Infection Process of Plectosporium tabacinum on False Cleavers (Galium spurium)

A native fungus, Plectosporium tabacinum (van Beyma) M. E. Palm, W. Gams et Nirenberg, has potential as a bioherbicide for the control of both herbicide-resistant and herbicide-susceptible false cleavers. Limited information is available on the infection process of P. tabacinum. P. tabacinum spore distribution pattern, germination, penetration, and colonization on false cleavers leaves were examined using confocal, light, and scanning electron microscopy. The results demonstrated that conidia were distributed over the entire surface of leaves and cotyledons. More than 90% of the conidia germinated on the leaf surface 6-8 h after inoculation. Penetration of the leaf epidermis by conidia started 8-10 h after inoculation. Histological observation showed that no appressoria were formed by P. tabacinum, but its hyphae produced appressed club-like structures that penetrated the cuticle and epidermal layers. No stomata or other natural openings were observed on the upper leaf surface of false cleavers seedlings. Penetration occurs directly on epidermal cells with more frequent intercellular penetrations. Hyphal penetration was visualized at a depth of 30 and 40 üm after 8 and 16 h of incubation, respectively. Secondary hyphae colonized mesophyll cells 16 h after inoculation. Even spore distribution, short spore germination time, club-like infection structure formation, direct penetration, quick colonization, and mucous secretion on false cleavers leaves may contribute to the kill of false cleavers by P. tabacinum. Slow spore germination and germ tube growth, low spore germination numbers, and no infection structure formation on Brassica napus leaves may be factors affecting the host selectivity of P. tabacinum.