Direct Root Penetration and Rhizome Vascular Colonization by Fusarium oxysporum f. sp cubense are the Key Steps in the Successful Infection of Brazil Cavendish

Fusarium wilt of banana, which is caused by Fusarium oxysporum f. sp. cubense, is one of the most serious diseases of banana. F. oxysporum f. sp. cubense race 1 (Foc1) and race 4 (Foc4) are the most prevalent pathogens of banana cultivars in the world. To understand the differences in the infection processes between Foc1 and Foc4, green fluorescent protein-tagged strains of F. oxysporum f. sp. cubense tropical race 4 (FocTR4) and Foc1 were used to inoculate 'Brazil Cavendish' banana. At 2 days postinoculation (dpi), it was observed that the spores and hyphae of both Foc1 and Foc4 attached to the root hairs and root epidermis. At 3 dpi, the hyphae of both Foc1 and Foc4 were found in the vascular tissues of roots. However, Foc4 was observed in the parenchymal cells of banana root, whereas Foc1 was not found in parenchymal cells at 7 dpi. Furthermore, few Foc1 hyphae were observed in a few xylems whereas many more Foc4 hyphae were present in many xylems and phloems. Foc4 was observed in the vascular tissues of banana rhizomes, whereas no Foc1 was found in rhizomes 2 months after inoculation. The attachment process in F. oxysporum f. sp. cubense infection was further studied with scanning electron microscopy. Foc4 was observed to penetrate into banana roots from the intercellular space of the epidermis and wounds, whereas Foc1 mainly penetrated from the wounds but not from the intercellular space of the epidermis. Therefore, direct root penetration and rhizome vascular colonization by F. oxysporum f. sp. cubense are the key steps in the successful infection of Brazil Cavendish.