Dissemination of Conidia of Peronospora Tabacina Adam

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Abstract
Conidia of P. tabacina in the atmosphere of tobacco plots and glass-houses were trapped by means of a Hirst spore trap. Greatest numbers of conidia were trapped at approximately the same time each day. In the field the number commenced to increase between 0545 and 0715 hours, reached a maximum between 1000 and 1100 hours, decreased to a few between 1430 and 1630 hours, and remained low until the next morning. In the glasshouse, maximum and mimimun numbers usually occurred later in the day. The number of conidia trapped during periods of increasing leaf movement was less than during the regular daily dispersal period. The daily cycle of dissemination is considered to be due to collapse of the conidiophore as a consequence of withdrawal of water into the mycelium in the leaf. Insofar as atmospheric temperature, humidity, and light are factors effecting changes in leaf turgidity, they are also factors in spore dispersal. The daily cycle of conidial discharge with rising temperature, increasing isolation, and decreasing humidity is considered to provide for distribution of conidia to nearby plants. A more widespread distribution of conidia would be obtained with leaf movement in a turbulent atmosphere at any time of day, but especially if such conditions occur during the normal daily cycle. Conidia trapped during each daily dispersal period were viable even though humidities at the time of trapping were as low as 35%.

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