Sporulation of Plasmopara viticola: Differentiation and Light Regulation

The development of grape downy mildew (Plasmopara viticola) was followed histologically during the entire latent period until the appearance of mature sporangia. Production of sporangiophores and sporangia was assessed using low‐temperature scanning electron (LTSEM) and fluorescent light microscopy. Time‐course studies using attached leaves of Vitis vinifera cv. Müller‐Thurgau revealed that the production of sporangiophores and sporangia is a highly coordinated process and is completed within 7 h. As this differentiation is assumed to occur only in darkness, the influence of light was investigated. For this purpose, different light regimes were applied to infected leaf discs of V. vinifera cv. Müller‐Thurgau. White light irradiation prevented formation of sporangia, although the growth of the mycelium was not affected. Many sporangiophores were observed that were abnormally shaped, i.e., short hyphae in clusters or thin, extremely elongated hyphae. For the formation of mature sporangia, a prolonged dark period was necessary. Light experiments suggest photosensitivity at the end of the latent period. A terminal white light irradiation caused an inhibitory effect, whereas a final phase of darkness promoted sporangium development. Different light qualities were tested, revealing an inhibition of sporangium development by blue light whereas neither red nor far‐red light were effective.