Variety mixtures can provide functional diversity that limits pathogen and pest expansion, and that makes use of knowledge about interactions between hosts and their pests and pathogens to direct pathogen evolution. Indeed, one of the most powerful ways both to reduce the risk of resistance break-down and to still make use of defeated resistance genes is to use cereal variety and species mixtures. The most important mechanisms reducing disease in variety and species mixtures are barrier and frequency effects, and induced resistance. Differential adaptation, i.e. adaptation within races to specific host genotypic backgrounds, may prevent the rapid evolution of complex pathotypes in mixtures. Mixtures generally stabilise yields and yield losses due to disease; abiotic stresses are also better buffered than in pure stands. When mixture components are carefully put together, product quality can be enhanced or at least equal that of the pure stands. Mixture use in practice worldwide is reviewed. functional diversity / induced resistance / differential adaptation / yield stability / evolutionary plant breedingRésumé -Les mélanges de variétés et les mélanges interspécifiques de céréales dans la pratique. Les variétés en mélanges, de par leur diversité génétique, limitent le développement des épidémies et des ravageurs. Cette diversité peut être organisée selon notre connaissance des interactions hôte -agent pathogène pour influer sur l'évolution des

Near‐isogenic lines differing in resistance genes were developed to study the effects of genes in barley (Hordeum vulgare L.) giving specific resistance to powdery mildew (Erysiphe graminis DC f.sp. hordei Em. Marchal). This paper describes breeding techniques, degree of similarity between isolines, and resistance genes present in the isolines. Twenty‐four donors of resistance genes were crossed and backcrossed five times to the spring barley cv. ‘Pallas’, and homozygous resistant BC5F2 plants were selected. Resistant lines were evaluated and selected for phenotypic similarity with the recurrent parent in the BC5F3, through BC5F7 generations. To select for genetic similarity in the region of the resistance locus Ml‐a, two linked hordein loci, Horl and Hor2, were studied in the BC5F4 generation. The resulting 24 near‐isogenic lines originated from single BC5Fs4 plants in each of 24 different progenies. With a few exceptions, the near‐isogenic lines were similar to the recurrent parent in morphological and physiological characteristics. Tests with the hordein genes indicated that double crossing‐over occurred very close to the resistance locus Ml‐a in five of the isogenic lines. With 22 powdery mildew isolates, 22 lines were similar to their donors in resistance reaction, while two lines gave reactions that indicated that they have lost a second resistance gene. The 24 near‐isogenic lines comprised 14 lines with one resistance gene each, and 10 lines with two or more resistance genes. All the isogenic lines differ in resistance. The degree of genetic similarity between the 24 isogenic lines and the recurrent parent Pallas is high according to the morphological and physiological observations and may be even higher than theoretically expected in lines with double recombination in the region of locus Ml‐a. These near‐isogenic barley lines are recommended for both practical and theoretical research work in the barley‐powdery mildew system.

The influence of increasing nitrogen supply (30, 60, 120 and 240 mg N per pot) on susceptibility was studied on seedlings of six cultivars of spring barley inoculated with virulent isolates of powdery mildew. The colony density (CD) measured as colonies per cm 2 was significantly increased with increasing application of nitrogen on all cultivars, and a significant interaction was found between N and cultivar. The different reactions of the cultivars could not be ascribed to lack of N uptake. In general, increasing N application enhanced the sporulation capacity of colonies (CSC) irrespective of increased CD and the cumulative production of spores per cm 2 leaf (CSCM) increased strongly with N application in all cultivars. No interaction between N and cultivar was found for the latter component. The increase in CSCM closely corresponded with the increase in N content and fresh weight of uninoculated leaves. No interaction between N treatment and powdery mildew isolates was found for infection efficiency and spore production per colony, when tested on one cultivar. The N-induced changes in infection and sporulation can explain the main part of the increasing effect of N fertilization on powdery mildew development in the field. The results indicate that it may be possible to breed for or select barley cultivars with low N impact on powdery mildew development.

Alternaria leaf blight disease caused by Alternaria brassicae in mustard (Brassica juncea (L) Czern and Coss) was studied in two crop seasons, 1992 and 1993 in Nepal at Nawalpur, Sarlahi (Tarai) and Khumaltar, Lalitpur (mid hill). At Nawalpur, epidemics of the disease was recorded for both seasons. Weather conditions like humidity, temperature and frequent rainfall played key role for the epidemics. In experimental fields, the disease appeared first in mid-December when the relative humidity was more than 80% with maximum temperature ranging between 18-25 0 C and minimum between 10-14 0 C. At Khumaltar, incidence of the disease was low in both seasons due to low average temperature from December to February. Among cultivars, Krishna and Pusabold were less susceptible than Varuna. Both mancozeb and iprodione had effectively reduced disease in the sprayed plots and increased seed yield by 48% and 130% respectively. The correlation between disease severity and yield, and yield components was negative and highly significant. Average yield loss was estimated to be in the range of 32 to 57%. Seed infection was also significantly higher in non sprayed treatment than sprayed one. The disease showed a negative effect on oil content causing losses on oil between 4.2 to 4.5%.

A radical shift in our approach to crop production is needed to ensure food security and to address the problems of soil degradation, loss of biodiversity, polluted and restricted water supplies, coupled with a future of fossil fuel limitations and increasingly variable climatic conditions. An interdisciplinary network of European scientists put forward visions for future crop production embracing the complexity of our socio-ecological system by applying the principle of diversity at all levels from soil micro-organisms to plant varieties and cropping systems. This approach, integrated with careful deployment of our finite global resources and implementation of appropriate sustainable technology, appears to be the only way to ensure the scale of system resilience needed to cope with many of our concerns. We discuss some of the most important tools such as (i) building soil fertility by recycling of nutrients and sustainable use of other natural and physical resources, (ii) enhancing biological diversity by breeding of crops resilient to climate change and (iii) reconnecting all stakeholders in crop production. Finally, we emphasise some of the changes in agricultural and environmental regulation and policy needed in order to implement the visions.