Bernward Märländer

Integrated Control of Root and Crown Rot in Sugar Beet : Combined Effects of Cultivar, Crop Rotation, and Soil Tillage

Rhizoctonia solani (AG 2-2IIIB), causing root and crown rot in sugar beet, poses an increasing problem in Europe. Agronomic measures have to be optimized to control disease and minimize yield and quality loss, because no fungicides can be applied. Resistant sugar beet cultivars have been introduced to reduce disease occurrence. Furthermore, crop rotation can influence R. solani occurrence. In contrast to other cereals, maize serves as a host of the fungus. In order to study the combined effect of these factors, a series of four field trials was established with crop rotations varying in the proportion of maize and comparing a resistant with a susceptible sugar beet cultivar in 2001-02 in southern Germany. Within crop rotations, cultivation methods were varied in the form of soil tillage, intercrops, or both. Sugar beet cultivar and crop rotation had the main impact on disease severity and sugar yield. With increasing proportion of maize, sugar yield decreased, whereas cultivation method had only a minor impact. Plowing directly before sugar beet increased sugar yield only within the unfavorable maize-maize-sugar beet rotation compared with mulching. These results give strong evidence that crop rotation of sugar beet with nonhost plants and cultivation of resistant sugar beet cultivars are adequate means for integrated R. solani control.

Characterization and mycotoxigenic potential of Fusarium species in freshly harvested and stored sugar beet in Europe.

Based on a 2-year field trial at two locations in Lower Saxony (Germany), 395 Fusarium isolates belonging to 13 species were collected from more than 3,000 sugar beet roots that were apparently healthy at harvest. In a comparative screen, subsamples were analyzed for Fusarium infection directly after harvest and after different storage conditions. Depending on the storage duration, a different species composition was observed. F. redolens was predominant in freshly harvested beets, while F. culmorum, F. cerealis, and F. graminearum comprised 50.0% (2006) and 84.8% (2007) of the Fusarium mycoflora of sugar beets subjected to long-term pile storage. Randomly selected isolates of all species detected were tested for pathogenicity to sugar beet, but only isolates of F. graminearum and F. sambucinum caused severe root symptoms. Overall, 34 isolates of all species detected were characterized for their mycotoxin profile in rice culture to determine potentially produced toxins for future analysis of sugar beet. A total of 26 Fusarium mycotoxins were detected by liquid chromatography-tandem mass spectrometry, including trichothecenes, zearalenone, and especially high amounts of beauvericin, enniatins, and moniliformin. Further work is required to analyze the natural occurrence of these mycotoxins in sugar beet.

Distribution of quality components in different morphological sections of sugar beet (Beta vulgaris L.)

The technical quality of sugar beet is essential for economical sugar manufacturing. In particular, it depends on the chemical composition of the beet. The concentrations of the quality components are influenced by agronomical conditions. Due to progress in breeding and cultivation methods over the past decades, the components and their distribution in different morphological sections may have changed. Therefore, the distribution of sucrose, dry matter, potassium, sodium, α-amino N, soluble N, nitrate, betaine, reducing sugars, glucose and raffinose were analysed and the standard molasses loss was calculated for the current agronomical conditions. Cultivation methods (variety, population density, N and K application) were varied in several field trials at two sites in 1997 to obtain heterogeneous beet material. The beets were segregated into four horizontal sections according to morphological criteria (root with root groove, hypocotyl, lower stem with dead petioles, upper stem with living petioles). On average, the sucrose concentration decreased from the root to the lower stem, and especially by one third to the upper stem. The dry matter concentration did not vary markedly between the four sections. In comparison to this, the concentration of potassium, sodium and α-amino N increased considerably from the lower to the upper stem. Therefore, the standard molasses loss (K, Na, α-amino N), calculated to evaluate the technical quality of sugar beet, increased as well. The concentration of soluble N, reducing sugars and glucose were approximately three times higher in the upper stem than in the root. In contrast to this, the concentration of raffinose, betaine and nitrate did not change markedly. There was a close relationship between soluble N and α-amino N in each section. The range of all components within the single sections differed extremely due to the variation in cultivation methods. But for most components the highest variation was found in the upper stem. Taking all components into account, the technical quality of sugar beet deteriorates the more shoot material (lower and upper stem) is included during harvesting and topping.

Field emergence dynamics significance to intraspecific competition and growth efficiency in sugar beet (Beta vulgaris L.)

Significance of field emergence dynamics on sugar beet growth was examined in order to determine the effect of individual date of emergence on early growth of single plants using plant growth analysis. The effect of a prolonged field emergence period on yield and quality of the sugar beet crop was investigated using simulated prolonged field emergence periods. Field trials were conducted on a loamy soil in Gottingen in 1998. Early growth of sugar beet was exponential. A delay of emergence had a significant negative effect on the mass of total dry matter production of single plants. Date of emergence had no significant effect on relative growth rate of single plants thus the efficiency of growth. Plants with delayed emergence were not able to compensate for the delay in dry matter production and therefore, contributed less to the dry matter formation of the crop. A prolonged field emergence period showed significant negative effects on yield parameters, but no effects on the quality of the beet. These effects on yield are formed by increased intraspecific competition to light caused by shading of delayed emerged plants.

New Generation of Resistant Sugar Beet Varieties for Advanced Integrated Management of Cercospora Leaf Spot in Central Europe

Cercospora leaf spot (CLS) epidemics in sugar beet have been increasing in recent years causing higher use of fungicides. Concomitantly, the availability of effective fungicides is at risk because of resistance development in the fungus, the lack of new active ingredients as well as restrictive approval practices. A key option for an integrated management of CLS is cultivation of resistant varieties. Because of the yield penalty in resistant varieties, acceptance in commercial practice so far has been low. The aim of our study was to characterize recent sugar beet varieties registered in Germany in terms of resistance and tolerance to CLS and their value for integrated pest management. The genetic basis of CLS resistance in varieties is protected by intellectual property rights even after variety registration and not open to the public due to economic competition. To gain reliable data for cultivation, varieties have to be tested for their resistance traits under field conditions at varying levels of infection with Cercospora beticola. In collaboration with variety related stakeholders, 15 sugar beet varieties were tested in 49 field trials in Germany from 2014 to 2016 for their yield response to CLS. The trials were set up in a split-plot design with and without infection (i.e., with and without fungicide). The classification of varietal reaction to CLS is based on symptomatic leaf area (susceptibility) and the resulting relative yield loss (tolerance). Since the relation between both parameters varied among varieties, it was used as an additional parameter to describe tolerance. On this basis, three groups of varieties were identified. They can be characterized as a susceptible, a resistant and a presumably tolerant cluster. A comparison of the data with an older dataset originating from 2009 to 2011 revealed that yield performance of recent varieties with resistance to C. beticola caught up with susceptible varieties due to breeding progress. They showed no yield penalty in the absence of the disease and better economic performance than susceptible varieties. It is assumed that these varieties will allow a substantial reduction of fungicide use for an advanced integrated pest management under central European conditions.

Control of Rhizoctonia solani in Sugar Beet and Effect of Fungicide Application and Plant Cultivar on Inoculum Potential in the Soil

Rhizoctonia solani (AG 2-2 IIIB) is the causal agent of Rhizoctonia root and crown rot, a disease that causes severe economic problems in sugar beet growing areas worldwide. In the United States, azoxystrobin is the most important active ingredient for fungicidal control of R. solani in sugar beet, showing efficacy superior to other substances. First reports on resistance development in R. solani, however, underline the importance of a careful fungicide resistance management. For this reason, the efficacy of a new fungicide mixture of azoxystrobin and difenoconazole was compared with a fungicide containing only azoxystrobin. Field trials were carried out under natural infection conditions as well as with inoculation in the years 2012, 2013, and 2014. Evaluation of the disease severity and the obtained white sugar yield of different sugar beet cultivars demonstrated that both fungicide treatments possess a similar efficacy, reducing the diseased beet surface by up to 78% and preventing yield losses. Additionally, a real-time PCR assay, based on DNA extracts from representative soil samples (250 g), was used to directly determine the effect of chemical treatment and plant cultivar on the soil-borne inoculum. Fungicide application significantly reduced the concentration of soil-borne inoculum by up to 97%. Furthermore, the results demonstrated that the cultivation of a susceptible cultivar significantly increases the concentration of R. solani in the soil by a factor of 200. In conclusion, the study implies that only a combination of resistant cultivar and fungicide application can prevent an accumulation of R. solani inoculum under conducive conditions in infested fields.

Effect of Sugar Beet Variety and Nonhost Plant on Rhizoctonia solani AG2-2IIIB Soil Inoculum Potential Measured in Soil DNA Extracts

A direct soil DNA extraction method from soil samples (250 g) was applied for detection of the soilborne sugar-beet-infecting pathogen Rhizoctonia solani anastomosis group (AG) 2-2IIIB using a newly developed real-time polymerase chain reaction assay that showed specificity to AG2-2IIIB when tested against various R. solani AG. The assay showed a good relation between cycle threshold and amount of AG2-2IIIB sclerotia detected in three spiked field soils and was also able to detect the pathogen in naturally infested field soil samples. A field trial was conducted to quantify R. solani AG2-2IIIB soil inoculum potential (IP) before and after growing a susceptible and a resistant sugar beet variety as well as after subsequent growth of an expected nonhost winter rye. Plants of the susceptible sugar beet variety displayed a higher disease severity. A more than sixfold increase of the R. solani AG2-2IIIB soil IP was observed in contrast to the resistant variety that resulted in a constant IP. Growing winter rye significantly reduced soil IP to the initial level at sowing. Further research is required to better understand the interaction between disease occurrence and soil IP as well as the environmental influence on IP development.