Observations on the epidemiology of Sooty Blotch in Organic orchards in the Netherlands

Summary

From 2002 to 2005 field observations where made on the occurrence of Sooty Blotch (SB) in unsprayed orchards on the apple variety Topaz. First SB symptoms in high inoculum orchards where found after on average 639 leaf wetness hours following the first rain 10 days after petal fall. Even then symptoms where present only on places on the fruits that probably stayed wet longer than the leaf wetness sensors have indicated. Weather stations that are set up to register average orchard conditions are unlikely to detect the wetness conditions that lead to SB infections in critical parts of the orchard.
From the delayed symptom development compared to the published data form USA we concluded that un-der Dutch conditions the lower temperature retards the symptom development.
In the first month after the first symptoms where found disease progress was slow, but increased towards the harvest. The gradual increase of both disease incidence and severity, and the appearance of the symp-toms suggest ongoing new infections on the fruits during summer rather than one major SB infection that spreads on the fruit.
Within a year the disease incidence between orchards varied form 0 to over 90%, but between years the disease incidence in an individual orchard stayed in the same range. This makes disease history an impor-tant factor in the panning of a control strategy.