Can biological products control <i>Neonectria ditissima</i> picking wound and leaf scar infections in apples?

Abstract

Neonectria ditissima is the causal agent of European canker in apple, and conidia infect wounds in apple trees. Several commercially available chemical fungicides and biological products (BPs) were tested for wound protection activity in three field experiments (2014—17). Freshly made wounds (rasp, picking, leaf and/or bud scar) were sprayed with the products at label rates. Wounds were challenge-inoculated with N. ditissima conidia after 24 hours and monitored monthly for lesion development. Only captan and copper oxide consistently protected wounds from infection and disease development in each experiment. The BPs tested in this study did not provide adequate wound protection when challenge-inoculated with N. ditissima at high concentration (&gt;1000 spores/wound) so are not recommended for wound protection during harvest and leaf fall when disease pressure is high.

How much captan is required for wound protection of <i>Neonectria ditissima </i>conidial infection in apple?

“…The leaf scars inoculated here were 1-day-old. In contrast to the low disease expression of leaf scars, more than 60% of picking and rasp wounds developed symptoms, which was consistent with earlier work Orchard et al 2017;Walter et al 2017b). Control of infection and disease development of these larger wounds was difficult to obtain.…”

“…The number of leaf scars showing symptoms was low, as was the case in 2015 (Dryden et al 2016;Walter et al 2017b). Only 7.9% of inoculated shoots expressed disease, with an average of 1.4 leaf scars/shoot developing symptoms.…”

“…Parallel grower experiments on three sites (data not shown) indicated that this is generally achieved using standard orchard sprayers with water rates of 1000 L/ha. The low expression of leaf-scar infections is disappointing, but this has occurred in other artificially inoculated and naturally infected leaf-scar experiments in the 2015 autumn-winter period (Dryden et al 2016;Walter et al 2017b). Normally, under New Zealand (Tasman) conditions, we would expect 10-20% of leaf scars to develop symptoms within 6 months of artificial inoculation Walter et al 2015;2017b) but in the 2015 autumn-spring period only 4.4 % of leaf scars expressed in the control treatment.…”

“…Dose-response assays were conducted in the field by creating and inoculating fresh leaf scars, and picking and rasp wounds (Dryden et al 2016;Walter et al 2017b). The leaf-scar experiment was done in 2015 on 2-year-old Royal Gala' trees, and the picking and rasp wound experiment in 2018 on 5-year-old 'Scilate' trees.…”

“…New leaf scars were created by pulling leaves from the tree during leaf fall (24 May 2015). These were marked and brush-inoculated (20-30 µL spore suspension per wound) the following morning (25 May 2015) using freshly prepared N. ditissima inoculum (1 × 10 5 conidia/ mL), as described by Walter et al (2015Walter et al ( , 2017b. Upon drying, spray applications were made using a Stihl back pack mist blower.…”