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 PRIMARY INFECTION BY ASCOSPORES

 

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Germinating AscosporeAscospore Infection graph of fieldclimate.com

Germination and Penetration of ascospores of Venturia inaequalis on leaves or fruits depend on leaf wetness periodes (wet leaves or rel. humidity >80% (holds the infection level till under 80% r.h.). and air temperature (above 2°C).  Higher temperatures lead under wet contitions to faster penetration, while at lower temperatures longer leaf wetness periods are needed.  This relationship was first published by MILLS and LAPLANTE (1945). Our calculation is carried out based on the publications of SCHWABE (1980). SCHWABE described the severity of scab infections depending on the temperature. These observations are in congruence with our own observations.   

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The Ascospore Infection Model calculates the progress of infection, depending on  leaf wetness and temperature, for weak, moderate and severe infections. To split the prediction into three "severity classes" is very traditional for apple scab models. The progress of infection is displayed as a graphic for hourly values. An infection has already taken place as soon as the progress curve for a weak infection reaches 100%. A moderate or severe infection is complete if the progress curves for this severity classes are reaching (100%). Often in practice or in reality an infection reaches 70% or 80% early in the morning and in dependence of the actual weather you will be able to know if this infection will finish or not. Displaying the daily maximum values of the infection process curve gives an overview over the expected infection dates in the season.   

For the market it is absolutely important to have disease free apple fruits to get a good price. Therefore growers tend to have no scab tolerance at all in their orchards. The model should be implemented into a curative application program (application of sprays containing compounds of cyprodinil or pyrimethanil at prebloom) when the  model pointed out an infection (no matter if weak or severe).  In the bloom and early post bloom periode, severe infections are most frequent and all humid periods have been covered by protective sprays. Further on plant tissue grows very fast, therefore a protection may only work for 4 days. In this case a curative spray after an incomplete infection period might be also needed. But in this case it is relevant if it has been a weak or severe infection and also if there was a low or high number of ascspores ready for infection.  

For more Information about the fieldclimate.com apple scab model please look at the Page for Biology or Ascospore Formation and Conidia Infection To learn how to use plant disease models for apple scab control read the next pages for pracical use.

Literature:

Schwabe WFS, 1980. Wetting and temperature requirements for apple leaf infection by Venturia inaequalis in South Africa. Phytophylactica, 69-0.

Schwabe WFS, 1980. Weather favouring apple scab infection in South Africa. Phytophylactica 12, 213-217.

Mac Hardy W.E., 1996. Apple scab biology, epidemiology and management. St. Paul, MN: Am. Phytopathol. Soc.: 545.