Europäisches Rassen-Monitoring und Pathogenesestudien zur Turcicum-Blattdürre (Exserohilum turcicum) an Mais (Zea mays L.)
European monitoring of physiological races and studies on pathogenesis of Northern Corn Leaf Blight (Exserohilum turcicum) on maize (Zea mays L.).
by Hendrik Hanekamp
Date of Examination:2016-04-25
Date of issue:2018-05-28
Advisor:Prof. Dr. Andreas von Tiedemann
Referee:Prof. Dr. Heiko C. Becker
Referee:Prof. Dr. Petr Karlovsky
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Name:Hendrik Hanekamp Dissertation.pdf
EnglishIn the last few years, the importance of Exserohilum turcicum, the causal agent of Northern Corn Leaf Blight (NCLB), has increased in European maize growing regions. The infestation of maize leafs by NCLB reduces the photosynthetically active leaf surface area and thereby leads to considerable yield losses. To ensure high and stable yields therefore, the use of resistant cultivars is the most effective method. The efficiency of monogenic resistances however is restricted, as virulent races of the pathogen which overcome the resistance keep emerging. One of the aims of this work was to determine the regional efficacy of NCLB re-sistance genes (R-genes) through race monitoring so as to recommend resistant cultivars to specific regions and provide relevant information that can be used in resistance breeding. Infested maize leaf samples were collected from 168 different locations of 10 European countries over the years 2011 and 2012. From these samples, 559 single spore isolates were successfully obtained and their virulence characteristics were thoroughly described. For the race classification, the disease reaction was assessed on a differential set of 5 near isogenic inbred lines: B37-Ht0 (with no Ht gene), B37- Ht1, B37- Ht2, B37- Ht3 and B37- HtN. For each tested R-gene, at least one corresponding virulent isolate was found. In total 12 different races were characterized. Of which, the four dominant races were race 0 (which made up 45% and is avirulent to all the tested R-genes), race 1 (22%), race 3 (15%) and race 3N (14%). The remaining 8 races (13, 123, 23, 2, 23N, 12, 1N and 13N) represented a cumulated two years percentage of 4%. The cumulative two years average proportion of the 3 most dominant races (1, 3 and 3N) was 52%. Interestingly, in the northern and coastal regions of France, Belgium and the Netherlands which are known for their intense maize cultivation, race 0 was predominant making up over 75%. In the Upper Rhine Valley, char-acterized by higher temperatures and long tradition of maize cultivation, 48 and 26% of the isolates were found to be virulent on Ht1 and Ht3, respectively. In the northern and southern regions of Germany, again with a history of intensive maize cultivation, the dominant race was race 0 directly followed by race 3. In the Inn valley in southeast Germany, 44% of the tested isolates were virulent on Ht3. In maize cultivation regions of Czech Republic and Poland, 37% of the isolates were virulent on Ht1. Isolates collected in south west France showed virulence frequencies of 45% for Ht3 and 38% for HtN. The unique characteristic of this region, in contrast to the other regions considered in this study, was the high virulence frequency on HtN mainly due to race 3N which had a virulence percentage of 38%. In southeast France, especially in the Rhône-Alpes region, the virulence frequency for Ht1 is higher than 50%. The region Steiermark in southern Austria is characterized by an intensive and long tradition of maize cultivation. The percentage of virulent isolates for Ht1 in this re-gion was higher than 83%, indicating that the Ht1 gene is no longer effective in the region. More importantly, the highest proportion of the highly complex race 123 was identified in this region. The maize cultivation region in north Italy was characterized by a high virulence frequency (60%) for Ht3. The present study revealed that the European E. turcicum population is characterized by regional very high virulence frequencies particularly against Ht1 and Ht3 in the southern regions where intensive maize cultivation have been practiced for long period of time. It is thus not worthwhile to use these R-genes in these areas. In the northern coastal regions however, Ht1 can probably deliver a medium-term protection against NCLB. Nevertheless, in order to prevent widespread E. turcicum epidemics and further slowdown the emergence of new virulent races, it is advisable to continuously use resistant cultivars that possess a combination of quantitative and qualitative resistance traits. Like in the colder coastal regions of northern Europe, an extended cultivation of maize is being take place in the warmer regions of southern Europe. Accordingly, the effect of tem-perature on the efficiency of the R-genes was also a point of interest of this work. Hence, climate chamber experiments were conducted using a differential set of near isogenic lines derived from B37. The results showed that the efficiency of two R-genes, namely Ht2 and Ht3, was reduced at temperatures below 20°C and 15°C, respectively. Furthermore, it was found that the race 123 appeared to be less virulent against the R-genes Ht2 and Ht3 at temperatures above 25°C. In addition to the temperature effect, the potential role of genetic background was evident. Due to their temperature sensitivity for example, the utilization of Ht2 in early ripening cultivars in the cooler northern regions seems to be not helpful. This study further addressed histological aspects of the compatible and incompatible inter-actions between E. turcicum and Maize. Efficient parameters (1. Xylem penetration efficien-cy, 2. Xylem colonization efficiency, 3. Mesophyll colonization efficiency ) that help to com-pare fungal colonization processes at different stages of infection were established. Using this method, it was found that the xylem wall was the crucial resistance barrier in the incom-patible interactions of all the tested Ht genes. The complete prevention of xylem penetra-tions was not observed in any of the tested genotypes with Ht genes. In contrast to the oth-er tested Ht genes, the HtN gene, in the compatible interactions, was characterized by a delayed xylem penetration and mesophyll colonization instead of a total loss of function of the R-gene.
Keywords: Setosphaeria turcica; R-genes; Virulence; Maize; Exserohilum turcicum