Penetration and Infection of Soybean Leaf Tissues byColletotrichum truncatumandGlomerella glycines

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Abstract
Conidial germination, infection peg penetration and establishment of the anthracnose-causing fungi (C. truncatum and G. glycines) on and in soybean (G. max) leaf tissues, was studied by using bright-field microscopy and scanning electron microscopy. Conidial suspensions of both fungi were atomized onto soybean leaves in a mist chamber at ambient temperature (24.degree. .+-. 2.degree. C). Seventy percent of conidia of C. truncatum germinated within 4 h; 25 and 40% of the conidia of G. glycines germinated at 6 and 12 h, respectively. Conidia of both fungi germinated terminally and subterminally with germ tubes that developed appressoria. The germ tubes of both fungi were significantly (P = 0.05) longer when conidia germinated on midribs than on leaf laminae. One and sometimes two appressoria developed from a single germ tube of conidia of C. truncatum. Penetration of epidermal cells by infection pegs from appressoria of both fungi was common. Indirect penetration through stomatal openings or direct penetration of guard cells was rare. Hyphae were observed in an between mesophyll cells at 2 days and in the vascular elements 3 days after inoculation. Acervuli were produced by conidia of C. truncatum and G. glycines at 48 and 72 h, respectively, and were common on leaf veins and petioles. Discrete veinal necrosis of inoculated leaves was evident only with conidia of C. truncatum at 30 h after inoculation of plants.