Summary• Striga hermonthica and S. asiatica are root parasitic weeds that infect the major cereal crops of sub-Saharan Africa causing severe losses in yield. The interspecific upland NEw RICe for Africa (NERICA) cultivars are popular amongst subsistence farmers, but little is known about their post-attachment resistance against Striga.• Here, we evaluate the post-attachment resistance levels of the NERICA cultivars and their parents against ecotypes of S. hermonthica and S. asiatica, characterize the phenotype of the resistance mechanisms and determine the effect of Striga on host biomass.• Some NERICA cultivars showed good broad-spectrum resistance against several Striga ecotypes, whereas others showed intermediate resistance or were very susceptible. The phenotype of a resistant interaction was often characterized by an inability of the parasite to penetrate the endodermis. Moreover, some parasites formed only a few connections to the host xylem, grew slowly and remained small.• The most resistant NERICA cultivars were least damaged by Striga, although even a small number of parasites caused a reduction in above-ground host biomass. The elucidation of the molecular genetic basis of the resistance mechanisms and tolerance would allow the development of cultivars with multiple, durable resistance for use in farmers' fields.

HighlightsStriga spp. resistance in NERICA cultivars, observed in vitro, was confirmed in situ.NERICA-2, -10 (S. asiatica) and -10, -17 (S. hermonthica) are most resistant.NERICA-1, -17, -9 (S. asiatica) and -1, -17, -10 (S. hermonthica) show tolerance.Some NERICA cultivars yielded 1.5–2 t ha−1 despite Striga spp. infestation.

Summary The parasitic weeds Striga asiatica and Striga hermonthica cause devastating yield losses to upland rice in Africa. Little is known about genetic variation in host resistance and tolerance across rice genotypes, in relation to virulence differences across Striga species and ecotypes.Diverse rice genotypes were phenotyped for the above traits in S. asiatica‐ (Tanzania) and S. hermonthica‐infested fields (Kenya and Uganda) and under controlled conditions.New rice genotypes with either ecotype‐specific or broad‐spectrum resistance were identified. Resistance identified in the field was confirmed under controlled conditions, providing evidence that resistance was largely genetically determined. Striga‐resistant genotypes contributed to yield security under Striga‐infested conditions, although grain yield was also determined by the genotype‐specific yield potential and tolerance. Tolerance, the physiological mechanism mitigating Striga effects on host growth and physiology, was unrelated to resistance, implying that any combination of high, medium or low levels of these traits can be found across rice genotypes. Striga virulence varies across species and ecotypes. The extent of Striga‐induced host damage results from the interaction between parasite virulence and genetically determined levels of host–plant resistance and tolerance. These novel findings support the need for predictive breeding strategies based on knowledge of host resistance and parasite virulence.

No abstract

The response of microbial functional diversity as well as its resistance to stress or disturbances caused by the introduction of an exotic tree species, Acacia holosericea, ectomycorrhized or not with Pisolithus albus, was examined. The results show that this ectomycorrhizal fungus promotes drastically the growth of this fastgrowing tree species in field conditions after 7 years of plantation. Compared to the crop soil surrounding the A. holosericea plantation, this exotic tree species, associated or not with the ectomycorrhizal symbiont, induced strong modifications in soil microbial functionalities (assessed by measuring the patterns of in situ catabolic potential of microbial communities) and reduced soil resistance in response to increasing stress or disturbance (salinity, temperature, and freeze-thaw and wet-dry cycles). In addition, A. holosericea strongly modified the structure of arbuscular mycorrhizal fungus communities. These results show clearly that exotic plants may be responsible for important changes in soil microbiota affecting the structure and functions of microbial communities.Forest loss and degradation through human disturbance as well as deterioration of land productivity is a major problem in large areas of arid and semiarid environments. Degraded soils are characterized by loss or disturbance of the vegetation cover, increased soil erosion, decreased in-water infiltration, loss of available nutrients and organic matter, loss of microbial propagules, and/or diminution in microbial activity (25,35,44). Restoration of ecosystem health and productivity was traditionally achieved via abandonment of land and subsequent natural forest succession. In recent decades, management options for acceleration of the recovery and restoration of the productivity and biodiversity of disturbed ecosystems have been considered, since fallow periods have been shortened or eliminated due to increased anthropogenic pressure and agriculture intensification (49). Concerning the techniques used for rehabilitating degraded areas, there is increasing evidence that forest plantations can play a key role in ecosystem rehabilitation or restoration (52). Among candidate plantation species, fast-growing leguminous tree species (e.g., Australian Acacia species) should have preference.

This paper provides a retrospective look at a systems-oriented research program, on the increasing occurrence of parasitic weeds in rainfed rice in sub-Saharan Africa, to qualitatively assess merits and identify challenges of such approach. We gained a broad contextual overview of the problem and different stakeholders' roles, which enabled identification of entry points for innovations in parasitic weed management. At the crop level parasitic weed infestation is associated with poor soil fertility and water management. Farmers' infrequent use of inputs to control them was caused by various factors, ranging from fears of undesired side effects (agronomic) to a lack of quality control of products (institutional). Furthermore, there may be enough extension agents, but they lack the required training on (parasitic) weed management to provide farmers with advice, while their organizations do not provide them with the necessary means for farm visits. At even higher organizational levels we observed a lack of coherent policies on parasitic weed control and implementation of them. Merits and challenges of an integrated multi-stakeholder and multi-level research project are discussed.

No abstract

No abstract