Effects of Shredding or Treating Apple Leaf Litter with Urea on Ascospore Dose ofVenturia inaequalisand Disease Buildup

Abstract

Ascospores produced on diseased leaves in the leaf litter constitute the primary inoculum causing scab in commercial apple orchards in the northeastern United States. Two sanitation practices, shredding the leaf litter with a flail mower and urea application, were evaluated for their potential to reduce the risk of primary scab. Three measures of a treatment's potential were made: leaf litter density was evaluated to determine reduction of the source or primary inoculum; trapped ascospores were counted to measure the reduction of primary inoculum; and scab lesions on trees and seedlings were counted to determine reduction in disease incidence and severity on leaves and fruit. The results show that in the northeastern United States, shredding the leaf litter in November or April will reduce the risk of scab by 80 to 90% if all of the leaf litter is shredded. If 10 to 35% of the leaf litter cannot be shredded because of the limited offset of the flail mower and spread of the tree canopy, then the risk of scab is reduced by 50 to 65%. Urea applied to the leaf litter in November (when approximately 95% of the leaves had fallen) reduced the number of ascospores trapped by 50%. Urea applied to the leaf litter in April (before bud break) reduced the number of ascospores trapped by 66%. The reductions are discussed in relation to a sanitation action threshold that links sanitation to a strategy that reduces the fungicide dose to control primary scab in an orchard assessed with a moderate amount of foliar scab the previous autumn.

Using plant extracts to reduce asexual reproduction of apple scab (Venturia inaequalis)

“…Populin had a similar effect as synthetic fungicides on both Golden and Jonathan in the second year (under wet climate conditions). The repeated application of synthetic fungicides throughout the growing season has been the only approach to managing apple scab (Sutton et al, 2000). According to Rosenberger and Cox (2010), several factors may enable ascosporous infection to become established in early spring and then progress during summer.…”

“…As our first populin applications were carried out in the early vegetation period, the populin treatment probably also had noxious effects on overwintering ascospores. Ascospores produced by sexual reproduction in the leaf litter constitute the primary inoculum causing scab in commercial apple orchards (Sutton et al, 2000). Populin application to the trees in autumn before leaf fall would probably further reduce the following year's scab infestation; however, further experiments in this direction need to be performed.…”

Non- Chemical Management of Apple Scab- A Global Perspective

“…Several studies have shown the effects of sanitary practices such as burning or burying leaves in the soil (Gomez et al, 2004), leaf shredding (Vincent et al, 2004;Holbet al, 2006) and a combination of shredding and using urea (Sutton et al, 2000) on reducing scab inoculum.These studies showed an ascosporic inoculum reduction of between 40 and 95% and a correlated scab reduction of 45 to 85%.Collecting leaves from the ground in the inter-rows in autumn along with burying the leaves left along the row has a positive effect in reducing primary contamination (Gomez etal., 2004). Gomez et al (2004) showed that for two consecutive years the practice of 'raking and ridging' reduced the severity of scab on the fruit by 68 to 74%, depending on the year.…”

“…The fungicidal properties of bicarbonates have long been known (Clayton et al, 1943) Sutton et al (2000) observed that in general, growers do not use sanitation techniques, although they can result in significant reductions in ascospore load. Reluctance to use sanitation practices is through:  the need for specialized equipment (shredder),  the failure of sanitation to provide complete disease control, and  lack of reliable relationships between sanitation measures and the degree to which fungicides can be reduced the following year.…”

“…In a survey of Ontario orchards, about 50% of the isolates of V. inaequalis were resistant to the eradicant fungicides currently used (Ontario Ministry of Agriculture and Food, 1993). For all these reasons, interest is increasing to develop alternative strategies to manage apple scab based on nonfungicidal methods that include genetic resistance, physical destruction of the pathogen, and biological controls (Carisseet al, 2000; Sutton et al, 2000).…”