Strategic irrigation is a method to prevent Apple scab (Venturia inaequalis). It is performed by irrigating the orchard floor in dry periods during spring, 24 hours before rain forecast. Irrigating the old leaves on the orchard floor will elicit the release of ascospores, but due to the dry trees and the dry weather, the ascospores dry out without causing infections.
The method relies on the occurrence of dry periods during the period of primary infection from April to mid-June.
Experiments were carried out at the University of Copenhagen and in an unsprayed Danish organic orchard in 2014 and 2015. Apple scab on fruit and leaves was reduced by strategic irrigation in ‘Elshof’ at the University field in both years. In the organic orchard the strategic irrigation reduced the infection of leaves by apple scab and Elsinoe leaf and fruit spot in some cultivars and some years.
Studies of the ejection of ascospores took place in 2015 using a glass slide trap. The reaction of ascospores to rain and to the amount of irrigated water was studied.