Effect of hosts on competition among clones and evidence of differential selection between pathogenic and saprophytic phases in experimental populations of the wheat pathogen Phaeosphaeria nodorum

<p>Abstract</p> <p>Background</p> <p>Monoculture, multi-cropping and wider use of highly resistant cultivars have been proposed as mechanisms to explain the elevated rate of evolution of plant pathogens in agricultural ecosystems. We used a mark-release-recapture experiment with the wheat pathogen <it>Phaeosphaeria nodorum </it>to evaluate the impact of two of these mechanisms on the evolution of a pathogen population. Nine <it>P. nodorum </it>isolates marked with ten microsatellite markers and one minisatellite were released onto five replicated host populations to initiate epidemics of Stagonospora nodorum leaf blotch. The experiment was carried out over two consecutive host growing seasons and two pathogen collections were made during each season.</p> <p>Results</p> <p>A total of 637 pathogen isolates matching the marked inoculants were recovered from inoculated plots over two years. Genetic diversity in the host populations affected the evolution of the corresponding <it>P. nodorum </it>populations. In the cultivar mixture the relative frequencies of inoculants did not change over the course of the experiment and the pathogen exhibited a low variation in selection coefficients.</p> <p>Conclusions</p> <p>Our results support the hypothesis that increasing genetic heterogeneity in host populations may retard the rate of evolution in associated pathogen populations. Our experiment also provides indirect evidence of fitness costs associated with host specialization in <it>P. nodorum </it>as indicated by differential selection during the pathogenic and saprophytic phases.</p

Mixed cropping of barley (Hordeum vulgare) and wheat (Triticum aestivum) landraces in the central highlands of Eritrea

A common cropping system in the central highlands of Eritrea is mixed cropping of barley (Hordeum vulgare) and wheat (Triticum aestivum); it is called hanfetz (Tigrigna word). Mixtures may give higher yield, better yield stability, better food quality and more animal feed. Factors affecting the productivity of mixtures include genotype combination, crop density and component crop ratio.Grain yields differed significantly among genotype combinations in certain years. A combination of Ardu 12/60 + Kenya + Mana gave high mean grain yield (2009 kg ha-1) and a relative yield advantage (RYT= 1.57) of 57% increase in grain yield over the sole crops. Harvest index, biomass, stand cover and thousand grain weight were well correlated with yield. Wheat plants were first suppressed by barley but later on grew taller than barley. The potential yield increase for mixtures over barley sole cropping may be associated with the relative height and higher light use (efficiency). Some genotype combinations also showed reasonable yield and resistance to stress with a drought susceptibility index Crop ratios of 100/50 (2275 kg ha-1) and 100/25 (2241 kg ha-1) were the best in grain yield when averaged over the three basic crop densities (100% = 100, 200 and 300 plants m-2) and years. Barley showed greater competitive ability than wheat. For barley the intra-specific competition was more important than the inter-specific competition. In such studies, yield advantage can be either due to the density effect or complementary use of resources. The drawback of the additive design is that yield advantage may be partly due to increased density. However, niche differentiation showed that mixtures shared resources efficiently and the yield advantage was the result of complementary use of resources. In the study under drought stress, an additive ratio (higher density) did not result in higher total yield compared to that in replacement series. The niche differentiation in both years under drought stress also showed that the yield advantage was due to complementary use of resources among the crop species.Stability analysis of barley and wheat mixtures on yield data from a large set of experiments showed that mixed cropping was significantly more stable than barley and wheat sole cropping. Some of the genotype combinations such as Ardu 12/60 + Kenya + Mana and Ardu 12/60 + Mana were more stable than others.The most promising genotype combinations and crop ratios obtained from this study have to be verified on farm and demonstrated to farmers before the technology is released for use.</p

Effect of drought stress on barley-wheat intercropping

The effects of drought or moisture stress (MS1- no stress; MS2- stress at seedling stage and MS3- stress at heading stage) was studied for different crop ratios of barley (Hordeum vulgare) and wheat (Triticum aestivum) mixtures sown in additive and replacement series at Halhale Research Station (Eritrea) during the off-seasons of 1998 and 1999. The objective was to identify crop ratios with higher productivity and to analyse the competition and niche differentiation of component crops in mixtures grown under stress. Competition and niche differentiation were assessed by analysing the data using a hyperbolic competition model. The best yields were obtained from the crop ratios 50% barley / 50% wheat and 25% barley / 100% wheat when averaged over two years. One barley plant was as competitive as about seven wheat plants. The relative competitive ability was higher in barley than in wheat. Inter-specific competition was larger than the intra-specific competition for wheat while for barley the intra-specific competition was greater than the inter-specific. The component crops shared the same resources in a complementary way. The Niche Differentiation Index (NDI) > 1 was related to Relative Yield Total (RYT) > 1 showing that the yield advantage was due to complementary use of resources. Key Words: Eritrea, Hordeum vulgare, mixed cropping, Triticum aestivu

Yield stability in barley-wheat mixed cropping in Central Highlands of Eritrea

Yield data of a large set of experiments with barley and wheat were analysed in order to assess whether yield stability was greater in mixed cropping than in sole cropping, and to identify which varietal mixture showed most stable grain yields. Stable cropping system were those having reasonably high mean yield, a regression coefficient b=1.0 of the relation between grain yield of the location and the mean yield of each genotype combination/crop ratio or cropping system and a deviation from regression (S2di) of the mean yield as small as possible. Mixed cropping with a mean grain yield of 1744 kg ha-1, regression coefficient (b) of 0.995 and a deviation from regression (S2di) of 0.277 was more stable in grain yield than either barley or wheat sole cropping. This stability test confirmed that mixed cropping was more stable than wheat or barley mono cropping and that some varietal mixtures were more stable than other

Competition and niche differentiation in barley (Hordeum vulgare) and wheat (Triticum aestivum) mixtures under rainfed conditions in the Central Highlands of Eritrea

Barley and wheat mixtures were grown in the field using additive and replacement ratios at two locations (Halhale and Mendefera) in Eritrea during the 1997 and 1998 seasons. The aim was to assess yield advantage and to analyse competition and niche differentiation using a hyperbolic regression model. It proved advantageous to grow barley and wheat in mixtures because more land area was required to obtain the same yield in sole crops. The hyperbolic regression approach confirmed that barley and wheat grown in mixtures resulted in yield advantages as a result of complementary use of resources. Barley showed greater competitive ability than wheat; for wheat, interspecific competition was larger than the intraspecific competition while for barley the intraspecific competition was greater than the interspecific competition. Niche differentiation indices were always above 1.0 indicating that the component crops did not inhibit each other from sharing resources in a complementary way

Effect of Drought Stress on Barley-Wheat Intercropping

The effects of drought or moisture stress (MS1-no stress; MS2-stress at seedling stage and MS3-stress at heading stage) was studied for different crop ratios of barley ( Hordeum vulgare ) and wheat ( Triticum aestivum ) mixtures sown in additive and replacement series at Halhale Research Station (Eritrea) during the off seasons of 1998 and 1999. The objective was to identify crop ratios with higher productivity and to analyze the competition and niche differentiation of component crops in mixtures grown under stress. Competition and niche differentiation were assessed by analyzing the data using a hyperbolic competition model. The best yields were obtained from the crop ratios 50% barley/ 50% wheat and 25% barley/ 100% wheat when averaged over two years. One barley plant was as competitive as about seven wheat plants. The relative competition ability was higher in barley than in wheat. Inter-specific competition was larger than the intra-specific competition for wheat while for barley the intra-specific competition was greater than the inter-specific. The component crops shared the same resources in a complementary way. The Niche Differentiation Index (NDI) > 1 was related to Relative Yield Total (RYT) > 1 showing that the yield advantage was due to complementary use of resources