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Conidial Germination, Infection Structure Formation, and Early Colony Development of Powdery Mildew on Poinsettia

Powdery mildew disease on poinsettias (Euphorbia pulcherrima) growing in commercial greenhouses was first observed in the United States in 1990 and has become an economically significant problem for poinsettia growers in the Midwest and northern United States since 1992. The temporal development of infection structures produced by conidial germ tubes of the pathogen (Oidium sp.) and the effect of high temperature on their development were investigated using poinsettia leaf disks placed in humidity chambers. Observations were made using light microscopy and scanning electron microscopy. At 20°C (85% relative humidity), conidia germinated and formed an appressorium within 6 h of inoculation. Germination over time followed a monomolecular curve (r² = 0.77, P ≤ 0.0001). Within 24 h postinoculation, germinated conidia had formed secondary germ tubes and a haustorium. The percentage of germinated conidia with appressoria and one or more secondary germ tubes increased linearly with time (r² = 0.92, P ≤ 0.0001), while the percentage of germinated conidia with appressoria and haustoria increased mono-molecularly (r² = 0.93, P ≤ 0.0001). Conidia had an ornamented appearance, and all conidiophores had arced basal cells. When the incubation temperature was 30°C, conidial germination and development of secondary germ tubes and a haustorium were reduced.