Comparative analysis of infection process in Upland cottons differing in resistance to Fusarium wilt caused by Fusarium oxysporum f. sp. vasinfectum race 4

Abstract Fusarium wilt ( Fusarium oxysporum f. sp. vasinfectum race 4, FOV4) in cotton is an early season soil‐borne vascular disease including seedling wilt and death. Its infection process in Upland cotton is currently unknown. This study compared the infection process among three Upland cotton cultivars ( Gossypium hirsutum ) using a confocal microscope. Seedlings were grown in a hydroponic system to the two‐true leaf stage and inoculated with a virulent local FOV4 isolate. Based on the infection process, the resistant cultivar FM 2334GLT was divided into resistant (R) and susceptible (S) groups. The susceptible PHY 725 RF and FM 2334GLT(S) had the highest numbers of conidia attached and germinated on the root surface, followed by susceptible Acala 1517‐08. FM 2334GLT(R) had the lowest numbers. Hyphae penetration and colonization inside the root followed a similar trend. In PHY 725 RF, FM 2334GLT(S), and Acala 1517‐08, hyphae started to penetrate into epidermal cells at 2 days post inoculation (dpi), spread in the cortex intercellularly and intracellularly at 3–5 dpi, and entered the xylem vessels at 7 dpi, followed by colonization at 11–15 dpi. FM 2334GLT(R) had the lowest numbers of penetrating hyphae inside the root, with no apparent hyphae observed in the xylem. This study demonstrated that FM 2334GLT(R) had delayed infection, reduced fungal growth and reproduction, and prevented the FOV4 fungus from invading the xylem. The resistance in FM 2334GLT(R) was similar to that in Pima PHY 841 RF but with fewer hyphae in the cortex, suggesting possible different resistant mechanisms.