Studies on the Characteristics of the Antagonistic Relationship between Radopholus similis (Cobb) Thorne and Mutualistic Endophytic Fungi in Nematode-Suppressive Banana Plants (Musa AAA)

The antagonistic relationship of Radopholus similis and seven mutualistic endophytes isolated from banana and plantain were investigated in a series of experiments. First, prior to field trials and further greenhouse experiments, the vegetative compatibility of endophytes identified as Fusarium oxysporum and known pathogenic F. oxysporum isolates was tested. Nitrate non-utilizing (nit) mutants were produced and crossed with complementary testers from pathogenic F. oxysporum f. sp. radicis-lycopersici, lycopersici and cubense strains. None of the endophytic F. oxysporum isolates formed heterokaryons with any of the pathogenic strains tested. The nematode-antagonistic endophytes are therefore not vegetatively compatible with the pathogenic strains, and can not exchange genetic information with them. Second, the role of systemic induced resistance to nematode penetration and reproduction was tested in a series of split-root tests carried out in greenhouses in Germany and Costa Rica. The results revealed that, of the three T. atroviride (MT-20, ET-35 and S2) and four F. oxysporum (MF-25, MF-40, S9 and P12) isolates tested, only MT-20 and S2 seemed to induce systemic effects in Musa AAA cv. ‘Valery’ plants. S2 significantly and consistently reduced R. similis penetration, 10 days after nematode inoculation, while MT-20 significantly reduced nematode reproduction over a 9 week period. None of the tested F. oxysporum isolates induced systemic resistance to nematode penetration or reproduction. Third, the possible transfer of nematode suppression from endophyte-inoculated Musa AAA cv. ‘Valery’ mother plants to daughter plants (suckers) in the field was studied. As part of a large scale field trial looking at the suppression of nematodes in the field, mother plants were inoculated with endophytes prior to field transfer, and nematode populations monitored for 7 months. At the first de-suckering, sword suckers were removed from the field and grown in the greenhouse, where they were later challenged with R. similis. Results from the field to greenhouse test indicated that, when nematode suppression is established in the field, this suppression is transferred from mother to daughter plants. Finally, the effects of single and combined inoculations of two F. oxysporum and two T. atroviride on R. similis biocontrol in Musa AAA cv. ‘Williams’ and on plant growth of Musa AAA cvs. ‘Grand Nain’, ‘Valery’ and ‘Williams’ were investigated. Combining inoculations of mutualistic endophytes increased nematode control levels and plant growth in ‘Williams’ plants inoculated with both R. similis and fungi. Plant growth in the absence of R. similis was promoted by single and dual inoculations of T. atroviride isolates and by single inoculations of F. oxysporum isolates in ‘Grand Nain’, ‘Valery’ and ‘Williams’ plants. Dual inoculations of F. oxysporum and of all four endophytes did not positively affect plant growth.Studien zur Characterizierung der Antagonistischen Beziehung zwischen Radopholus similis (Cobb) Thorne und Mutualistischen Endophytischen Pilzen in Nematoden-Suppressiven Bananen Pflanzen (Musa AAA) Die antagonistische Beziehung von Radopholus similis und sieben aus Bananen und Kochbananen isolierten mutualistischen Endophyten wurden in einer Serie von Versuchen durchleuchtet. Vor weiteren Gewächshausversuchen und Feldversuchen, wurde erstens die vegetative Kompatibilität der als Fusarium oxysporum identifizierten Endophyten und bekannte pathogenen F. oxysporum Isolate getestet. Nitrate nicht verwendende (nit) Mutanten wurden produziert und mit komplementären Testern der pathogenen Stämme von F. oxysporum f. sp. radicis-lycopersici, lycopersici und cubense gegenüber gestellt. Keiner der getesteten endophytischen F. oxysporum Isolate formte einen Heterokaryon mit pathogenen Stämmen. Die Nematoden antagonistischen Endophyten sind daher nicht mit pathogenen Stämmen vegetativ kompatibel, und können keine genetische Information austauschen. Zweitens wurde die Rolle der systemisch induzierten Resistenz zu Nematoden Penetration und Reproduktion in einer Reihe Split-Root Versuchen im Gewächshaus in Deutschland and Costa Rica nachgeprüft. Von den drei T. atroviride (MT-20, ET-35 und S2) und den vier F. oxysporum (MF-25, MF-40, S9 und P12) getesteten Pilzen, induzierten nur MT-20 und S2 eine systemische Reaktionen in Musa AAA cv. ‘Valery’ Pflanzen. S2 reduzierte in signifikanterweise und immer wieder die Penetration von R. similis, 10 Tagen nach Nematoden Inokulation, wehrend MT-20 in signifikanterweise die Reproduktion über eine 9-Wöchige Periode reduzierte. Keins der getesteten F. oxysporum Isolate induzierte eine systemische Resistenz, weder zur Penetration noch zur Reproduktion von R. similis. Drittens wurde die mögliche Übertragung der durch Endophyten hervorgerufene Nematoden Suppression in Musa AAA cv. ‘Valery’ Mutterpflanzen an deren Töchter (Wurzelschössling) im Feld erforscht. Als Teil eines groß angelegten Feldversuches, wurden Mutterpflanzen mit Endophyten vor der Aussaat im Feld inokuliert, und der Nematoden Bestand über 7 Monate überwacht. Bei der ersten Schösslingsentfernung, wurden Schwertschösslinge vom Feld entnommen und ins Gewächshaus verpflanzt, wo sie später mit R. similis inokuliert wurden. Die Ergebnisse des Versuchs deuteten darauf hin das, dort wo sich die Nematoden Suppression im Feld etabliert, wird diese auch von Mutter- zu Tochterpflanze übertragen. Letztlich wurden die Wirkung einzelner und kombinierter Inokulationen von zwei F. oxysporum und zwei T. atroviride auf die Biokontrolle von R. similis in Musa AAA cv. ‘Williams’ und auf das Wachstum von Musa AAA cvs. ‘Grand Nain’, ‘Valery’ und ‘Williams’ untersucht. In ‘Williams’ Pflanzen die mit R. similis sowie Pilzen inokuliert wurden, wurde das Niveau der Nematoden Biokontrolle und des Pflanzenwachstums durch kombinierte Inokulationen gesteigert. In der Abwesenheit von R. similis, wurde der Wachstum in ‘Grand Nain’, ‘Valery’ und ‘Williams’ Pflanzen durch die Inokulation mit einzelnen oder beiden T. atroviride und mit einzelnen F. oxysporum gesteigert. Die Inokulation mit beiden F. oxysporum und mit allen Pilzen förderte nicht das Wachstum der Pflanzen.</p

A literature review on yield gaps of various root, tuber and banana crops as a background for assessing banana yield reductions due to pests and diseases at a field site in western Burundi

Banana pests (corm weevil and root nematodes) and diseases (Xanthomonas wilt of banana, banana bunchy top disease and fusarium wilt) are major constraints to banana production in Central Africa. The pests cause various degrees of yield reduction, while plants affected by three of the diseases eventually die before producing an edible bunch. Studies on yield gaps for most of these constraints are currently limited. This paper reviews yield gap studies of some root, tuber and banana crops broadly and with a specific focus on biotic constraints. It also presents an initial case study conducted in Burundi to understand yield gaps due to various banana pests and diseases. Bunch weights of banana varied widely at production zones in western Burundi due to biotic constraints. Boundary line analysis revealed large yield gaps due to the various pests. The often sub-optimal, medium and small bunch sizes found in visibly healthy fields however indicate that in addition to mitigating effects of biotic constraints, significant improvements in bunch weights could be attained through the application of agronomic/field management practices that enhance soil fertility, soil moisture content and soil health. Simple and robust methods (such as the boundary line analysis) for estimating yield gaps caused by pests and diseases, and abiotic constraints on farm are crucial for informing/guiding on the need to apply agronomic and/or disease control efforts. In addition, continuous/sustained field monitoring, with the involvement of farmers, over time will be necessary for a more accurate assessment of yield gaps caused by diseases and pests

Banana pest risk assessment along banana trade axes running from low to high altitude sites, in the Eastern DR Congo and in Burundi

Pests and diseases greatly contribute to the decline in banana yields, food and income insecurity in the Great Lakes region of Africa. Understanding people’s role in pest and disease spread at landscape level is crucial for effective pest and disease management. To determine this, focus group discussions (FGDs) targeting 10 experienced farmers (50% female and 50% male) were conducted in 27 villages along four banana trade routes in western Burundi and eastern DR Congo. FGDs determined the presence and risk of spread of key banana pests and diseases via the movement of banana bunches, planting material and other products, labourers, traders and farm tools. Black leaf streak (BLS), Fusarium wilt and banana bunchy top disease (BBTD) were reported to have been in the landscape for over 40 years while Xanthomonas wilt (XW) was a more recent introduction (1-7 years). BBTD, XW and weevils were the most prevalent constraints. BBTD was observed at previously unsuitable high-altitude zones, which should be a cause of concern, especially with the current risk of climate change. Climate change, and linked temperature increases, could also worsen the prevalence of XW, weevils, nematodes and BLS. Movement of farming tools by labourers and traders, of planting material/suckers and banana bunches emerged as the most common human practices potentially responsible for the spread and/or build-up of banana pests/diseases. Strengthening farmer’s knowledge and institutional capacities of actors on these different modes of disease spread in banana value chains in the region is recommended