Burkina Faso, Mali and Niger by scientists from McGill University (Montreal, Canada). More than 200 fungal isolates were recovered from diseased Striga in 31 sites. Amongst these, ten isolates of Fusarium were found to reduce Striga emergence in pots. An isolate of F. oxysporum (M12-4A) was the most effective and gave 100% control of Striga2. A toxigenic analysis of the isolate revealed the production of small amounts of fusaric acid in a specific culture medium but, and most important, the absence of mycotoxins. Fusarium oxysporum does not belong to the group of Fusarium spp. producing fumonisin B1. In addition, fusaric acid was not detected in the inoculum produced on sorghum straw3. Inoculum production of the fungus using glumes as a substrate yielded highest values of CFU (colony forming units) compared to sorghum straw pieces but chlamydospore production was maximized on straw4. A collaborative research programme between McGill University, the Institut d·Economie Rurale (IER) Mali, and ICRISAT (the International Crops Research Institute for the Semi Arid Tropics), funded by IDRC (the International Development Centre of Canada), facilitated field testing with this isolate in 1994. In treatments involving ground inoculum of F. oxysporum M12-4A, Striga biomass was reduced by 70% and sorghum yield increased by 97%5. In a combined treatment of F. oxysporum inoculum (grown on sorghum straw) and ammonium nitrate, the total number of emerged Striga was reduced by more than 90% while sorghum yield was increased by 100%6. In controlled environment, a reduction of 72% of Striga germination was observed when seeds were submitted to a fresh solution of macrospores of F. oxysporum M12-4A. Signs of radicle infection were recorded. Electron microscopy (TEM) observations revealed the presence of the fungus within the seed structures and some enzymatic activity was demonstrated with the disruption of the cellulose microfibril structure at the tip of the fungal hyphae7.

A follow-up 1992 survey conducted in northern Ghana resulted in 13 fungal species isolations from infected S. hermonthica plants, with Fusarium spp. the most prevalent8. Two isolates of F. oxysporum proved to be highly pathogenic to S. hermonthica9. This work is being followed up in a joint project between ICRISAT, the Universities of Hohenheim and Giessen, Germany, and the supra-regional GTZ project -Ecology and Management of Parasitic Weeds·, funded by the Federal Ministry of Cooperation and Development (BMZ), Germany. Extensive surveys in Burkina Faso, Mali and Niger also demonstrated the occurrence of highly pathogenic and host specific isolates of F. oxysporum. For pot and field experiments, the fungus was propagated on sorghum grains and incorporated preplanting into the soil. Total (100%) control of the weed was achieved in the pot experiments, and, encouragingly, field trials conducted in the less predictable environment of farmers· fields in a year of poor rainfall gave good results at five locations in three countries; in one trial, an isolate known as 121N gave more than 90% control of the weed, and a three-fold increase in sorghum biomass.

A key element in all this work is successful technology transfer leading to adoption by farmers, and for this reason, major efforts by McGill scientists are in progress to develop methods of low-tech and/or village-scale production. A dramatic increase in sorghum yields has already been recorded with one such system. The fungus is grown on sorghum straw or sorghum glumes for mass production and then small amounts are applied with the crop seed in farmers· fields at planting time: initial results indicated that decreases of 75-90% were achieved in Striga populations, and yield increases of up to 100% were obtained. The technique is now being field tested in collaboration with the Institut d·Economie Rurale (IER), Mali.

Two methods of mass producing the fungus are being assessed. The -straw· method involves placing chopped straw under plastic sheeting and solarizing it for 25 days to kill other organisms; then, sheet-wrapped, the straw is taken inside to cool and after four or five days the inoculum is incorporated. A -composting· method, which is used by ICRISAT and the Universities of Giessen and Hohenheim, involves digging a pit which is then filled with crop residues, animal dung and water; this is mixed and turned for three months after which the mixture is sterilized and the fungus inoculum added. Although the composting method is the more complex and labour intensive, compost making is traditional in some parts of West Africa, and this method also fulfils a dual role - adding the biocontrol agent at the same time as compost which improves soil fertility. However, straw or any biomass is very rare in the Sahel region and often used for other purposes.

The group at McGill University are investigating the production (through fermentation process) of a dry form of inoculum composed mainly of the resting structures (chlamydospores) of Fusarium. Sorghum straw powder is used as a substrate to grow the fungus in liquid for a 14-day period. The material is then air-dried and retains its viability for more than eight months at room temperature. Preliminary field trials with this dry powder material at a rate of 31 kg/ha was effective in reducing Striga emergence by 83%. It is believed that a local production -cottage-industry· of this material, using locally grown sorghum straw, would benefit farmers of several remote areas struggling with Striga.

However, a -cottage·-level production of the fungus may not prove an appropriate approach to reach a broad community of farmers. An approach which is based on the production of the fungus on a national or regional basis is envisaged in a collaborative venture between the University of Hohenheim, the supra-regional GTZ project -Ecology and Management of Parasitic Weeds· and McGill University. Different formulations have been developed (pasta, alginates) and are now under investigation at the University of Hohenheim, where pot experiments are being undertaken to assess their efficacy for control of S. hermonthica. First results are encouraging but also show that the quantity of inoculum (more than 1000 kg/ha) is still too high for transport and field application. Further investigations to improve the formulation and to assure a better distribution of the fungus in the soil are planned.

There is some hope that Fusarium could be developed for Striga control. It attacks all stages including ungerminated seeds, and reducing the seedbank is crucial for long term control. It also appears to have a residual effect in the field: just one year after the field experiments and the application of the fungus, emerging Striga plants were still heavily attacked and killed by the fungus in sorghum fields. The strains tested so far are host specific to S. hermonthica and do not cause wilt in cereals such as sorghum and pearl millet or in dicot crops. There is also evidence that the presence of the fungus may even enhance crop growth. Sorghum plants associated with it grew taller than those grown alone. The mechanism is unclear but may involve phytohormone production by the fungus. And finally, but crucially, because they are soilborne organisms Fusarium species are better protected against the environmental extremes of the Sahelian climate than other above-ground bioherbicides.

Striga has long been a major constraint for farmers in marginal areas of the Sahel region, attacking the very crops they rely on for food security. If the early promise shown by this research is fulfilled, there may at last be a real hope of improving Striga control, and a chance for dramatically increasing cereal crop yields in sub-Saharan Africa.

1 Abbasher, A.A.; Sauerborn, J. (1992) Fusarium nygamai: a potential bioherbicide for Striga hermonthica control in sorghum. Biological Control 2, 291-296.

2 Ciotola, M. et al. (1995) Discovery of an isolate of Fusarium oxysporum with potential to control Striga hermonthica in Africa. Weed Research 35, 303-309.

3 Savard, M.E. et al. (1997) Secondary metabolites produced by a strain of Fusarium oxysporum used for Striga control in West Africa. Biocontrol Science and Technology 7, 61-64.

4 Diarra, C. et al. (1996) Mass production of Fusarium oxysporum (M12-4A), a biocontrol agent for Striga hermonthica. In: Moran, V.C.; Hoffmann, J.H. (eds) Proceedings of the IX International Symposium on Biological Control of Weeds, Stellenbosch, South Africa, January 1996. University of Cape Town, pp.149-152.

5 Diarra, C. et al. (1996) Field efficacy of Fusarium oxysporum for the control of Striga hermonthica. Nuisibles - Pests - Pragas Vol. 4 No 1, pp.257-263.

6 Ciotola, M. et al. (1996) Fusarium oxysporum isolated M12-4A controls Striga hermonthica in the field in west Africa. In: Moran, V.C.; Hoffmann, J.H. (eds) Proceedings of the IX International Symposium on Biological Control of Weeds, Stellenbosch, South Africa, January 1996. University of Cape Town, p.508.

7 Ciotola, M. et al. (1996) Impact of Fusarium oxysporum isolate M12-4A upon seed germination of Striga hermonthica in vitro. In: Moreno, M.T. et al. (eds) Sixth International Parasitic Weed Symposium, Cordoba, Spain, April 1996, pp.871-875.

8 Abbasher A.A. et al. (1995) Microorganisms of Striga hermonthica in northern Ghana with potential as biocontrol agents. Biocontrol Science & Technology 5, 157-161.

9 Kroschel, J. et al. (1996) Pathogenicity of fungi collected in northern Ghana to Striga hermonthica. Weed Research 36, 515-520.

Contacts: Dr Alan K. Watson
[e-mail: a.watson@cgnet.com] and Marie Ciotola
[e-mail: cyk6@musica.mcgill.ca], Bioherbicide Research Group, Macdonald Campus, McGill University, 21111 Lakeshore Road, Ste-Anne-de-Bellevue, Québec, H9X 3V9, Canada
Fax: (514) 398-7897
J. Kroschel, Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ) Gmbh, University of Hohenheim (380), 70593 Stuttgart, Germany
E-mail: kroschel@uni-hohenheim.de
Fax: +49 711 459 3843

Solanum Skeletonized in South Africa

There are promising indications that a Leptinotarsa species from North America may become an important component in programmes for containing the spread in South Africa of the New World species Solanum elaeagnifolium (silverleaf nightshade, known as satansbos in South Africa). The weed, which is native to the southern USA, Mexico and Argentina, is an important invasive weed of arable and pasture land in South Africa, and is also naturalized in India, Australia, Chile, North Africa and some North American states. It spreads by seed, but locally vegetative propagation is more important - it reshoots vigorously after mechanical land clearance, and regenerates from small root sections after ploughing in arable land.

In South Africa, a programme funded by the Directorate of Resource Conservation of the National Department of Agriculture had introduced and tested eight biocontrol agents, since 1972, but until recently little progress had been made. However, Leptinotarsa texana and Leptinotarsa defecta were released in 1992, and became established at several sites in the main regions of infestation - the first insects to be established against a solanaceous weed anywhere in the world. Evaluations conducted since 1994 by Dr J. H. Hoffmann of the University of Cape Town at release sites in the Eastern Cape have indicated that populations of one of the species, L. texana, have increased significantly, reaching such levels of abundance that they have been collected and redistributed elsewhere. The damage they have inflicted has been spectacular, as they have stripped plants of foliage and most of their bark, creating -waves· of complete defoliation leaving only the skeletonized plants with their inedible fruits. The damage inflicted has caused a downward trend in terms of plant production and biomass in S. elaeagnifolium populations since monitoring began in 1994, and has reduced the density and spread of existing infestations. Leptinotarsa texana may prove to be an important control option in non-arable areas of infestation, where neither chemical nor mechanical control are economically justified. However, it could also be a useful part of an integrated programme in arable areas: plants growing the along boundaries of arable land could support reservoir populations of the beetle, which could move into the arable land to clear regrowth occurring following land preparation.

Source: Plant Protection News, the quarterly bulletin of the Plant Protection Research Institute, South Africa, No. 49.
Contact: Dr Terry Olckers, Weeds Research Division, Cedara (KwaZulu-Natal), Private Bag X9059, Pietermaritzburg 3200, South Africa
E-mail: NTTO@NATAL1.AGRIC.ZA
Fax: + 27331431017

Ascension Island Stamps on Weeds

Insect biological control agents released against weeds on Ascension Island have been featured on a set of stamps. The stamps show two species introduced in the 1970s: Cactoblastis cactorum from Argentina released to control Opuntia species, and Teleonemia scrupulosa also from Argentina introduced against Lantana camara. Both agents are credited with having contributed to reductions in the abundance of the target weeds on Ascension Island. Also featured are two seed-feeding bruchids originating from the New World, Neltumius arizonensis and Algarobius prosopis, released more recently, in 1996, for control of Prosopis juliflora [see BNI 18(2)].