Do NERICA rice cultivars express resistance to Striga hermonthica (Del.) Benth. and Striga asiatica (L.) Kuntze under field conditions?

Abstract

The parasitic weeds Striga asiatica and Striga hermonthica cause high yield losses in rain-fed upland rice in Africa. Two resistance classes (pre- and post-attachment) and several resistant genotypes have been identified among NERICA (New Rice for Africa) cultivars under laboratory conditions (in vitro) previously. However, little is known about expression of this resistance under field conditions. Here we investigated (1) whether resistance exhibited under controlled conditions would express under representative Striga-infested field conditions, and (2) whether NERICA cultivars would achieve relatively good grain yields under Striga-infested conditions. Twenty-five rice cultivars, including all 18 upland NERICA cultivars, were screened in S. asiatica-infested (in Tanzania) and S. hermonthica-infested (in Kenya) fields during two seasons. Additionally, a selection of cultivars was tested in vitro, in mini-rhizotron systems. For the first time, resistance observed under controlled conditions was confirmed in the field for NERICA-2, -5, -10 and -17 (against S. asiatica) and NERICA-1 to -5, -10, -12, -13 and -17 (against S. hermonthica). Despite high Striga-infestation levels, yields of around 1.8 t ha^-1 were obtained with NERICA-1, -9 and -10 (in the S. asiatica-infested field) and around 1.4 t ha^-1 with NERICA-3, -4, -8, -12 and -13 (in the S. hermonthica-infested field). In addition, potential levels of tolerance were identified in vitro, in NERICA-1, -17 and -9 (S. asiatica) and in NERICA-1, -17 and -10 (S. hermonthica). These findings are highly relevant to rice agronomists and breeders and molecular geneticists working on Striga resistance. In addition, cultivars combining broad-spectrum resistance with good grain yields in Striga-infested fields can be recommended to rice farmers in Striga-prone areas.

Keywords: Africa; Oryza glaberrima; Oryza sativa; Parasitic weeds; Tolerance; Upland rice.

Fig. 1

Maximum number of emerged Striga plants m⁻² per cultivar for Kyela 2011 (A), 2012-1 (B) and 2012-2 (C) – S. asiatica – and for Mbita 2010 (D) and 2011 (E) – S. hermonthica. Left side: means and standard errors of means; right side: cluster analyses.

Fig. 2

Contrasting Striga infection levels in the S. hermonthica screening trial at Mbita, Kenya (July 2011) replicate 6 (A) and replicate 3 (B); Sub-plots, representing cultivars are delimited by white lines.

Fig. 3

Rice grain dry weights (t ha⁻¹) per cultivar for Kyela 2011 (A), 2012-1 (B) and 2012-2 (C) – S. asiatica – and for Mbita 2010 (D) and 2011 (E) – S. hermonthica. Left side: means and standard errors of means; right side: cluster analyses.

Fig. 4

The relationship between resistance of the cultivars (maximum number of emerged Striga m⁻²) and rice grain yield (t ha⁻¹). (A) Mbita field trial 2011 (R² = −0.28); (B) Kyela field trial 2012-2. NERICA cultivars are abbreviated by ‘N’ following the specific number, Mwangulu is abbreviated as ‘MG’ and Supa India as ‘SI’.

Fig. 5

Post attachment resistance of selected NERICA rice cultivars (N1, N7, N9, N10 and N17) and their parents to (A) Striga hermonthica (Sh-Mbita) and (B) S. asiatica (Sa-Kyela) ecotypes collected from the field sites at Mbita Point, Kenya and Kyela, Tanzania respectively. Striga dry weight was assessed at 21 days after infection. Data are means of four replicates ± SE. Means with the same letter do not differ significantly from each other (Tukey multiple comparison test, P > 0.05).

Fig. 6

Relationship between the biomass of Striga-infected plants compared to uninfected plants (%) and the dry weight of (A) Striga hermonthica (Sh-Mbita) and (B) S. asiatica (Sa-Kyela), attached to the roots of NERICA rice cultivars (N1, N7, N9, N10 and N17), parental lines and checks, 21 days after infection. Data are presented as means ± SE of four replicates.