Primary infection by ascospores: Formation and dischare of ascospores to infect plant tissue

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mature ascospore                        release of ascospores 

Vernturia inaequalis hibernate as so called "pseudothecia" (fruit body). The pseudothecia are dicariontic (diploid). When forming ascospores a meiotic cell division reduces the set of chromosomes and they become haploid. After meiosis a mitosis takes place and numerous asci containing 8 ascospores each are formed.  Forming Ascospores (Asco) is assessed by a linear degree day accumulation on the base of 10°C (= 1440 °hours). Maturation of ascospores (Asc Reif) takes place in spring and early summer, in some apple production areas weeks before buds break or in other areas often when buds burst. Comparing different modelling approaches to assess the duration of Ascospore release showed, that ascospore release is delayed by periods of low moisture. This means ascospore maturation is only possible if the top soil or leaf litter is moist enough.  

In fieldclimate.com modelling of ascospore maturation is predicted when the relative humidity is higher than 70%. The speed and the quantity of mature ascospores is assumed to be proportional to the accumulated temperature above 0°C. More than 3 hours above  -3°C are assumed to reduce the matured ascospore to 0.  

Factors, which are influencing the discharge of ascospores (Asc Frei) are leaf wetness and light. If leaves are dry no ascospore discharge takes place. Also light is needed that high numbers of ascospores are being discharged. For example: starting from 2 hours after sunrise to 2 hours after sunset high numbers of ascospore can be released, when leaves are wet. So without sun (night and early morning) the quantity of released ascospores is strongly reduced. 

In fieldclimate.com ascospore discharge is modelled when the humidity and temperature for ascospore maturation is given as well as leaves are frequently wet. Temperature has to be above 10°C. In most apple growing areas this will not take place due to the fact that in moist periods during spring temperatures are limited. 


Quelle:" rel="external nofollow">http://www.depi.vic.gov.au/agriculture-and-food/pests-diseases-and-weeds/plant-diseases/fruit-and-nuts/pome-fruit-diseases/apple-scab


For more information about the fieldclimate.com apple scab model please refer to the following pages explaining the Infection by ascospores and conidia.