Antagonistic microbes for the management of Cosmopolites sordidus and Radopholus similis in East African bananas

Abstract

The East African highland banana (EAHB, Musa spp.) is a key staple in East and Central Africa (ECA), where they provide up to 60% of total calorie consumed per capita. Banana production has declined since the 1970s, with the current production being a fraction of its potential. The banana weevil (BW), Cosmopolites sordidus, and a complex of plant-parasitic nematodes (PPN) are among the key biotic constraints to banana production. Radopholus similis is the most damaging nematode parasite of banana in ECA, often appearing in a complex with Helicotylenchus multicinctus, Pratylenchus spp. and Meloidogyne spp. Thus, developing proper and effective management strategies against both BW and PPN is crucial for the improvement and restoration of banana production in ECA. Therefore, this PhD study aimed at: 1) assessing the efficacy of indigenous entomopathogenic fungi (EPF) against BW, 2) evaluating the potential of local endophytic fungal isolates for the management of R. similis infection of banana and 3) testing the potential of multiple endophytic isolates for the management of R. similis infection of banana, 4) studying the possible mode of action of fungal endophytes against PPN infection of banana and 5) assessing the field performance of endophyte enhanced banana tissue culture plantlets with respect to suppression of PPN and BW infection, persistence of the endophytic fungus in the banana roots and impact on yield and longevity of the banana field. Amongst the 20 EPF isolates screened (15 Beauveria bassiana, 4 Metarhizium anisopliae and 1 Isaria fumosorosea), 8 B. bassiana isolates caused >50% mortality of adult BW, while Metarhizium and Isaria isolates were less pathogenic. Beauveria bassiana isolates ICIPE 273, ICIPE 648 and ICIPE 660 were the most pathogenic compared to other isolates, killing ≥80% of adult BW in the shortest time (LT50 ≤ 11.1 days) and at the lowest spore concentration (LC50 ≤ 5.49 × 107 spores ml-1). Furthermore, the three Beauveria isolates, ICIPE 273, ICIPE 648 and ICIPE 660 were able to sporulate on BW cadavers, producing ≥ 3.6 × 107 spores per BW cadaver after 14 days of incubation, an indication of the potential of these fungal isolates for self-dissemination. On the other hand, the non-pathogenic Fusarium oxysporum (isolate V5w2) and the B. bassiana (isolate WA) were endophytically introduced into tissue cultured (TC) plants of the cooking banana cv. Mbwazirume and the dessert banana cv. Grande Naine which were then grown in the field for two cropping cycles. The fungal endophytes were inoculated separately and in combination. Colonisation of the banana plants by the fungal endophytes reduced from 45% root colonisation at field establishment to <3% colonisation at 6 months post field establishment, while no root colonisation was recorded beyond 6 months. Inoculation of the

banana plants with V5w2 alone significantly reduced nematode (R. similis and H. multicinctus) infection in the first cropping cycle by 35%, compared to 13% and 36% reductions when the plants were inoculated with WA alone or a combination (V5w2 and WA), respectively. Similarly, plants inoculated with V5w2 alone or a combination (V5w2 and WA) had higher yield (≥11 t ha-1) compared to those inoculated with WA alone (8.6 t ha-1) or the uninoculated plants (7.9 t ha-1). This revealed that inoculation of the TC plants with V5w2 alone is ideal for nematode control and improvement of crop yield demonstrating the benefits of using fungal endophytes in improving the yield of both cooking and dessert banana through nematode suppression. However, these benefits were not observed in the ratoon crop, an indication that repeat in-field application of the fungal isolate is required to boost the endophyte inoculum in successive crop cycles. To understand the mode of action of fungal endophytes in suppression of R. similis infection of banana, the EAHB cv. Ng’ombe was inoculated with the fungal endophytes Trichoderma asperellum (ICIPE 700) and Hypocrea lixii (ICIPE 697) individually and in combination. Inoculation of banana TC with the fungal endophytes significantly suppressed R. similis infection by >81% and led to enhanced upregulation of the defence-related gene PR-1, the cell wall strengthening gene β-1,3-glucan synthase and the cell signalling gene calmodulin-Ca2+, an indication that defence genes play a critical role in the suppression of R. similis infection of endophyte-inoculated banana plants.