The usefulness of mixtures (multiline cultivars and cultivar mixtures) for disease management has been well demonstrated for rusts and powdery mildews of small grain crops. Such mixtures are more useful under some epidemiological conditions than under others, and experimental methodology, especially problems of scale, may be crucial in evaluating the potential efficacy of mixtures on disease. There are now examples of mixtures providing both low and high degrees of disease control for a wide range of pathosystems, including crops with large plants, and pathogens that demonstrate low host specificity, or are splash dispersed, soilborne, or insect vectored. Though most analyses of pathogen evolution in mixtures consider static costs of virulence to be the main mechanism countering selection for pathogen complexity, many other potential mechanisms need to be investigated. Agronomic and marketing considerations must be carefully evaluated when implementing mixture approaches to crop management. Practical difficulties associated with mixtures have often been overestimated, however, and mixtures will likely play an increasingly important role as we develop more sustainable agricultural systems.

Use of multiline cultivars and cultivar mixtures for disease management

In an experiment that directly manipulated grassland plant species richness and composition, decreased plant species richness (''diversity'') increased pathogen load (the percentage of leaf area infected by species-specific foliar pathogens across the plant community) in 1998. Pathogen load was almost three times greater in the average mono- culture than in the average plot planted with 24 grassland plant species, an approximately natural diversity. Eleven individual diseases increased in severity (percentage of leaf area infected by a single disease) at lower plant species richness, and severity of only one disease was positively correlated with diversity. For 10 of the 11 diseases whose severity was negatively related to diversity, disease severity was positively correlated with host abun- dance, and in six of these cases, species diversity had no effect on disease severity after controlling for the effects of host abundance. These results suggest that increased abun- dances of individual host species at lower species diversity increased disease transmission and severity. In 1996 and 1997, similar results for a smaller number of diseases sampled were found in this experiment and another similar one. Although the effect of diversity on disease was highly significant, considerable variance in pathogen load remained among plots of a given diversity level. Much of this residual variance was explained by community characteristics that were a function of the species composition of the communities (the identity of species present vs. those lost). Specifically, communities that lost less disease- prone species had higher pathogen loads; this effect explained more variance in pathogen load than did diversity. Also, communities that lost the species dominant at high diversity had higher pathogen loads, presumably because relaxed competition allowed greater in- creases in host abundances, but this effect was weak. Among plant species, disease prone- ness appeared to be determined more by regional than local processes, because it was better correlated with frequency of the plant species' populations across the region than with local abundance or frequency across the state. In total, our results support the hypothesis that decreased species diversity will increase foliar pathogen load if this increases host abun- dance and, therefore, disease transmission. Additionally, changes in community character- istics determined by species composition will strongly influence pathogen load.

/pdf/effects-of-grassland-plant-species-diversity-abundance-and-3b9ygrxqxw.pdf

Effects of grassland plant species diversity, abundance, and composition on foliar fungal disease

Diversification of cereal cropping systems with alternative crops, such as oilseed, pulse, and forage crops, furnishes producers with a range of agronomic and economic options. Crop diversification also improves management of plant diseases through manipulation of host factors such as crop and cultivar selection; interruption of disease cycles through crop rotation, fungicide application, and removal of weeds and volunteer crop plants; and modification of the microenvironment within the crop canopy using tillage practices and stand density. Management practices, such as seed treatment, date and rate of seeding, balanced fertility, control of weeds, field scouting, harvest management, and record keeping, can also be utilized to manage plant diseases. This review evaluates the risks to diversified crop production systems associated with the major plant diseases in the northern Great Plains and the influence of host, pathogen, and environmental factors on disease control. Principles to help producers reduce and manage the risk from plant diseases are presented, and discussion includes strategies for countering fusarium head blight (Fusarium spp.), commonly called scab; and leaf spot diseases in cereals; sclerotinia stem rot [Sclerotinia sclerotiorum (Lib.) De Bary] in oilseed and pulse crops; and ascochyta blight (Ascochyta lentis Vassil.; teleomorph: Didymella lentis Kaiser, Wang & Rogers) and anthracnose blight [Colletotrichum truncatum (Schwein.) Andrus & W.D. Moore] in pulse crops. Producers should not rely exclusively on a single management practice but rather integrate a combination of practices to develop a consistent long-term strategy for disease management that is suited to their production system and location.

/pdf/managing-plant-disease-risk-in-diversified-cropping-systems-q8y7gh8ubg.pdf

Managing Plant Disease Risk in Diversified Cropping Systems

Les varietes en melanges, de par leur diversite genetique, limitent le developpement des epidemies et des ravageurs. Cette diversite peut etre organisee selon notre connaissance des interactions hote - agent pathogene pour influer sur l'evolution des populations parasites. La culture de varietes en melange ou de melanges interspecifiques est une des methodes les plus efficaces a la fois pour limiter le risque de contournement des resistances, et pour utiliser avec benefice des resistances deja contournees. Les principaux facteurs de reduction de la severite des epidemies dans les melanges sont les effets de barriere, la proportion de plantes sensibles et la resistance induite. L'adaptation d'isolats de meme race au fond genetique d'un hote, ou adaptation differentielle, pourrait ralentir la selection de races complexes dans les melanges. D'une facon generale, les melanges stabilisent les rendements. Le risque de pertes de rendement dues aux stress biotiques et abiotiques est plus limite dans un melange que dans une culture pure. Un choix judicieux des composants du melange peut permettre d'obtenir une qualite du produit de recolte equivalente ou superieure a celle d'une culture pure. L'utilisation des melanges au niveau mondial est discute.

/pdf/cereal-variety-and-species-mixtures-in-practice-with-1vm5fsrw2v.pdf

Cereal variety and species mixtures in practice, with emphasis on disease resistance

The use of pesticides made it possible to increase yields, simplify cropping systems, and forego more complicated crop protection strategies. Over-reliance on chemical control, however, is associated with contamination of ecosystems and undesirable health effects. The future of crop production is now also threatened by emergence of pest resistance and declining availability of active substances. There is therefore a need to design cropping systems less dependent on synthetic pesticides. Consequently, the European Union requires the application of eight principles (P) of Integrated Pest Management that fit within sustainable farm management. Here, we propose to farmers, advisors, and researchers a dynamic and flexible approach that accounts for the diversity of farming situations and the complexities of agroecosystems and that can improve the resilience of cropping systems and our capacity to adapt crop protection to local realities. For each principle (P), we suggest that (P1) the design of inherently robust cropping systems using a combination of agronomic levers is key to prevention. (P2) Local availability of monitoring, warning, and forecasting systems is a reality to contend with. (P3) The decision-making process can integrate cropping system factors to develop longer-term strategies. (P4) The combination of non-chemical methods that may be individually less efficient than pesticides can generate valuable synergies. (P5) Development of new biological agents and products and the use of existing databases offer options for the selection of products minimizing impact on health, the environment, and biological regulation of pests. (P6) Reduced pesticide use can be effectively combined with other tactics. (P7) Addressing the root causes of pesticide resistance is the best way to find sustainable crop protection solutions. And (P8) integration of multi-season effects and trade-offs in evaluation criteria will help develop sustainable solutions.

Eight principles of Integrated Pest Management.

Mixtures of winter barley cultivars containing up to six components were grown over three years with and without fungicide treatment. Yield increases were recorded for mixtures compared with the mean of their monoculture components and there was a significant trend towards greater benefit from increased number of components. These benefits were partially attributable to a corresponding increase in control of Rhynchosporium secalis as component number increased. The potential for exploitation of mixtures in cereals for control of splash-dispersed pathogens is discussed.

The effect of component number on Rhynchosporium secalis infection and yield in mixtures of winter barley cultivars

Soil tillage effects on the efficacy of cultivars and their mixtures in winter barley

A local population of Rhynchosporium secalis was sampled and the race structure, pathogenicity, RAPD profile and rDNA ITS sequence examined. RAPDs were also used to compare this population with a larger sample of isolates including some isolated more than 30 years ago. Both the local population and the wide sample revealed much variation for all characteristics with no two isolates identical. The reliability of detached leaf pathogenicity tests was improved over previous studies by pre-washing spores and brushing the leaves prior to inoculation. Isolate aggressiveness was highly correlated with lesion number but not lesion size. There was no correlation between race and aggressiveness. RAPD profiles showed high levels of variability but more similarity was observed within a site than between sites. Clusters of similarity were primarily associated with samples from the same locality but samples taken 8 years apart from within the same field were not closely related. ITS2 sequences were all identical whereas ITS 1 sequences showed 0.8 % polymorphism. ITS polymorphisms fell mostly within one RAPD cluster. ITS sequence comparisons with the EMBL database gave some indications of the taxonomic placement of R. secalis.

Variability in pathotype, aggressiveness, RAPD profile, and rDNA ITS1 sequences of UK isolates of Rhynchosporium secalis

A range of mixtures including one set made from cultivars grown in the UK and one from cultivars grown in Poland, were included, along with their component cultivars, in nine trials at Scottish Crop Research Institute, Dundee, UK or at the Experimental Plant Breeding Station of the IHAR, Bakow near Kluczbork, Poland, over five seasons. The effects of inoculum pressure, plot size, fertiliser level and germplasm on mixture efficacy were investigated. In the four trials where inoculum pressure was controlled, mixtures reduced infection more at lower inoculum pressures, but this did not translate into yield benefit. Smaller plots increased mildew in monocultures but not mixtures. Fertiliser levels increased mildew levels but did not affect mixture efficacy. There were large differences between both Polish and UK germplasm, and between Polish and UK trial sites, but the performance of the mixtures compared with their respective monoculture components was similar within both germplasm groups and trial sites. Mixtures reduced lodging and affected plant height and heading date. The advantages of mixtures for improving yield, reducing fungicide applications and improving agronomic characteristics was demonstrated and there seems to be great potential for their further improvement and exploitation.

The Effect of Inoculum Pressure, Germplasm Selection and Environment on Spring Barley Cultivar Mixtures Efficacy

A range of winter barley cultivars was grown, either as pure stands or as components of mixtures, in a trial in 1995-96. For malting quality characters, mean values of mixtures did not differ from mean values of the appropriate monocultures, except for decreases in homogeneity, as determined by a fluorescence test of cell wall modification. When disease pressure was modified by fungicide, hot water extract was not significantly altered, although this may have been due to the restriction of malting to grain retained by a 2.5 mm sieve. One mixture, comprised of three related winter malting cultivars, gave higher hot water extracts than its components, as pure stands, with no adverse effects on homogeneity. This was not simply attributable to lower grain nitrogen content.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1998.tb00973.x

D. C. Guy

Papers

The effect of component number on Rhynchosporium secalis infection and yield in mixtures of winter barley cultivars

Soil tillage effects on the efficacy of cultivars and their mixtures in winter barley

Variability in pathotype, aggressiveness, RAPD profile, and rDNA ITS1 sequences of UK isolates of Rhynchosporium secalis

The Effect of Inoculum Pressure, Germplasm Selection and Environment on Spring Barley Cultivar Mixtures Efficacy

The effect of cultivar mixtures on malting quality in winter barley