| dc.description.abstracteng | In the last 60 years, maize production has increased worldwide, reaching 1.14 billion tons in 2018. Maize production in Europe and South America was about 110 and 130 million tons in 2018, respectively. The demand for highly productive maize is observed in both tropical and temperate zones. Thus, the selection of plants from different maturity groups and high yield production are required from breeding programs. Besides highly productive plants, other agronomical traits such as resistance to pest and diseases needs to be considered during selection. Globally, some of the most important diseases affecting maize are northern corn leaf blight (NCLB), and Gibberella and Fusarium ear rot (GER and FER, respectively). Host resistance to E. turcicum is based on qualitative or quantitative sources, while for GER and FER only quantitative resistance is available in commercial hybrids. The quantitative resistance is more durable; however, it is more laborious to introgress into breeding lines.
Northern corn leaf blight (NCLB) is an important disease in maize-producing areas worldwide. The symptoms of NCLB, whose causal agent is the ascomycete Exserohilum turcicum (teleomorph Setosphaeria turcica), are characterized by elliptical grey-green lesions. High disease severity can cause yield losses up to 40% (Levy und Pataky 1992). The main control methods applied for NCLB control are fungicide applications and the cultivation of resistant hybrids. Qualitative resistance has been widely used to control NCLB in many countries through the deployment of Ht genes. The race assessment from isolates collected in Argentina and Brazil during 2017, 2018 and 2019 revealed a high frequency of race 0 isolates (83% and 65% in Argentina and Brazil, respectively). In those countries, Ht genes are not being used extensively to control NCLB. This information is important for breeding programs and may help with disease management.
Favorable weather conditions for NCLB development are long dewy periods and moderate temperatures. These optimum conditions for disease development can be observed in temperate regions as well as in mid-altitude regions in the tropics. The comparison of E. turcicum isolates in response to temperatures varied in vitro and in vivo between 15 and 30°C demonstrating that the aggressiveness of South American isolates was higher than that of European isolates. The multivariate analysis confirmed that South American isolates are better adapted to higher temperatures by grouping them separately. In conclusion, E. turcicum populations may adapt quickly to environmental changes. The plasticity in adapting to environmental conditions of E. turcicum may decrease the durability of resistance.
Studies on the pathogenesis of E. turcicum in the differential maize line B37 with and without the resistance genes Ht1, Ht2, Ht3 and Htn1 were conducted for different stages of infection and disease development from penetration (0-1 dpi), until full symptom expression (14-18 dpi). Symptomological analysis demonstrated that Ht1 expressed necrotic lesions with chlorosis, Ht2 displayed chlorosis and small lesions, Ht3 resulted in chlorotic spots and Htn1 express wilt-type lesions. Histological studies conducted with Chlorazol Black E staining indicated that the pathogen was able to penetrate xylem vessels at 6 dpi in compatible interactions and strongly colonized the mesophyll at 12 dpi, which is considered the crucial process differentiating susceptibility from resistance. Additionally, lower disease levels, low fungal DNA content at 10 and 14 dpi, and the delayed progress of infection in compatible interactions with resistant lines imply that the Ht genes are associated with or confer additional quantitative resistance. Physiological studies showed a reduction in the photosynthetic rate, transpiration, stomatal conductance and instantaneous carboxylation efficiency in the incompatible interaction at 5 dpi. At 14 dpi, the strong necrosis displayed in the resistance reaction by B37Ht1 resulted in the reduction of photosynthesis as observed for B37. However, leaf area, aerial and root dry biomass were not reduced in inoculated plants at 28 dpi. Additionally, high rates of peroxide localization were observed in inoculated plants at 3 and 6 dpi, corroborating data on peroxidase activity. In fact, Ht1, Ht3 and Htn1 reduced pathogen sporulation whereas Ht2 reduced the number and size of lesions. All phenotypical studies demonstrated that Ht genes confer distinct resistance mechanisms.
The resistance phenotype expressed by Ht2 may change according to environmental conditions. There are reports on the influence of low post-inoculation temperature (22/18°C) and low light intensity (324 and 162 µmol m-2 s-1) on resistance expressed by this gene. Our objective was to prove that temperature has no influence on the resistance conferred by the Ht2-gene against E. turcicum. Therefore, maize plants were pre-exposed to warm (30/25°C) and moderate (20/15°C) temperature regimes for 10 days before inoculation. There was no influence of pre-inoculation temperature on the expression of resistance by Ht2. The resistance conferred by the Ht2 gene was confirmed by quantifying the fungal DNA in planta at 21 dpi. Changes in resistance phenotypes may be related to pathogen aggressiveness factors.
GER and FER can cause qualitative yield losses due to mycotoxin production. GER is mainly caused by Fusarium graminearum and FER by F. verticillioides. GER is more frequent in regions with colder temperatures and high precipitation, and is more prevalent in Germany, while FER occurrence is favored by warm and dry weather conditions and is more prevalent in Brazil. In general, F. graminearum was more aggressive than F. verticillioides, which support affirmations about systemic colonization by F. verticillioides. With regard to tropical and temperate hosts, the German isolates were more aggressive than the Brazilian isolates when inoculated in the tropical lines. Additionally, tropical lines pre-exposed to higher temperatures presented higher disease severity when compared to plants exposed to mild temperatures. In general, the cultivation of resistant hybrids remains a successful strategy for controlling NCLB, GER and FER. The optimization of resistance resources is fundamental for maintaining the durability of resistance. | de |