Epidemiologie von Cercospora beticola Sacc. und Befalls-Verlust-Relationen bei Zuckerrüben (Beta vulgaris L.) in Abhängigkeit von der Anfälligkeit von Sorten und Konsequenzen für sortenspezifische Bekämpfungsschwellensysteme
Epidemiology of Cercospora beticola Sacc. and disease-loss relationship in sugar beet (Beta vulgaris L.) depending on the susceptibility of cultivars and consequences for cultivar specific threshold systems
by Ulrike Kaiser
Date of Examination:2007-07-19
Date of issue:2007-10-09
Advisor:Prof. Dr. Bernward Märländer
Referee:Prof. Dr. Bernward Märländer
Referee:Prof. Dr. Mark Varrelmann
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EnglishThe most important pathogen of leaf spots in sugar beet is Cercospora beticola (Sacc.). The fungus reduces the assimilation rate due to a decreasing photosynthetically active leaf area. As a consequence yield and quality losses occur. The wide spectrum of sugar beet varieties offered in Germany includes varieties with medium to high susceptibility to Cercospora beticola and varieties with low susceptibility. As a result of intensive breeding efforts the less susceptible varieties have in the meantime reached a nearly identical yield performance in absence of infection compared to susceptible varieties. The conduction of comparative epidemiological, variety-specific studies within official variety trials is impossible because of technical reasons. In order to study the breeding progress of the resistance to Cercospora beticola and its importance for the integrated pest management the following questions were asked: How does the epidemic progress of Cercospora beticola differ among varieties with different susceptibility? Which differences exist among varieties with different susceptibility in terms of the disease-loss relationship? Is it possible to develop variety-specific threshold systems for fungicide application in sugar beet?The aim of this work was to study the epidemiology of the pathogen and its effects on yield and quality, especially white sugar yield (WSY), of old less susceptible and in absence of infestation lower yielding varieties in comparison with new less susceptible and in absence of infestation higher yielding varieties, and susceptible varieties. Furthermore, on the basis of disease-loss relationship the possibility of developing variety-specific threshold systems was investigated. For this purpose, in 2004 and 2005 field trials on seven sites per year were conducted in Germany and Austria with three susceptible, one old less susceptible and two new less susceptible varieties, and eight control strategy treatments. The control strategies included an untreated control, a healthy one treated repeatedly with static fungicide applications, and six strategy treatments based on the summary threshold system 5/15/45 % disease incidence (DI) of infected leaves (Cercospora beticola, Erysiphe betae and Ramularia beticola). Fungicide applications were conducted one and two times. The first application was either conducted in time (when exceeding the threshold), or with 14, or 28 days delayed. The second application was also conducted in time, or with 14, or 28 days delayed (first application to these three treatments in time). From the end of June DI and disease severity (DS) of Cercospora beticola were assessed regularly and root yield, sugar content, and tec! hnical quality determined after harvest. The development of DI followed a sigmoid curve until reaching 100 %. The summary threshold was reached in nearly every environment (site x year) and by all varieties. By fitting a logistic growth curve to the measured values of DI no shown differences existed among varieties and all varieties within the same environment reached the threshold nearly at the same time. With increasing infestation level susceptible varieties reached considerably higher DS than less susceptible varieties. By fitting a logistic growth curve to the measured values of DS and subsequent statistical analysis at three DS progress times varieties differed significantly when DS at harvest (DS-harvest) exceeded 20 %. Under low (DS-harvest < 5 %) and medium infestation level (DS-harvest 5 - 20 %) varieties did not differ. Variety differences for the area under disease progress curve (AUDPC) were found with increasing infestation levels but were less clear than for the DS-harvest. With increasing infestatio! n level varieties differed considerably in WSY, however, differences between susceptible and less susceptible varieties were only occasionally significant as a result of the mostly late initiating infestation. The fungicide application based on the summary threshold system caused a significant higher WSY averaged over all varieties than in the untreated control even under low infestation level (DS-harvest < 5 %). With decreasing sugar beet price and increasing seed price and under the assumption that fungicide application are based on the summary threshold system this WSY gain would diminish but still mostly have a positive economic effect. Under medium (DS-harvest 5 - 20 %) but especially also under high infestation level (DS-harvest > 20 %) the application twice in time minimized WSY losses, whereas the single application in time reduced WSY. Significantly negative effects of the delayed fungicide applications in comparison to the application in time were not found, whereas WSY evidently decreased the more the application was delayed. Due to the lack of interactions between variety and control strategy it was impossible to statistically evaluate variety-specific reactions to the delayed treatments. At increasing infestation level less susceptible varieties tended to show higher WSY and more stable yield performance with the delayed treatments than susceptible varieties. For all varieties the disease-loss relationship between WSY and DS-harvest, or AUDPC was nearly the same. However, less susceptible varieties compensated for negative infestation effects and showed nearly identical WSY as under no infestation even at higher DS-harvest and AUDPC. Therefore, at high infestation levels less susceptible varieties had a yield advantage over susceptible varieties. It was impossible to develop variety-specific threshold systems which in practice are not considered necessary. In general, it could be determined that for less susceptible varieties the time window for fungicide application is wider than for susceptible varieties. Thus, farmers could be unburdened in times with work peaks. Furthermore, the cultivation of less susceptible varieties could reduce the use of plant protection chemicals as requested for the integrated pest management.
Keywords: sugar beet; foliar diseases; Cercospora beticola; susceptibility of cultivars; disease-loss relationship; threshold systems
Der bedeutendste Erreger von Blattflecken in Zuckerrüben ist Cercospora beticola (Sacc.). Der Pilz reduziert durch eine Verringerung der photosynthetisch aktiven Blattfläche die Assimilatbildung und -einlagerung, woraus letztlich Verluste an Ertrag und Qualität resultieren. Das Sortenspektrum für Zuckerrüben in Deutschland umfasst sowohl Sorten mit mittlerer bis hoher als auch mit geringerer Anfälligkeit für Cercospora beticola. Die weniger anfälligen Sorten verfügen infolge intensiver Züchtung mittlerweile über ein ähnlich hohes Leistungspotential unter Nichtbefall wie anfällige Sorten, wobei vergleichende epidemiologische Untersuchungen sortenspezifisch im Rahmen der Wert- und Sortenprüfungen aus technischen Gründen nicht möglich sind. Um die Bedeutung des Züchtungsfortschritts bei der Resistenz gegen Cercospora beticola für den integrierten Pflanzenschutz zu untersuchen, wurden folgende Versuchsfragen bearbeitet: Wie unterscheidet sich der Epidemieverlauf von Cercospora beticola bei Sorten mit unterschiedlicher Anfälligkeit? Welche Unterschiede bestehen zwischen Sorten mit unterschiedlicher Anfälligkeit hinsichtlich der Befalls-Verlust-Relation? Lassen sich sortenspezifische Bekämpfungsschwellensysteme für den Fungizideinsatz in Zuckerrüben ableiten?
Schlagwörter: Zuckerrübe; Blattkrankheiten; Cercospora beticola; Sortenanfälligkeit; Befalls-Verlust-Relation; Bekämpfungsschwellensysteme