Histopathology of Susceptible and Resistant Soybean Roots Inoculated with Zoospores ofPhytophthora megaspermaf. sp.glycinea

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Abstract
Histopathological comparisons of the sequence and timing of pre- and postpenetration events of pathogenesis of P. megasperma f. sp. glycinea zoospores on resistant (''Harosoy 63'' [H-63]) and susceptible (''Harosoy'' [H]) soybean roots revealed that resistance is effective during the colonization stage in the resistant cultivar. Roots were inoculated with zoospores and samples were excised at intervals to 72 h after inoculation and prepared for light microscopy. Scanning electron microscopy was used to examine prepenetration events. Lateral roots of other plants were inoculated with zoospores and were examined macroscopically at 24-h intervals for 8 days. Hypocotyl and tap root sections of these plants were examined for oogonia, oospores or mycelium. The timing and sequence of pre- and early postpenetration phenomena were similar on H and H-63 roots. Zoospore accumulation, encystment, germination, and penetration from simple germ tubes between anticlinal walls of epidermal cells occurred within 1.5 h after inoculation. Hyphae grew inter- and intracellularly within cortical tissue and the stele. Significant differences in ramification hyphae lengths in resistant vs. susceptible cultivars were apparent in roots at 48 h. Ramification hyphal lengths in H roots were twice the lengths in H-63 roots at 72 h. Ramification hyphal lengths in H roots were twice the lengths in H-63 roots at 72 h. Fewer oogonia and oospores per root section were observed in H-63 roots than in the more susceptible cultivar. Hyphal ramification from lateral roots to tap roots and hypocotyls of H, but not H-63, plants occurred within 8 days after zoospore inoculation.