Model for First Possible Infection in Fall
The development of a phoma stem canker epidemic is seperatet into three stages.1) In the first stage, the date when phoma leaf spot epidemics start in autumn was predicted from the summer weather data. Since phoma stem canker is a monocyclic disease (one cycle per growing season), the date in autumn when leaf spotting starts to develop is a crucial factor affecting the severity of phoma stem canker epidemics on stems the following summer (West et al. 2001). The date when phoma leaf spotting starts in autumn is estimated from the temperature and rainfall during the intercrop period between the harvest of the previous crop and the establishment of the new crop. Where approximately 4 mm of rain will make the occurence of Black Leg Disease one day earlier and the impact of temperature is higher at the begin of the period in mid summer than in autum.
If the ascospore infection is possible do to the autumn climate, we have to look for the climate needs of ascospore infeciton.2) In this second stage we can look for mature ascospores. To mature the ascospores it needs depending on temperature more than 288 hours of air temeprature in between 5 and 25°C and relative humidity higher than 85%. Now it needs 4 mm or rain or more to distribute the ascospores. A leaf wetness period has to start an ascopore infection and if can be completed within 8 hours under optimum temperature.
In later autum and in spring conidia can be formed on mature lesions of Black Leg Disease.
3) In this third stage we have to expect conidia infections, which are started by a leaf wetness period and which are comleted by periods of relative humidity higher than 85% for longer than 8 hours at optimum temperature.