Root exudates from banana and their relationship to strains of the Fusarium causing Panama wilt

Abstract

SUMMARYA method is described for obtaining sterile exudates from banana roots still attached to the parent plant. Root exudates from the varieties Gros Michel (susceptible to Panama wilt) and Lacatan (resistant) were tested for their effects on spore germination of three strains of the pathogen, Fusarium oxysporum f. cubense.In a medium otherwise optimal for spore germination, root exudates from 3–6‐month‐old Lacatan plants inhibited spore germination whereas root exudates from Gros Michel did not. The extent of the inhibition depended on the strain of the Fusarium tested and it decreased with ageing of the plants from which the root exudates came. In weak sucrose or soil‐extract media, root exudates from Gros Michel promoted spore germination of each strain. Exudates from young Lacatan plants did not promote germination in these two media, but exudates from 7–9‐month‐old did, although less than Gros Michel exudates.Chromatograms showed eighteen amino acids in the root exudates, thirteen common to both banana varieties. Gros Michel root exudates contained no detectable cystine or threonine, and Lacatan no leucine, serine or tyrosine. Root exudates from young Lacatan contained more aspartic acid, glutamine and proline than those from young Gros Michel, but the differences decreased as the plants aged.Total carbohydrates were maximal in root exudates from 6‐month‐old plants of both varieties. Gros Michel had 1.5 times more carbohydrate than Lacatan when young, but plants 3 and 9 months old contained almost equal amounts. Chromatograms showed that eight sugars were present and Gros Michel exudates had more of each sugar than did Lacatan at all stages of sampling.Materials that inhibited spore germination on Dox‐agar media were demonstrated at RF 0.7 on chromatograms of Lacatan exudates. Neither ninhydrin‐reactive spots nor spots that fluoresced under ultra‐violet light occurred at RF 0.7 with Gros Michel exudates; nor did any materials at that RF inhibit spore germination.The relationships between materials in the root exudates from the wilt‐susceptible and wilt‐resistant plants and differences in spore germination responses are considered as part of a wilt‐resistance mechanism in the rhizosphere of banana.