Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/25/2002
Publication Date: 2/1/2003
Citation: LAPAIRE, C.L., DUNKLE, L.D. MICROCYCLE CONIDIATION IN CERCOSPORA ZEAE-MAYDIS, THE CAUSAL AGENT OF GRAY LEAF SPOT OF MAIZE. PHYTOPATHOLOGY. 2003. v. 93. p. 193-199. Interpretive Summary: The asexual spores of the fungus Cercospora zeae-maydis are the primary structures causing the gray leaf spot disease of corn, but spore production from diseased leaf tissue normally requires 2 to 3 weeks after infection. We have discovered that the spores are capable of more than doubling their numbers by producing viable secondary spores within 2 to 3 days in the absence of nutrients and even while anchored to leaf hairs above the leaf surface. This developmental process is termed microcycle conidiation because the formation of spores (conidia) does not require a significant amount of vegetative growth. Microcycle conidiation requires RNA and protein synthesis and occurs only on an air-substrate interface. It is inhibited by ammonium salts but not by nitrate salts, amino acids, or simple sugars. Understanding the processes involved in spore formation and the conditions that influence it will contribute important information about disease epidemiology and may lead to improved strategies to manage gray leaf spot.
Technical Abstract: Conidia of the fungal pathogen causing gray leaf spot of maize, Cercospora zeae-maydis, can directly produce morphologically mature, viable secondary conidia without an intervening vegetative state. Within 48 hours after germinating on an air-water or air-substratum interface, conidia produce 1 to 3 new conidia on conidiophores that develop from germ tubes or directly from internal cells of the original conidium. This microcycle conidiation process is independent of exogenous nutrients, is not affected by nitrate salts, amino acids, or simple sugars, but is suppressed by ammonium salts. Based on the effects of inhibitors of macromolecular biosynthesis and radiolabeled precursor incorporation, microcycle conidiation requires RNA synthesis during the first 6 hours after activation of germination and protein synthesis throughout the developmental process. On leaves in a humid atmosphere, conidia adhering to trichomes germinate and produce new conidia, whereas conidia in contact with the leaf surface germinate and the hyphae grow but fail to produce conidia. During conidiogenesis, a nucleus from the tip of the conidiophore migrates into the incipient conidium and undergoes a series of nuclear divisions accompanied by formation of septa resulting in production of multicellular conidia with primarily uninucleate cells.