Varietal Mixtures Research Articles

Diversity of different farmer and modern wheat varieties cultivated in contrasting organic farming conditions in western Europe and implications for European seed and variety legislation

The importance of genetic diversity in cultivated varieties for organic and low-input agriculture has attracted increasing attention in recent years, with a need to identify relevant sources of diversity and strategies for incorporating diversity in plant breeding for organic systems. However, the regulatory system in many countries, particularly in the European Union, restricts the varieties available to farmers to those registered in an official catalogue, and most countries require varieties to go through official tests under conventional management, which has resulted in a lack of suitable varieties available to organic farmers. This study characterized a sample of wheat (Triticum aestivum L.) landraces, historic varieties and varietal mixtures currently of interest to organic farmers in a diverse range of organic conditions on farms in Italy, France and the Netherlands. These varieties were assessed for individual plant and spike characteristics and compared to modern registered wheat varieties grown under the same on-farm conditions. Significant differences in mean values were found among varieties for many plant and spike traits, as well as significant variety-by-environment interactions. There were often similar levels of intra-varietal variability between farmer and modern varieties, indicating that the strong selection for genetic homogeneity to meet regulatory criteria has little impact on the phenotypic variability of certain traits when assessed on-farm. Several farmer varieties had high values of traits related to productivity outside their region of origin, which underlines the need for experimentation with diverse types of varieties in order to find and develop appropriate varieties for organic systems.

Analyzing the Influence of Qualitative Resistance Selection Pressure on Variation of Aggressiveness in Plasmopara Halstedii

Variation of aggressiveness in populations of race 710 of Plasmopara halstedii ((Farl.) Berl. et Toni) (sunflower downy mildew) was measured under different strategies of qualitative resistance selection pressure: mixture, alternation and monoculture of major resistance genes in comparison with a population under no selection pressure. Two sunflower lines showing different levels of quantitative resistance were used to measure four aggressiveness criteria: percentage infection, latent period, sporulation density and reduction of hypocotyl length. P. halstedii strains multiplied under varietal mixtures presented the greatest sporulation densities and shortest hypocotyl lengths, those multiplied under alternation presented a reduced latent period and shorter hypocotyl lengths compared with those not influenced by selection pressure. There were no significant differences between populations multiplied under monoculture of resistance genes and those under no selection pressure. These changes appear as being linked to the number of infected plants present. The results suggested that the method of Pl gene management affects aggressiveness because it determines the number of susceptible plants harbored by the parasite.

Factors shaping on-farm genetic resources of sorghum [Sorghum bicolor (L.) Moench] in the centre of diversity, Ethiopia.

Ethiopia is cited as one of the centres of sorghum diversity. In order to assess the on-farm genetic resources management of sorghum various research methodologies were employed. These were focus group interviews with 360 farmers, key informant interviews with 60 farmers and development agents and semi-structured interviews with 250 farmers. Besides, diversity fair was done with over 1200 farmers. For quantifying on-farm diversity, direct on-farm monitoring and participation with 120 farmers were made. Quantification of varietal diversity per farm was counted by a participatory zigzag sampling in the diagonal direction of the plot with the farmer and all encountered varieties were counted. Soil samples were taken from 120 farms and were subjected to analyses of soil pH, P, available nitrogen, organic matter and exchangeable potassium. Altitude and other related climatic data were collected. The number of varieties conserved by farmers ranged from one to twenty per farm and this is affected by socio-economic and biophysical factors. The mean numbers of 8.3 and 6.3 varieties were grown by Oromo and Amhara farmers respectively. The minimum and maximum range did not vary for both ethnic groups. There was no significant difference in the number of varieties held by various wealth groups. With respect to farm size as explained by the quadratic model, it significantly accounted and predicted for the variation in the number of varieties. The role of soil pH, P, available nitrogen, organic matter, and exchangeable potassium on-farm genetic diversity is described. P was a positive limiting factor for varietal diversity. As to the effect of crop ecology, there were higher number of varieties in the intermediate altitudes than in the lowland and highland. Both the quadratic and linear equation expressed that distance from the house and town and showed non-significant relationship to the number of varieties planted per farm. Varietal mixture is one of the strategies used by the farmers for improved on-farm genetic diversity management. Farmers’ underlying principles for conserving genetic diversity is described. Three models developed, namely; Bioecogeographic genetic diversity model, Farmer induced genetic diversity model and Farmer-cum-bioecogeographic genetic diversity model are explaining the processes shaping on-farm genetic diversity of sorghum in Ethiopia. Key words: Biophysical factors, farmer varieties, germplasm, genetic diversity, genetic diversity model, socio-economic factors, on-farm.

Crown Rust Development and Selection for Virulence in Puccinia coronata f. sp. avenae in an Oat Multiline Cultivar.

Crown rust, caused by Puccinia coronata f. sp. avenae is the most important disease of cultivated oat in North America. Numerous race-specific (Pc) genes for crown rust have been found in Avena spp. but this type of resistance has not been durable when used in oat cultivars. Increasing diversity for resistance within a crop by the use of multiline cultivars or varietal mixtures has been proposed as a means of achieving durable resistance to highly variable pathogens such as P. coronata f. sp. avenae. Multiline cv. E77 was evaluated over multiple seasons in the University of Minnesota buckthorn nursery in St. Paul. Buckthorn (Rhamnus cathartica, the alternate host of P. coronata) supports a sexually recombining, highly diverse crown rust population in the St. Paul nursery. Crown rust severity on flag leaves was measured multiple times on E77 and its 10 component lines during grain filling. Single-urediniospore isolates taken from crown rust samples during early stages of the epidemic and at the end of the epidemic were tested for virulence on the 10 component lines of E77 in greenhouse seedling tests. Crown rust development was reduced in E77 compared with the weighted mean of the component lines at all stages of the crown rust epidemics. The mean virulence of single-urediniospore isolates tended to increase late in the epidemic on E77. These data suggest that multilines may select for complex virulence or "super races" and any resistance effect may not be durable.

Does resource complementarity or prevention of lodging contribute to the increased productivity of rice varietal mixtures in Yunnan, China?

We conducted an experiment in Gejiu, Yunnan Province of China during the wet season of 2002 to examine the importance of resource complementarity and prevention of lodging in the fast-spreading practice of growing rice varietal mixtures in China to suppress rice blast disease and its associated damage. The hybrid rice variety Shanyou 63 and the glutinous or sticky rice variety Huangkenuo were used to study intra- and inter-varietal competition in the rice intercropping system. The experiment was laid out as a randomized complete block design in four replications with pure stands and mixtures of both varieties at different total planting densities and mixing ratios. In half of the pure stand treatments of glutinous rice a metal frame was installed to prevent lodging. The results showed that resource complementarity between the two rice varieties, resulting from niche differentiation, was marginal and did not greatly enhance the productivity of the rice varietal mixtures. In contrast, prevention of lodging of glutinous rice was identified as an important additional advantage of growing these rice varieties in mixture.

Identificação de raças fisiológicas de Magnaporthe grisea em áreas de arroz irrigado no Estado do Tocantins

Identification of physiological races of Magnaporthe grisea in areas of rice irrigated in the State of Tocantins In Brazil, the most important disease in rice is blast, which is caused by the fungus Magnaporthe grisea. This disease is considered to be a limiting factor for rice productivity. This work aimed to determine the prevalent races of the rice blast pathogen M. grisea in the state of Tocantins, in order to study the variability in the region and make it possible to develop and recommend blast-resistant rice cultivars. To assess this information, infected leaves were collected from three experimental sites, each consisting of multilines and a varietal mixture of irrigated rice in Tocantins state. A total of 250 monosporic isolates were obtained. Subsequently, these isolates were inoculated in the International Standard Differential (ISD). A total of 45 races were identified, where the most prevalent race was IA1 (24.8% of the isolates), followed by IA-65 (11.2%) and IA-33 (6.4%), respectively. The highest number of races was found in the experimental site located at Lagoa da Confusao, followed by Formoso do Araguaia (Projeto Formoso) and Unitins-Agro, respectively. In the population of M. grisea under study, a high variability was found with prevalence of the race groups: IA, IB and ID.

Farmers' Seed System of Sorghum [Sorghum bicolor (L.) Moench] in the Center of Diversity: II. Seed Quality, Storage, Protection and Security

ABSTRACT Farmer's seed system is defined as a system in which seed selection, seed production, seed storage, management and seed diffusion are integrated with crop production. The seed quality, seed storage, seed protection and seed security are important parts of seed system. The objectives of this study were to assess in sorghum: 1) the quality of farmer-produced seed, 2) the seed management, storage and protection, and 3) level and factors of seed security in the region. The on-farm seed quality for sorghum was comparatively good. The average germination for the three varieties tested was 82.8%. The conventional purity (varietal) level of 98% was not met due to the dominant practice of varietal mixture in the region. However, varietal component-wise it met the standard. However, if we suppose that the varieties mixed as a normal component of a farmers' seed lot, then the mean purity of 97% of 10 farmers met the national and regional West Asia and North Africa (WANA) standard. Farmers have various on-fa...

Farmers' Breeding of Sorghum in the Center of Diversity, Ethiopia: I. Socioecotype Differentiation, Varietal Mixture and Selection Efficiency

ABSTRACT Sorghum is one of the main staple crops for the poorest and most food insecure people of the world. Ethiopia being the center of origin and diversity for sorghum, the crop has been cultivated for many thousand years. In order to quantify socioecotype differentiation, varietal mixture, and selection efficiency, both survey research and field experiments were undertaken. Sorghum is found to be widely grown in the study area and adaptively differentiated into races, ecotypes, and varieties. The typical Durra race is found in the lowlands; Durra, Durra-Caudatum, and Caudatum are found in the intermediate altitude and Bicolor-Caudatum, Guinea-Caudatum, and Bicolor races are found in the highlands. Ecotype differentiation resulted in ecotypes and varieties specifically adapted to the crop growing environments. A transect of variety by altitude showed a genotype substitution over altitudinal ranges from the lowlands to the highlands. Socioecotypic differentiation, introduced as a new concept, is the spa...

Comparative evaluation of exotic and adapted germplasm of spring wheat for floral characteristics in the Indo‐Gangetic Plains of northern India

AbstractFour hundred spring wheat genotypes, comprising exotic and indigenous germplasm and adapted cultivars for commercial cultivation, were investigated during four crop seasons for six floral characteristics related to outcrossing behaviour. Genetic variability and heritability were investigated along with phenotypic correlation coefficients among these characteristics. A significant quantitative variation existed among the characteristics in all three groups. High heritability for stigma length, openness of florets, anther extrusion and duration of floral opening indicated that selection for these characteristics may be effective. The adapted cultivars were found more promising than the exotic and indigenous germplasm. Most of the characteristics showed significantly positive correlation with each other and high correlation was observed between anther and stigma length, and anther extrusion and duration of floral opening. Superiority of varietal mixtures and segregating generations (F3 and F4) of two crosses over parental lines suggested that heterozygosity and heterogeneity can provide yield advantages. Germplasm lines possessing superior expression of floral characteristics may be used to improve yielding ability through enhanced heterozygosity or the development of hybrids.

Strategies for controlling cassava mosaic virus disease in Africa

Cassava mosaic disease (CMD) is caused by whiteflyborne viruses of the genus Begomovirus (family Geminiviridae). The disease has long been regarded as the most important of those affecting cassava in sub‐Saharan Africa, and has been the subject of much research, especially since the onset of the current very damaging pandemic in eastern and central Africa. This review considers the main features of CMD and the various possible means of control. The main emphasis to date has been on the development and deployment of virus‐resistant varieties. These are widely adopted in countries where CMD has caused serious problems, and provided a powerful incentive for farmers to abandon some of the most susceptible of their traditional varieties. Only limited use has been made of phytosanitation involving CMD‐free planting material and the removal (roguing) of diseased plants. Cultural methods of control using varietal mixtures, intercrops or other cropping practices have also been neglected, and there is a need for much additional research before they can be deployed effectively. Nevertheless, the severe losses now being caused by CMD in many parts of sub‐Saharan Africa could be greatly decreased through the application of existing knowledge.

Geographic variation in phenotypic traits in Phratora spp. and the effects of conditioning on feeding preference

AbstractThe leaf beetle genus Phratora (L.) (Coleoptera: Chrysomelidae) includes important pests of cultivated willows and poplars. The feeding preference of P. vulgatissima and P. vitellinae collected from different geographical locations was studied under laboratory conditions. There were geographic differences in the amount consumed of different willow host genotypes by each beetle species. Correlation analysis showed that, regardless of these individual differences between willow genotypes and locations, the ranking of preferred host genotypes was significantly related between locations. There were differences in the preference of host genotypes by P. vulgatissima adults and larvae after being confined to a specific willow genotype for a month, suggesting the possibility of a conditioning effect on food preference. This study also permitted the assessment of possible geographic differentiation in morphological traits. Both beetle species generally showed an increase in body size, width, and percentage fat of dry body weight with a more northerly location. These results are discussed in relation to using varietal mixtures as a pest management strategy.

Foliar insect pest management on cowpea ( Vigna unguiculata Walpers) in simulated varietal mixtures. : I. The suitability of partial insecticide applications

This research described in this paper examined the impact of varietal mixtures on pest management utilising a model system of partial insecticide applications. In six field experiments conducted in 1995 and 1996 in Igalaland, Kogi State, Nigeria varietal mixtures were simulated through the application of systemic insecticide (carbofuran or furathiocarb) to a pre-determined percentage of a cowpea ( Vigna unguiculata) crop. The objective was to examine and compare the pest damage on untreated cowpea plants grown in plots in which varying percentages of the cowpea plants were treated with insecticide. A secondary objective was to determine whether action of the foliage pests influenced flower production. The results showed that the presence of insecticide-treated plants reduced the level of leaf damage by Ootheca mutabilis Sahlberg and the densities of Aphis craccivora Koch populations on untreated plants. The greater the percentage of insecticide-treated plants the greater this reduction on the untreated plants. Meanwhile, the number of flowers found on the untreated plants increased suggesting the foliage pest damage reduced flower production.

Foliar insect pest management on cowpea ( Vigna unguiculata Walpers) in simulated varietal mixtures : II. Pest resistance management implications

Field experiments were established in which cowpea ( Vigna unguiculata Walpers) was treated with a series of partial insecticide (carbofuran) application treatments to simulate (susceptible/resistant) varietal mixtures and a series of inter-crops with soybean ( Glycine max). Treatments were sampled for damage by the foliage pests Ootheca mutabilis Sahlberg and Aphis craccivora Koch and for flower production. The soybean is likely to have provided both a mechanical barrier to pest movement and disruption to O. mutabilis through volatile allelochemicals. The comparison between the two treatment types would suggest whether similar mechanisms were the cause of pest reductions observed in partial insecticide applications on cowpea. The results suggested that the inter-cropping with soybean was not as effective in managing pest populations as a partial insecticide application and could even increase pest numbers. Further examination of the data from the partial insecticide application treatments appeared to show that the pest infestation was lower on the untreated plants closest to the insecticide-treated plants. This could be as a result of pest redistribution onto insecticide-treated plants after initial colonisation. The implications of this, in terms of pest resistance management are discussed.

Progress of cassava mosaic virus disease and whitefly vector populations in single and mixed stands of four cassava varieties grown under epidemic conditions in Uganda

SummaryProgress curves of cassava mosaic virus disease (CMD) and populations of the whitefly vector (Bemisia tabaci) were assessed using four cassava varieties grown alone and as a random mixture in two experiments established under epidemic conditions at a site near Kampala in southern Uganda. There were significant differences in final CMD incidence and in the areas under the disease progress curves between varieties when grown alone and as a mixture in both experiments. Variety Ebwanateraka had the highest incidence and SS4 the lowest, even though it supported the largest populations of adult whiteflies. The overall incidence of CMD in the mixture was similar to that in pure stands of the partially resistant Nase 2 and greater than in the resistant Migyera and SS4. Compared to pure stands, incidence of CMD in each component of the mixture was reduced significantly only in Ebwanateraka, whereas vector populations were less only in SS4 and Nase 2. On several observation dates the actual incidence of CMD and populations of adult whiteflies in the mixture were significantly less than expected values estimated from the results for the four varieties when each was grown alone. A highly significant positive relationship was established for each variety between peak populations of adult whitefly and leaf area index at the time. The implications of the findings and the scope for future research on the use of varietal mixtures for the management of CMD are discussed.

Varietal mixture of poplar clones: Effects on infection by Melampsora larici‐populina and on plant growth

The effects of poplar varietal mixture on infection levels caused by Melampsora larici‐populina and on plant growth were studied during two consecutive growing seasons. The varietal mixture consisted of an intimate mixture of three poplar clones, Populus x interamericana‘Boelare’, ‘Rap’ and Populus candicans‘NNT’, expressing different race‐specific resistances. Infection levels of M. larici‐populina were assessed in monoclonal and polyclonal plots. Plant heights were determined at the end of each growing season. Infection levels in the growing season of the first year were not significantly different between monoclonal and polyclonal plots at the end of the epidemic. During the second growing season, a naturally occurring complex pathotype, able to infect clones ‘Boelare’ and ‘Rap’, invaded some plots prematurely and changed the composition of pathogen population. During this season, four infection assessments were conducted. Infection reduction in polyclonal plots compared with monoclonal plots was observed on ‘Boelare’ and P. candicans plants. Nevertheless, no significant difference was observed for growth between monoclonal and polyclonal plots. Biological factors which may be involved in the reduction of infection levels in polyclonal plots are dilution of susceptible individuals, physical barriers caused by resistant plants and cross‐protection. A high auto‐infection ratio may have partly hidden the positive effects of the clonal mixture.

Fusarium Wilt Disease Caused by Fusarium oxysporum f. sp. phaseoli on a Common Bean Cultivar, G 2333, in Rwanda and the Democratic Republic of Congo.

The common bean (Phaseolus vulgaris L.) is an important source of dietary protein for poor resource families in Rwanda and the Kivu region of the Democratic Republic of Congo (DRC). During the second season of 1990, symptoms consisting of chlorosis, premature defoliation, stunting, wilting, and vascular discoloration were observed for the first time on an improved popular climbing bean cultivar, G 2333, in the prefectures of Butare and Gikongoro, in southwest Rwanda. Between 1991 and 1994, surveys to determine disease occurrence were conducted in the two prefectures and in Kigali North, Gitarama, Cyangugu, and Kibuye in Rwanda, and in the south Kivu region of the DRC. Aseptic isolation from diseased plant samples yielded the fungus Fusarium oxysporum f. sp. phaseoli. Pathogenicity tests on G 2333 seedlings by the root-dip inoculation method resulted in symptoms similar to those observed in the field, and recovery of the fungus, fulfilling Koch's postulate. Control seedlings gave no symptoms. Disease incidence was 44 and 53% in Butare and Gikongoro, respectively, during the first season of 1991 and 50 and 70% in the second season, while crop loss in some fields was as high as 100%. Between 1992 and 1994 the disease was also observed in the four other prefectures in Rwanda, and in south Kivu. Climbing bean cultivars Gisenyi-2-bis, G 865, Flora de Mayo, and Puebla 444 Criollo, and several bush local varietal mixtures grown where the disease occurred, were unaffected. CIAT bean Fusarium wilt differentials IPA 1, RIZ 30, A 211, Mortiño, Diacol Calima, BAT 477, HF 465-63-1, and Ecuador 650, and lines A 300 and XAN 112, were inoculated with three isolates (FOP-RW1, FOP-RW2, FOP-RW3) from Rwanda. Only IPA 1, RIZ 30, and A 211 were susceptible, indicating the isolates' likeness to the Brazilian race (1). This was the first report of Fusarium wilt disease on G 2333 in the two countries despite the cultivar having been grown for about 5 years. Reference: (1) R. L. D. Ribeiro and D. J. Hagedorn. Phytopathology 69:272, 1979.

Evolution of a pathogen population in host mixtures: rate of emergence of complex races

The development of a pathogen population in a crop varietal mixture was studied with an epidemic simulator based on the model EPIMUL. The pathogen population was composed of simple races, able to develop on only one genetic component of the mixture, and a complex race, which developed on all mixture components. The complex race was modelled with a very low initial frequency compared to simple races, to simulate the emergence of a complex race in the field. Several successive epidemics were simulated as if the pathogen population reproduced on the same plot for several years. The development of the complex race on the simulated plot was either focal or uniform. The effects of the cost of virulence, of density dependence and of differential adaptation to host genetic background on the simple race-complex race competition were studied. Experimentally measured values of the cost of virulence and differential adaptation were incorporated into the model, and both factors were shown to greatly reduce the increase in frequency of the complex race over time. Density dependance also influenced race competition, but mainly for high values of the parameter. Our results suggest that the cost of virulence is probably not the only mechanism that may influence the simple race-complex race competition in host mixtures. In our simulations, differences in the spatial distribution of the initial inoculum between parasite races led to large differences in their final frequencies. Thus, more investigations, including randomized disease distributions, would be of interest to judge the potential importance of spatial effects in the field.

Effects of induced resistance on infection efficiency and sporulation of Puccinia striiformis on seedlings in varietal mixtures and on field epidemics in pure stands

An inducer race ofPuccinia striiformis inoculated two days before a challenger race on wheat seedlings of cv. Clement, reduced infection efficiency by 44% and lesion expansion by 7.7%; there was no effect on sporulation rate. In field epidemics, induced resistance restricted the disease intensity on cvs. Clement and Austerlitz in pure stands by 44% and 57% respectively. In the glasshouse at the seedling stage, disease intensity in a varietal mixture was reduced by 82% compared to the susceptible pure stands; one third of the total disease reduction was attributable to induced resistance.

Use of deepwater varietal mixtures to minimize hydrological risk in basin lands of Bihar, India

Use of deepwater varietal mixtures to minimize hydrological risk in basin lands of Bihar, India

Evolution of a pathogen population in host mixtures: simple race–complex race competition

The evolution of the genetic structure of a pathogen population was studied in a varietal mixture with an epidemic simulator based on the model EPIMUL. The pathogen population was composed of simple races able to develop on only one component of the mixture and a complex race which developed on all mixture components. The effects on the simple race–complex race competition of a cost of virulence, of density dependence and of differential adaptation were studied. The selection for simple or complex races in the pathogen population did not depend on initial race frequencies. For a given multiplication rate, complex race frequency increased faster when the spore dispersal gradient was shallow, when distribution of initial disease was generalized, when amount of initial disease was reduced and when the number of mixture components was increased. This was attributed to a better efficacy of the mixture in controlling simple races, resulting in a higher relative fitness of the complex race. For measured values of density dependence or differential adaptation effects, the complex race was at a higher frequency after a mean number of pathogen cycles between 2.5 and 5. The effect of the cost of virulence was stronger and, in certain situations, could result in selection for simple races. In the conditions of our simulations and with the effects tested, stabilization of the pathogen population in host mixtures was unlikely to occur. However, more information is needed concerning the rate at which complex races could evolve and how quickly mixture resistance could be eroded.