Heterogeneity of the aerial concentration and deposition of ascospores of Venturia inaequalis within a tree canopy during the rain

Abstract

Scab is an important disease of apple and its control depends almost exclusively on frequent use of fungicides. Primary scab infection in the spring assumes several steps: ascospore maturation, liberation of ascospores that become airborne, deposition on susceptible tissues, and infection. However, the spatial heterogeneity of ascospores within the tree canopy is unknown. Aerial concentration of ascospore (ACA), ascospore concentration in rain water (ACR) and ascospore deposition (AD) were therefore measured at six heights (20-257 cm from the ground) with rotating-arm air samplers, funnels, and greased glass slides, respectively, during five rain events in 2001 and in 2002. In addition, ACR and AD were measured at eight locations within tree canopy at 196 cm height. Apple scab was assessed at the end of the primary infection period in each sampling location within the apple tree. A similar experimental design was used in 2003 to study the spatial heterogeneity of both AD and primary scab lesions. ACA and AD decreased with increasing height, while ACR increased with increasing height. Based on both variance to mean ratio and the power law relationship in both years, the ACR was heterogeneous, while AD was heterogeneous only during the peaks of ascospore release. The ACR was significantly higher at the centre of the trees and the AD was significantly higher at the centre and at the western edge of the trees. Only the cumulative AD was significantly correlated with apple scab lesions at the same location (r = 0.83). In 2003, a similar pattern of spatial heterogeneity within the tree canopy was observed for AD and primary scab lesion counts and there was a linear relationship (R 2 = 0.84) between these two variables. It was concluded that ACR and AD within the tree canopy are not randomly distributed at least during peaks of ascospore release and that AD is a good estimate of primary scab lesion development. This spatial heterogeneity should be considered when estimating ascospore deposition using mathematical models or when quantifying ascosporic inoculum using spore samplers.

Spatial heterogeneity, incidence-incidence and incidence-lesion density relationship of apple scab (Venturia inaequalis) in managed orchards

“…1) and relationship between the logarithm of the observed variance s 2 f and logarithm of mean lesion density y f (b) (Eq. 2) for apple scab assessments on leaves sampled at different periods in Quebec, Canada from 1998 to 2007 dose (PAD), was aggregated within an orchard, by Carisse et al (2006) who reported that ascospore deposition within an apple tree was not randomly distributed following peaks of ascospore ejection, as well as by Carisse et al (2009) who found that betabinomial parameters p and θ could be calculated in 65% of data sets on number of scabbed leaves per shoot, while the binomial distribution provided the best fit for only 22% of the data sets. In the present study, a covariance analysis indicated that sampling season had a significant effect on the intercept parameter at the leaf scale meaning that the relationship between incidence and aggregation changed significantly over the course of the season.…”

“…In a recent study, Charest et al (2002) showed that the primary inoculum is not uniformly distributed within an orchard. Furthermore, the same authors showed that airborne ascospore concentrations are aggregated within an orchard, and Carisse et al (2006) showed that ascospore deposition within an apple tree was aggregated during the peak of ascospore release. From these results, one may assume that apple scab incidence and lesion density are spatially aggregated in orchards; however, the robustness of this assumption must be evaluated.…”

Disease Decision Support Systems: Their Impact on Disease Management and Durability of Fungicide Effectiveness

“…Ascospore maturity could be assessed by visual observations of pseudothecia and release by spore trapping. Work by Carisse et al (2006) and Charest et al (2002) on spatial distribution of airborne ascospores suggested that several traps would be required because of spatial heterogeneity in inoculum density and that the detection threshold may be lower than the density of airborne ascospores in a well-managed orchard. In addition, information of airborne ascospore concentration has no predictive value because by the time the information become available, infection already took place.…”

“…Finally, the last ascospores mature until exhaustion of the stock in pseudothecia. In practice, there is so much variation in the proportion of mature ascospores between leaves and among cultivars (Fig.5.) (Carisse et al, 2006) that degree days were used to determine the period of availability of ascospores from about 50 to 350 CDD. At 350 the DDSS assume that over 95% the ascospore supply is depleted if sufficient rain events have occurred).…”

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).…”