The epidemiology of apple scab (Venturia inaequalis (Cke.) Wint.)

Abstract

SUM MARYThe prevalence of Venturia inaequalis ascospores in orchards was compared both in terms of the number of spores per volume of air (dose) and the number produced per area of dead leaf (productivity). The two parameters often gave divergent estimates, probably because dose depends on total leaf per unit area of ground whereas productivity does not. Differences in the amount of dead leaf surviving until bud-burst in different orchards or years were enough to explain the anomalies and suggested that because earthworms often removed over 90% of the fallen leaves by spring, they exercised an important control of ascospore number.Assessments of fruit surface scabbed, an accepted way of judging scab control, were not related to ascospore productivity the following spring, but estimates on the percentage of all leaves infected at leaf-fall were. Other natural variables, like the dates when leaves were formed or fell, seemed of minor importance, especially when compared with effects of earthworms or chemicals. Applying dinitro-ortho-cresol to dead leaves in spring decreased the number of ascospores liberated by at least 9076 and there is evidence of a similar effect from broadcast ammonium sulphate. The spring and summer spray programmes were also important. Measurements in several orchards during an %year period showed that dose and productivity both declined to less than a hundredth of the highest levels, but in a single wet year productivity in sprayed orchards increased several times and by 40-fold on unsprayed trees. Ways to prevent high ascospore doses occurring are discussed, together with possible causes of past severe attacks of apple scab at Wisbech.

The ascospore production season ofVenturia inaequalis(Cke.) Wint., the apple black spot fungus

“…In their work with V. inaequalis, Hirst and Stedman (1962) found that when leaves were wetted at temperatures above 6°e the majority of mature ascospores present were liberated within three hours of wetting. Ascospore catches in the present investigation are therefore believed to represent closely the total numbers of mature ascospores ready for release at the time of the test.…”

“…and a vacuum pump. The wind tunnel was of almost the same dimensions and pattern as that described by Hirst and Stedman (1962). The impactor for trapping spores released in the tunnel was described by Brook (1963), the spores being caught as a single deposit on a microscope slide faced with petroleum ielly.…”

Mycosphaerella ribisleaf spot on black currants in New Zealand: Perithecial maturation, ascospore release, and symptom development

“…Rain is known to be an important factor controlling ascospore release in ascomycete fungi, e.g. Venturia inaequalis (Hirst & Stedman 1962), and is probably important for M. ribis. Variations in the distribution of spore catches in different years, within the broad seasonal pattern, may have resulted from the distribution of rainfall.…”

A method to monitor airborne Venturia inaequalis ascospores using volumetric spore traps and quantitative PCR

“…Airborne inoculum was assessed in association with apple scab incidence in a study by Carisse et al (2007), but was not found to be correlated with incidence when monitoring was conducted only at the end of the primary infection period (green tip to calyx stage). The two main construction types of traps available are the vacuum or volumetric spore trap (VST) and the rotating-arm impaction spore sampler (Rotorod; Hirst and Stedman 1961;Zuck and Caruso, 1984;Aylor 1993). The vacuum method has the advantage of collecting spores continuously through a rotating disc or on a roll of tape, therefore allowing the user to compare spores collected with weather data of the same time period (Rossi et al 2001).…”