Subcuticular–Intracellular Hemibiotrophy of Colletotrichum lupini in Lupinus mutabilis

Anthracnose caused by Colletotrichum lupini is the most important disease affecting lupin cultivation worldwide. Lupinus mutabilis has been widely studied due to its high protein and oil content. However, it has proved to be sensitive to anthracnose, which limits the expansion of its cultivation. In this work, we seek to unveil the strategy that is used by C. lupini to infect and colonize L. mutabilis tissues using light and transmission electron microscopy (TEM). On petioles, pathogen penetration occurred from melanized appressoria, subcuticular intramural hyphae were seen 2 days after inoculation (dai), and the adjacent host cells remained intact. The switch to necrotrophy was observed 3 dai. At this time, the hyphae extended their colonization to the epidermal, cortex, and vascular cells. Wall degradation was more evident in the epidermal cells. TEM observations also revealed a loss of plasma membrane integrity and different levels of cytoplasm disorganization in the infected epidermal cells and in those of the first layers of the cortex. The disintegration of organelles occurred and was particularly visible in the chloroplasts. The necrotrophic phase culminated with the development of acervuli 6 dai. C. lupini used the same infection strategy on stems, but there was a delay in the penetration of host tissues and the appearance of the first symptoms.