A. von Tiedemann

A crop loss-related forecasting model for sclerotinia stem rot in winter oilseed rape.

Sclerotinia stem rot (SSR) is an increasing threat to winter oilseed rape (OSR) in Germany and other European countries due to the growing area of OSR cultivation. A forecasting model was developed to provide decision support for the fungicide spray against SSR at flowering. Four weather variables-air temperature, relative humidity, rainfall, and sunshine duration-were used to calculate the microclimate in the plant canopy. From data reinvestigated in a climate chamber study, 7 to 11 degrees C and 80 to 86% relative humidity (RH) were established as minimum conditions for stem infection with ascospores and expressed as an index to discriminate infection hours (Inh). Disease incidence (DI) significantly correlated with Inh occurring post-growth stage (GS) 58 (late bud stage) (r(2) = 0.42, P </= 0.001). Using the sum of Inh from continuous infection periods exceeding 23 h significantly improved correlation with DI (r(2) = 0.82; P </= 0.001). A parallel GS model calculates the developmental stages of OSR based on temperature in the canopy and starts the model calculation at GS 58. The novel forecasting system, SkleroPro, consists of a two-tiered approach, the first providing a regional assessment of the disease risk, which is assumed when 23 Inh have accumulated after the crop has passed GS 58. The second tier provides a field-site-specific, economy-based recommendation. Based on costs of spray, expected yield, and price of rapeseed, the number of Inh corresponding to DI at the economic damage threshold (Inh(i)) is calculated. A decision to spray is proposed when Inh >/= Inh(i). Historical field data (1994 to 2004) were used to assess the impact of agronomic factors on SSR incidence. A 2-year crop rotation enhanced disease risk and, therefore, lowered the infection threshold in the model by a factor of 0.8, whereas in 4-year rotations, the threshold was elevated by a factor 1.3. Number of plants per square meter, nitrogen fertilization, and soil management did not have significant effects on DI. In an evaluation of SkleroPro with 76 historical (1994 to 2004) and 32 actual field experiments conducted in 2005, the percentage of economically correct decisions was 70 and 81%, respectively. Compared with the common practice of routine sprays, this corresponded to savings in fungicides of 39 and 81% and to increases in net return for the grower of 23 and 45 euro/ha, respectively. This study demonstrates that, particularly in areas with abundant inoculum, the level of SSR in OSR can be predicted from conditions of stem infection during late bud or flowering with sufficient accuracy, and does not require simulation of apothecial development and ascospore dispersal. SkleroPro is the first crop-loss-related forecasting model for a Sclerotinia disease, with the potential of being widely used in agricultural practice, accessible through the Internet. Its concept, components, and implementation may be useful in developing forecasting systems for Sclerotinia diseases in other crops or climates.

Broadening the Genetic Basis of Verticillium longisporum Resistance in Brassica napus by Interspecific Hybridization.

ABSTRACT Verticillium wilt caused by the vascular fungal pathogen Verticillium longisporum is one of the most important pathogens of oilseed rape (Brassica napus sp. oleifera) in northern Europe. Because production of this major oilseed crop is expanding rapidly and no approved fungicides are available for V. longisporum, long-term control of the disease can only be achieved with cultivars carrying effective quantitative resistance. However, very little resistance to V. longisporum is available within the gene pool of oilseed rape, meaning that interspecific gene transfer from related species is the only possibility for broadening levels of resistance in current varieties. The amphidiploid species B. napus can be resynthesized by crossing the two progenitor species Brassica oleracea and Brassica rapa, hence resistant accessions of these two diploid species can be used as resistance donors. In this study a total of 43 potential B. rapa and B. oleracea resistance donors were tested with regard to their reaction to a mixture of two aggressive V. longisporum isolates, and resistances from diverse lines were combined by embryo rescue-assisted interspecific hybridization in resynthesized rapeseed lines. Progenies from crosses of the two B. rapa gene bank accessions 13444 and 56515 to the B. oleracea gene bank accessions BRA1008, CGN14044, 8207, BRA1398, and 7518 showed a broad spectrum of resistance in pathogenicity tests. Of 45 tested resynthesized lines, 41 lines exhibited a significantly higher level of resistance than the moderately V. longisporum-tolerant oilseed rape cultivar Express. These lines represent a promising basis for the combination of different resistance resources in new varieties.

Plant pathogens, insect pests and weeds in a changing global climate: a review of approaches, challenges, research gaps, key studies and concepts

Climate change biology is witnessing a significant quantity of new publications each year, which compromises efforts to keep up-to-date on the rapidly growing body of climate change biology literature. The present paper provides an overview on research approaches and challenges in climate change biology with respect to plant pathogens, insect pests and weeds (collectively termed ‘pests’ here). It also summarizes the suggestions of researchers about how to conceptualize and prioritize future research strategies. Recently published key studies demonstrate that climate change research is qualitatively advancing and that the interactions among environmental and biotic factors which have been found are complex. This complexity hinders attempts to generalize responses of pests to changes in climate. The challenge remains to identify the most significant causal relationships and to separate them from other factors such as crop management practices, which may also influence the observed changes in pest distribution and prevalence in managed ecosystems. In addition, the present overview shows that there are still gaps in many research areas, while other fields have been intensively investigated. For example, the identification of potential benefits in plant protection that may emerge from future climate change has not been explored as extensively as the potential threats. However, encouraging developments can be observed in recent climate change research, for instance the increased number of studies performed under subtropical and tropical climatic conditions, the increased availability of results from multi-factorial field experiments and modelling studies do consider increasingly pest–crop–climate interactions. Further progress can be expected, provided that researchers, sponsors and other stakeholders maintain their interest in climate change biology research.

Identification of Brassica accessions with enhanced resistance to Verticillium longisporum under controlled and field conditions

Verticillium longisporum (VL) is a soil-borne vascular fungal pathogen with host-specificity to cruciferous plants such as oilseed rape, threatening its production particularly in the Northern European countries. In a comprehensive screening conducted under greenhouse conditions, 1348 accessions of Brassica napus as well as the progenitor species B. oleracea and B. rapa (syn. campestris) were tested for their resistance to VL. While most of the tested B. napus accessions showed a susceptible to moderate resistant phenotype and the majority of the B. rapa genotypes were highly susceptible, an elevated level of resistance was found in the B. oleracea pool. Resynthesized oilseed rape lines produced on the basis of these data by interspecific hybridization of B. oleracea and B. rapa also exhibited enhanced resistance to VL. In order to verify the greenhouse data, a subset of B. napus breeding lines was further tested for resistance in the field at different sites in North and Northeast Germany. Overall, there was no statistically significant relationship, neither between greenhouse and field data nor between results of different field sites. However, genotypes highly resistant in the greenhouse generally performed well also in the field. Thus, the combination of resistance tests conducted under controlled as well as field conditions is a powerful and reliable approach in the practical selection of resistant breeding lines.ZusammenfassungVerticillium longisporum (VL) ist ein bodenbürtiges, pilzliches Gefäßpathogen mit Wirtsspezialisierung auf kruzifere Pflanzenarten wie Raps, dessen Produktion durch diesen Erreger insbesondere in den nordeuropäischen Ländern gefährdet ist. In einem umfangreichen Screening unter Gewächshausbedingungen wurden 1348 B. napus-Akzessionen sowie die Elternspezies B. oleracea und B. rapa (syn. campestris) auf ihre Anfälligkeit gegenüber VL getestet. Während die meisten der getesteten B. napus-Akzessionen einen anfälligen bis mäßig resistenten Phänotyp zeigten und die Mehrheit der B. rapa-Genotypen hoch anfällig war, wurde ein erhöhtes Resistenzniveau im B. oleracea-Pool gefunden. Resynthetisierte Rapslinien, welche auf der Basis dieser Daten durch interspezifische Hybridisierung von B. oleracea und B. rapa erzeugt worden waren, wiesen ebenfalls eine erhöhte Resistenz gegenüber VL auf. Um die Daten aus dem Gewächshaus zu verifizieren, wurden ausgewählte B. napus-Zuchtlinien weiteren Resistenztests im Feld an verschiedenen Standorten im Norden und Nordosten Deutschlands unterzogen. Insgesamt gab es weder zwischen Gewächshaus- und Felddaten noch Feldversuchsstandorte einen statistisch signifikanten Zusammenhang. Allerdings schnitten Genotypen, welche hoch resistent im Gewächshaus waren, im Allgemeinen auch unter Feldbedingungen gut ab. Somit ist die Kombination von Resistenztests sowohl unter kontrollierten als auch unter Feldbedingungen ein leistungsfähiger und zuverlässiger Ansatz in der praxisbezogenen Selektion von resistenten Zuchtlinien.

Pathogenicity and host specificity of Brazilian and African isolates of the acaropathogenic fungus Neozygites tanajoae to mite species associated with cassava.

Neozygites tanajoae is a host‐specific acaropathogen introduced in the late 1990s from Brazil into cassava fields in West Africa for biocontrol of the cassava green mite (Mononychellus tanajoa). Local African isolates of N. tanajoae are morphologically similar to introduced Brazilian strains but the two origins can be distinguished by diagnostic PCR. In this study it was explored whether differential traits in acaropathogenic performance and host specificity exist between Brazilian and African isolates. Pathogenicity and host specificity of two Brazilian and two African isolates of N. tanajoae were compared in leaf discs laboratory bioassays or on whole cassava plants in the greenhouse by exposing adult female mites to inoculum delivered from N. tanajoae infested mite mummies. The results show that all four N. tanajoae isolates caused significant mortality of M. tanajoa, both on leaf discs and on whole plants. However, on leaf discs the Brazilian isolate collected directly from Alto Alegre in Brazil (Brazil–Brazil isolate) caused a significantly higher mortality (80%) than the Brazilian isolate collected about 9 years post‐release from a field in Adjohoun (Brazil–Benin isolate) (61.3%). The two African isolates collected in Benin and Tanzania caused a mortality similar to the African–Brazilian isolate (62.3% and 61.3% respectively). On whole cassava plants, the four fungal isolates did not show any significant differences in incidence of M. tanajoa infection. The host specificity study was conducted on leaf discs against a collection of mite species typically associated with cassava plants in Africa, Typhlodromalus aripo, Euseius fustis, Tetranychus urticae and Oligonychus gossypii. It demonstrated that the African and Brazilian isolates of N. tanajoae had similar host specificity for M. tanajoa.