Infection of Sugarcane Leaves byPuccinia melanocephala

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Abstract
Sequential development of P. melanocephala was studied from germination of urediospores to the establishment of the pathogen within sugarcane leaves. The optimal temperature range for germination of urediospores and formation of appressoria was between 15.degree. and 30.degree. C. Penetration occurred only after germ tubes formed appressoria over stomata. Development of infection structures followed the general pattern observed with other rust fungi. Morphology of the appressoria, substomatal vesicles, infection hyphae, and haustoria of P. melanocephala were defined. Haustoria of P. melanocephala have a central point of attachment and are lobed with fingerlike branches. Plastic leaf replicas and scratched cellophane elicited sequential development of the germ tube and formation of appressorium, penetration peg, substomatal vesicle, infection hypha and haustorial mother cell. Haustoriumlike structures also were seen on a cellophane membrane. These observations support the hypothesis that a contact stimulus triggers development of appressoria and the subsequent infection structures. Guard cells or stomata on leaves provide that contact stimulus in nature. Penetration of P. melanocephala into leaves of the nonhosts, oat and wheat, was similar to penetration of sugarcane leaves. However, the fungus did not develop beyond the infection hypha stage in wheat or the haustorial mother cell stage in oats.

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