Genetic, environmental and cultural factors influencing the resistance to septoria tritici blotch (Mycosphaerella graminicola) in wheat

KeyWord:Genetic, environmental and cultural factors influencing the resistance to septoria tritici blotch (Mycosphaerellagraminicola ) in wheat.Bread wheat is the most widely grown and consumed food crop in the world. It is the staple food of nearly 35%. The forecasted global demand for wheat in the year 2020 varies between 840 to 1050 million tonnes. The global average yield must increase from the current 2.5 t ha - 1  to3.8 t ha -1. Wheat breeding is focused on developing widely adapted, disease-resistant genotypes with high yields and grain quality that are stable across a wide range of environments. Incorporating durable resistance is a priority since breeding for stable yields without adequate resistance against the major diseases would be impossible. One of the major diseases in wheat production worldwide is septoria tritici blotch caused byMycosphaerella graminicola (Fuckel) Schroeter (anamorph,Septoria tritici Rob. ex Desm.).New sources of resistance are required as only a few varieties currently available have adequate levels of resistance. Location of resistance in synthetic wheats is interesting because they are relatively easy to cross with common wheats and their resistance can be introgressed into agronomically acceptable genotypes and combined with other resistances.  Chromosomal location of the resistance is the step preceding the development of recombinant chromosome lines and mapping genes.           Higher levels of resistance are supposed to be genetically or epidemiologically linked to late heading and tallness. The presence of genetic linkages can complicate the breeding for early heading, short cultivars resistant to septoria tritici blotch. It is also necessary to know to what extent resistance is present against a wide spectrum of isolates and whether that resistance is expressed at all stages of plant development. Furthermore, some cultural practices, such as N-fertilisation may modify the expression of the disease. Also the relation between disease response and yield loss is not fully clarified.The aim of this thesis was to look for new sources of resistance, identifying the chromosomal location of that resistance in some materials and to study the effects of morphophysiological and environmental factors that may modify the expression of that resistance. Several materials with resistance in seedling stage to septoria tritici blotch were identified using a set of 10 isolates of the fungus. Synthetic 6x (Triticum dicoccoides xTriticum   tauschii ) was resistant to all isolates.T. spelta , Cappelle-Desprez, Hobbit Sib and Mara were resistant to 7 out of 10 isolates. Bezostaya and Cheyenne were resistant to four isolates. Some levels of resistance were also found in adult stage. Chromosomal location was investigated in four sets of substitution lines in the seedling stage (Synthetic 6x, Cheyenne, Cappelle-Deprez andT. spelta in Chinese Spring) and in two sets in the adult stage (Synthetic 6x and Cheyenne in Chinese Spring). Isolates IPO 92067 and IPO 93014 were used for all sets except for Cheyenne where IPO 92067 and IPO 92064 were inoculated. The most consistent effects were found in Synthetic 6x which showed to carry genes for resistance on chromosome 7D in the seedling stage and adult stages to isolate IPO 92067 and in the seedling stage to isolate IPO 93014. Chromosomes 5A and 5D also showed to carry minor gene effects in the adult stage for both isolates. Chromosome 1B from Cheyenne in the seedling stage and in the adult stage and chromosome 5D in the adult stage showed levels of resistance to both isolates similar to the resistance parent or better than the susceptible parent did. For Cappelle-Desprez, chromosomes 2B, 3A and 3B showed to carry minor gene effects for both isolates in the seedling stage. ForT. spelta , chromosome 6D carried genes for resistance for both isolates and 7D also showed clear effects for isolate IPO 92067 in seedling stage. Some other chromosomes for the different sets showed to carry resistance to one of the isolates.            Variation in quantitative resistance to a virulent Argentinean isolate (IPO 99013) was found in a set of 50 cultivars. Cultivars Klein Volcán, Klein Estrella and Klein Dragón showed good levels of resistance in the seedling stage and the adult stage, whereas some other cultivars showed good levels of resistance either in the seedling stage (Buck Chambergo, ProINTA Puntal, Klein Don Enrique, Buck Panadero) or in the adult stage (Cooperación Millán, Granero INTA). When a set of 16 of these cultivars was tested with 7 isolates in the adult stage, specific cultivar × isolate interactions were found, although Klein Dragón and Klein Volcán showed acceptable levels of resistance to all seven isolates.            Our results showed no genetic associations between the resistance toMycosphaerella graminicola and heading date or plant height within a broad spectrum of Argentinean cultivars tested with one virulent Argentinean isolate. When the disease severity was evaluated at the same developing stage in all cultivars, associations depended on how weather conditions predisposed the disease development in early or late cultivars. Negative correlations with height in this set of cultivars were only significant when weather conditions were less conducive to the disease development. No associations with heading date or plant height were found when resistance was assessed in controlled conditions and plants inoculated at the flag leaf stage. Isogenic lines of the wheat cultivar Mercia carryingRht 3 andRht 12 and isogenic lines of the cultivar Cappelle-Desprez carryingRht 3 showed the highest reductions in plant height (between 39.7 and 41.3 cm) and the highest values of susceptibility to septoria tritici blotch. Isogenic lines of both cultivars carryingPpd genes for insensitivity to photoperiod showed the shortest heading time and the lowest values of severity. Plant height was negatively associated with necrosis percentage and heading date showed some positive associations which can be attributed to weather conditions being more conducive to the development of the disease in the late lines. Negative or positive associations between resistance and heading date were also found in a set of six Argentinean cultivars grown in 1996 and 1997. These associations also depended on weather conditions.   Generally, nitrogen fertilisation caused an increase in the development of the disease under conducive weather conditions, but cultivar × N-fertilisation interactions proved to be significant. Despite the increase of the area under disease progress curve under N fertilisation when environment was conducive, the percentage of reduction in yield, yield components and test weight due to septoria tritici blotch was similar in N fertilised and non-fertilised conditions suggesting the presence of tolerance mechanisms.             Identification of chromosomes carrying resistance genes to septoria tritici blotch facilitates the location of genes for resistance through the development of recombinant or introgression lines for those particular chromosomes. We are actually testing introgression lines with the 7D and 5D chromosomes of Synthetic 6x in Chinese Spring to map the resistance genes. Knowledge about precise location of resistance genes and the finding of markers linked to them will allow breeders to select for resistance without inoculating crops with the pathogen and avoiding the influence of environmental effects on the expression of septoria tritici blotch.            Although the presence of genetic associations between resistance, heading date and plant height may depend on the genetic materials used, results of this work demonstrated that within a wide spectrum of materials it would be possible for breeders to select for short and early heading cultivars with resistance to the disease. The use of molecular markers will also be useful to identify associations between heading date, plant height and resistance toMycosphaerella graminicola . Our further research allowed us to identify in recombinant lines of a Synthetic 6x (T .tauschii × Altar 84) × Opata 85 some QTLs accounting for variation in resistance to septoria tritici blotch in the seedling and adult stages. These QTLs did not coincide with the regions where QTLs for flowering time were previously mapped for the same population, indicating that at least within some germplasm these traits are not linked.            Tolerance of wheat cultivars to septoria tritici blotch is another topic for further investigation. Although it is clear that some cultivars suffer less from reduction in yield than expected based on the severity of septoria leaf blotch, the mechanisms of this tolerance is not well understood. Further work will focus on molecular marker analysis to investigate resistance and to incorporate this resistance into materials with good agronomic characters.