Abstract
The study presents the results of nine years of field observations of ascospore release of Venturia inaequalis in the Skierniewice area in central Poland. In total, spores were trapped 221 times during 2005–2008 and 2010–2014. Of these, 142 episodes lasted for less than 8 hours, 69 lasted from 8 to 29 hours, and 10 episodes lasted from 30 to 93 hours. Spore releases started in spring from 25 March to 28 April and ended from 27 May to 17 June, and the season for ascospore release lasted from 43 to 76 days, with an average of 58 days. During 139 ascospore releases, less than 1 000 spores per cubic meter of air were collected and during 25 discharges more than 10 000 spores were trapped. Releases of ascospores were highly related to rainfall and daylight. Two-thirds (67%) of the spores were trapped during rain. Only 12% of the discharges occurred without any registered rain, accounting for 7% of all trapped spores. Nearly three fourth (73%) of all ascospore release hours occurred in daylight, and 91% of the spores were trapped in daytime. Rainy nights with constant leaf wetness were observed, during which no spore releases occurred despite the rainfall. Ascospore releases were also less prominent at the beginning and end of the season and after weak rains. Rain was most effective as the trigger of discharges at temperature between 5 and 13 °C and when global radiation coinciding with rainfall was below 700 W/m2. In conclusion, the study confirms the dominant role of daytime rainfall in the release of ascospores by Venturia inaequalis.
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