Treatment of overwintering apple leaves to reduce primary inoculum of apple black spot

Abstract

Field trials were conducted in Hawke's Bay and Lincoln into methods of treating over-wintering apple leaves to reduce ascospore production by the apple black spot pathogen, Venturia inaequalis. The leaf treatments comprised three levels of nutrient amendments (including a water control) and five levels of saprophytic fungal isolates (including a nofungus control), in a factorial design. Leaves were left to over-winter on the orchard floor, and in spring the V. inaequalis ascospores released were trapped on glass slides and counted. Ascospore numbers were reduced (P<0.05) by the leaf amendment urea, which alone caused 73% reduction, but not by the Bio-Start™ product. The effect of fungal isolates was not significant (P=0.12), although when combined with the water treatment, the isolates, Chaetomium, Phoma and Epicoccum spp. and Trametes versicolor reduced numbers of ascospores by 33, 27, 15 and 28%, respectively, compared to the no-fungus control. When combined with urea, the Chaetomium isolate reduced ascospore numbers by 92 and 82% compared to the nil fungus/water control treatments in Hawke's Bay and Lincoln, respectively, indicating that this treatment has potential for reducing primary inoculum of apple black spot.