Two to 3 weeks after pollination, high levels of disease severity caused yield losses between 40 and 70% ( Levy and Pataky, 1992). Yield losses depend on the level of host resistance, disease severity, plant phenological growth stage during infection, and position of the infected leaves ( Levy and Pataky, 1992). Yield losses up to 44% were recorded in susceptible hybrids at high disease severity levels between 52 and 100% during the full dent stage ( Bowen and Paxton, 1988). Northern corn leaf blight (NCLB) caused by the ascomycete Exserohilum turcicum, synonym Setosphaeria turcica has spread worldwide into regions where maize is cultivated. turcicum are associated with or confer additional quantitative resistance. Lower disease levels and a delayed progress of infection in compatible interactions with resistant lines imply that maize R genes to E. Following the distinct symptom expressions, resistance mechanisms conferred by Ht1, Ht2, Ht3, and Htn1 genes apparently are different. In contrast to incompatible interactions, however, the pathogen was able to penetrate into xylem vessels at 6 dpi in compatible interactions and strongly colonized the mesophyll at 12 dpi, which is considered the crucial process differentiating susceptible from resistant interactions. turcicum formed appressoria and penetrated the leaf surface directly in both types of interaction. Histological studies with Chlorazol Black E staining indicated that E. At 10 dpi, DNA content was elevated in all compatible interactions. In incompatible interactions, fungal DNA was only detected in very low amounts. Symptoms of resistance responses conveyed by the different Ht genes considerably differed between Ht1 (necrotic lesions with chlorosis), Ht2 (chlorosis and small lesions), Ht3 (chlorotic spots) and Htn1 (no lesions or wilt-type lesions). Leaf segments from healthy and inoculated plants were collected at five different stages of infection and disease development from penetration (0–1 days post inoculation - dpi), until full symptom expression (14–18 dpi). Maize plants from the differential line B37 with and without resistance genes Ht1, Ht2, Ht3, and Htn1 were inoculated with either incompatible or compatible races (race 0, race 1 and race 23N) of E. turcicum races in maize lines exhibiting compatible or incompatible interactions. Therefore, histological studies and DNA quantification were performed to compare the pathogenesis of E. Symptom expression and resistance mechanisms differ in plants carrying different resistance genes. Disease control is based on fungicide application and resistant cultivars displaying monogenic resistance. Exserohilum turcicum, causal agent of NCLB, infects and colonizes leaf tissue and induces elongated necrotic lesions. Northern corn leaf blight (NCLB) is an important leaf disease in maize ( Zea mays) worldwide and is spreading into new areas with expanding maize cultivation, like Germany. 2Plant Protection Office, Chamber of Agriculture Lower Saxony, Hannover, Germany.1Division of Plant Pathology and Crop Protection, Department of Crop Sciences, Georg-August-Universität Göttingen, Göttingen, Germany.Barbara Ludwig Navarro 1*, Hendrik Hanekamp 2, Birger Koopmann 1 and Andreas von Tiedemann 1
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