Venturia inaequalis (Cke.) Wint. is the incitant of apple scab, a disease of great economic importance throughout the world. This pathogen, in the early stages of infection, is confined to the area between the cuticle and the epidermis of the host and thrives there in intimate contact with these layers. This rather peculiar growth habit seems to indicate that lipids might have an important role in determining the host specificity and the pathogenicity of V. inaequalis. [...]

Cercospora beticola Sacc. is a necrogenic pathogene which produces brown small nearly circular spots on the foliage of table beets, sugar beets, mangolds and Swiss chard. On beet leaves the spots have a reddish-purple border, while on mangolds there is usually a pale green border around each spot. The center of the spot is grayish due to long septate conidia produced on conidiophores which come out through the stomata.

The so-called age of antibiotics is generally considered to be of recent origin but the foundation of its beginning goes back to the last century. The phenomenon of antibiosis, the production by one organism of a specific chemical substance or substances which have an injurious effect upon another organism, was noticed by many of the early microbiologists. Although Tyndall, Pasteur, and DeBary worked in different fields of biology, each mentioned this phenomenon of antibiosis in his writings.

Red clover (Trifolium pratense L.) is one of the most important and most widely grown cultivated legumes in Canada. The early flowering varieties are the most popular in the clover growing areas of Quebec, Ontario and the Maritime provinces because they are able to make a rapid recovery after the first cutting. Red clover is subject to numerous diseases and insects, some of which cause severe injury to the plant, while others inhibit plant growth and thus reduce the yield or quality of the bay.

Wheat, an important food crop of the world, suffers from a number of diseases which cause a heavy loss to the growers every year. Stem rust (Puccinia graminis tritici Eriks. and Henn.) is one of these serious diseases and it often occurs in an epiphytotic form. It is a common observation that some wheat varieties are very susceptible to a particular rust race and are sometimes completely destroyed while other varieties prove to be resistant. The knowledge of these facts led plant pathologists to realise the importance of resistant varieties and to study the nature of resistance to this disease.

Disease resistance in a plant is a manifestation of a complex phenomenon which involves among other things, the inherent and acquired physiologic properties of not only the host but also those of the pathogen influenced by ever-changing conditions of both the macro- and micro-environment.

The study of various aspects of the wilt diseases of plants which are caused by fungi inhabiting the vascular systems of the host plants has engaged the attention of a number of research workers for over half a century. Various theories have been proposed to explain the wilt symptoms in several species of plants, yet none can be considered to fully explain how the fungus in the vascular elements causes the plant to become diseased and die. The wilt disease caused by Fusarium oxysporum f. lycopersici (Sacc.) Snyder & Hansen is specific to the tomato, Lycopersicon app.

Autoradiograms of plants which had one of their leaves (presentation leaf) photosynthesizing in C14O2 show high concentration of radioactivity in the growing point and roots and low concentration in leaves below the presentation leaf. Infection on two of these lower leaves increases the concentration of radioactivity in them, especially around lesions, but has no marked and consistent effect on the concentration in other organs. Infection on the presentation leaf reduces photosynthesis and export of C14 to other parts. Heat girdling of petioles of healthy and infected leaves show that in both cases, C14 export takes place through phloem and its import through xylem. Radioactivity in detached leaves exposed to C14O2 , either in light or in darkness, is uniformly distributed in healthy leaves, but accumulates around lesions in diseased ones. Starch has similar distribution. Pathological increase in transpiration and metabolic activity are probably responsible for the altered cl4 distribution in diseased leaves.

INTRODUCTION Sunflower is the most important oilseed crop in South Africa. The sunflower oil market has shown a steady increase of approximately three percent per year in the past few years, with a current demand of 600 000 tons of seed for oil extraction (Pakendorf, 1998). In the past, sunflower in South Africa was considered to be an alternative crop to maize, i.e. if a maize crop could not be successfully produced due to drought or any other constraint. This led to a situation where sunflower cultivation was not done under optimal conditions, leading to low and erratic yields and consequently gaining a reputation of being uneconomical compared to maize. The areas planted during the 2002/2003 season were, Free State 275 000 ha, Mpumalanga 40 000 ha, Limpopo 37 000 ha, Gauteng 10 000 ha and North West 220 000 ha with a total of approximately 582 000 ha (Beukes, 2003). It is evident that the largest concentration of sunflower is in the Free State and North West province. This is generally the drier or western part of South Africa with more sandy soils. However in the Limpopo province most of the sunflower is planted very late in Arcadia type soils with very high clay content. Another factor typical to these areas is that the evaporation is up to three times the value of the annual rainfall. Economics is an important factor that influences the expansion of sunflower. In areas where maize has a low average yield, sunflower is a good alternative crop (Parkendorf, 1998). The above-mentioned areas of cultivated sunflower vary considerably in soil, climate and elevation. Although it is widely accepted that sunflowers have a good general adaptability, the planting date and rainfall have an influence on the performance of hybrids. The instability of hybrids creates difficulty in selection in breeding programs. Most decisions are based on limited information from one or two years with a normal ANOVA and cross site analysis. No effects of environment x genotype interaction are taken into consideration. According to Becker and Leon (1988) successful new varieties must show good performance for yield and other essential agronomic traits. Their superiority should be reliable over a wide range of environmental conditions. Plant breeders generally agree on the importance of high yield stability, but there is fewer consensuses on the most appropriate definition of �stability� and on methods to measure and to improve yield stability. The basic cause of differences between genotypes in their yield stability is the wide occurrence of genotype x environment interactions, i.e. the ranking of the genotype depends on the particular environmental conditions where it is grown. Very few researchers use statistical measures of yield stability in their breeding programs. A deeper insight into the relation among the numerous stability parameters and their similarity may be obtained by comparing the resulting stability rank orders of different genotypes which are derived by applying different concepts of phenotypic stability (Huehn, 1990). The aim of this study was to compare various statistical procedures � For assessing genotype x environmental interaction and yield stability of South African sunflowers. � To determine the most suitable parametric procedure to evaluate and describe sunflower genotype performance under dryland conditions in South Africa. � To recommend to breeders the most appropriate procedure to estimate genotype performance and stability most accurately. Individuals and seed companies plant the trials co-coordinated by the Agricultural Research Council (ARC) as a trade for participation to the research. This trial system ensures good quality hybrids in the market since intercompany competition is very active and the advantage of having hybrids with good yields and good ranking in this trial setup would ensure good sales. Part of the system requirements is to have all entries registered on the cultivar list after a Difference in Uniqueness System (DUS) test run by the Registration Department in Roodeplaat. This, in turn, ensures the quality of the seed reaching the millers and the oil press. The independent evaluation of data run by the ARC, gives the farmer an advantage of a choice of improved hybrids, proven to have good yields without extra cost. In the map of general agricultural regions (Fig.1) it is evident that the Free State is mostly utilized for cereal production and to the west for mixed farming. In the areas of cereal cultivation, sunflowers are used in rotation with wheat and maize. The western areas are traditionally maize areas. During the last five years the percentage of sunflower hectares has greatly increased in the North West and decreased in the Mpumalanga province. Fig. 1 Map of agricultural activities. The main sunflower production areas are indicated (â) and the test sites by (â) (Dept. of Foreign Affairs and Information, 1982) The large circle depicts the area actually planted in the Free State and part of the adjoining North West and contains two of the ARC trial sites namely Potchefstroom and Koster. The bottom smaller circle would be the very early plantings in the southern Free State. The circle above Johannesburg represent the area with the dark Arcadia type soils known as the �Springbok flats� with the Warmbaths site and the circle west of Johannesburg would represent the North West province and contains the Lichtenburg site.

Drought and low N stresses are factors most frequently limiting maize production in the tropics, where the vast majority of farmers have limited access to improved techno logy. Previous studies confirmed good performance of the crop under these conditions by improving tolerance to drought occurring at flowering, which is essential for resource constrained farmers in countries like Ethiopia. In addition to screening at sites with rain free seasons, the available genotypic variability for drought adaptive traits within adapted maize populations is crucial for successful improvement of tolerance. At Melkasa in Ethiopia, 196 randomly derived S1 lines from Population A-511 were tested for high yield potential, and drought and plant density adaptive traits under two moisture regimes, using two plant densities. Both plant density and drought stress significantly affected the tested primary and secondary traits of the S1 lines. However, drought effects were more effective in reducing grain yield and its components, as well as in increasing anthesis-silking interval or in delaying days to 50% silking. Significant genotypic variability within Population A-511 was observed for grain yield and most stress (drought and high plant density) adaptive traits. The association of yield with stress adaptive traits that included ears plant -1 , anthesis-silking interval, kernels ear -1 , and kernels plant -1 increased with increased stress. In contrast, the genotypic variance and heritability of grain yield and its components increased with decreasing stress, except for ears plant -1 . Regarding the use of suitable environments for drought tolerance screening in areas with unpredictable rainfall patterns, indirect selection under drought stressed high plant density conditions and direct selection under drought stressed normal density conditions were found to be equally efficient. CIMMYT drought tolerant lines and their crosses were tested separately at two plant densities under both drought stressed and well watered conditions managed by irrigation, and also in rainfed environments. This was mainly to determine their potential in performance, combining ability and heterosis under both stress and reduced stress environmental conditions. Considerable differences in performance were observed among drought tolerant inbreds, and among their crosses under contrasting growing conditions. However, most crosses from drought tolerant lines provided higher yields than local hybrids, indicating their suitability for the environments in which they were tested. In rainfed environments, higher yields were recorded for most genotypes at high plant density (» 88 800 plants ha -1 ) than the recommended density (» 44 400 plants ha -1 ) for local hybrids. The opposite held true when unimproved S1 lines derived from population A-511 were tested in another experiment. This indicated the increased potential in performance and tolerance to high plant density stress, mainly due to improvement made in drought tolerance. CML442, Mex102, CML202 and Mex101 were superior lines in per se performance and GCA effects, especially for yield. Crosses in which these lines were involved were also superior in yield and most other traits as well as for the corresponding SCA effects. Both additive (GCA) and non-additive (SCA) effects were important for expression of the tested traits in rainfed environments. However, additive effects were relatively more important than non-additive effects for expression of these traits. Drought tolerant lines with high per se performance in yield across rainfed environments gave high yielding hybrids under similar conditions, reflecting an association between crosses and their parental lines. Similarly, for drought tolerant genotypes evaluated across drought stressed and irrigated environments, the highest grain yield was recorded under well- watered high plant density conditions. On the contrary, under drought stressed high plant density conditions, the yield of lines and their crosses was reduced by 73 and 56%, respectively. Furthermore, the estimated heterosis increased with increased stress. These results confirmed increased tolerance of the improved lines and their crosses to high plant density stress, and increased tolerance of the crosses to drought stress compared to their parental lines. Combining ability tested in contrasting environments showed a predominant role of additive (GCA) effects for secondary traits and non-additive (SCA) effects for yield. For drought adaptive traits and yield, lines CML202 and Ken were superior in GCA and per se performance across diverse environments, while Mex101 was superior under drought stress. Among crosses, CML440 x Ken, CML440 x CML442, and Mex103 x CML202 were relatively superior in each and across environments. Drought tolerant genotypes that perform well in a range of soil N and moisture regimes are expected to give better yields with reduced genotype by environment interaction (GEI) across diverse environments. The AMMI 2 analysis model efficiently estimated the drought tolerant GEI patterns over environments. Crosses developed from drought tolerant lines showed variability in mean yield and GEI across environments. High yielding hybrids such as Mex103 x CML442, Mex103 x CML202, CML440 x CML442, Mex101 x CML445, CM202 x CML445, CML202 x Ken, CML440 x Ken and BH140 showed minimum GEI, indicating their wide adaptation across stress and reduced stress environments. In contrast, the unstable, high yielding hybrids like Mex101 x Mex102, Mex101 x CML442 and Mex102 x CML442 were adapted to unfavourable environments that included drought stress, and eroded topsoil. When compared to conventional hybrids, 22 and 26 crosses produced from drought tolerant parents were better than BH540 in mean yield and stability, respectively. On the contrary, the other local hybrid, BH140 was superior in terms of both parameters, due to one of its parents being improved for reduced plant height by CIMMYT.