Combination of Crotalaria spectabilis with Rhizophagus irregularis MUCL41833 decreases the impact of Radopholus similis in banana

The past decade has seen substantial progress in our knowledge on microorganisms (e.g. arbuscular mycorrhizal fungi-AMF) and push-pull plants (e.g. Crotalaria spectabilis) that contribute to the biocontrol of nematodes. However, the application of microorganisms together with push-pull plants has seldom been considered. Therefore, the objective of the study was to investigate the combined effect of AMF and C. spectabilis on the control of the nematode Radopholus similis in banana. Banana plants, pre-colonized or not with the AMF Rhizophagus irregularis MUCL 41833 were grown in 3 L pots in presence/absence of Crotalaria spectabilis. Above-ground parts were separated with a fixed talpa net to separate shoots and leaves of the two plants. Similarly, the pots were divided below-ground in two compartments by growing the banana roots in a pocket nylon mesh (30 mm), to avoid roots of both plants to intermingle. The banana plants were established first and were followed three weeks later by C. spectabilis. Inoculation of nematodes was done in parallel to the planting of C. spectabilis. Half of the banana plants received 1000 monoxenically produced juveniles and adults of R. similis. Eight treatments were set up with 6 replications: mycorrhizal banana plantlets with/without C. spectabilis and with/without nematodes (+M + C + N, + M + C - N, + M - C + N, + M - C - N) and non-mycorrhizal banana plantlets with/without C. spectabilis and with/without nematodes (- M + C + N, - M + C - N, - M - C + N, - M - C - N). Plant growth parameters, root colonization by the AMF and infection by the nematode were evaluated. R. similis did not impact banana root colonization by the AMF. Conversely, the fungal symbiont as well as C. spectabilis significantly decreased the total number of nematodes as well as their multiplication rate. The multiplication rate in the controls (i.e. - M - C + N) was 280.3, while it decreased to 176.5, 106.7 and 83.8 in the - M + C + N, + M - C + N and + M + C + N treatments, respectively. The root necrosis index (RNI) was significantly decreased in presence of the AMF, C. spectabilis and the combination of both. The RNI of the control (i.e. - M - C + N) was 61.7% while it was 33.7, 19.8 and 17.2 in the - M + C + N, + M - C + N and + M + C + N treatments, respectively. Banana root fresh weight was significantly increased in presence of C. spectabilis and shoot dry weight in presence of AMF, but no increase was noticed in presence of both organisms together. This study reaffirmed that AMF and C. spectabilis are effective in decreasing the pressure caused by R. smilis in banana roots. It demonstrated further, for the first time, that their combination decreased even more drastically the surface of necrotic cortical tissues caused by the nematodes, opening new avenues for their concomitant use in an integrated pest management strategy. (C) 2016 Elsevier B.V. All rights reserved.