by Courtney Llewellyn

In the sphere of apple orchards, the use of exclusion netting may result in more than one positive outcome. The netting not only keeps pests away, it can also reduce thinning needs as well as fruit set.

This topic was researched by Mokhles Elsysy and Todd Einhorn of the Department of Horticulture at Michigan State University, spurred by the fact that use of netting in tree fruit production systems is increasing and that thinning is an important but challenging practice in apple production, especially for organic producers.

Their research noted that exclusion netting can also prevent fruit damage from climatic events such as hail, solar radiation and wind. Netting can reduce or eliminate insecticide applications and improve resource efficiency by way of reduced evapotranspiration as well. Elsysy and Einhorn’s multi-year project aimed to determine if exclusion netting could additionally reduce fruit set and thinning requirements of apple trees in Michigan.

They enclosed entire apple trees in Alt’Carpo netting (with 10% shading, 2.8-by-4 mm weave and Helios® anti-hail systems, produced in Bergamo, Italy, and very effective against excluding codling moth) at specific percentages of open king blossom – 20%, 50% and 80% open. (The center and largest blossom in the 5 blossom cluster is the king blossom.) They did this to reduce pollination, fruit set and thinning. They then evaluated the effect of the nets on productivity, fruit size, shape and quality. They enclosed entire apple tree canopies in several experiments with this netting over several years.

What were the results? First, netting treatments significantly reduced fruit set and seed content when applied at different bloom stages. Applying the netting at 20% opening of king blossom seemed to work the best in reducing fruit set in both Honeycrisp and Fuji cultivars. Second, netting resulted in a significant reduction in fruit number at harvest compared to non-netted control trees.

When it came to fruit shape, however, fruit symmetry ended up being affected by seed number. A greater proportion of fruit of lower seed content were asymmetrical compared to fruit of high seed content. (Higher seed number generally compensates for lower fruit set, and in this case, higher seed number fruits were more symmetrical and therefore more visually appealing.)

Less clear was the connection between seed number and weight. While fruit weight was statistically greater for fruit of higher seed content (more than nine seeds in Fuji and more than six seeds in Honeycrisp), it wasn’t the case often enough for the results to be definitive.

“These findings implore a re-examination of the influence of fertilization and seed content on apple fruit size, shape and quality,” the study stated, concluding, “Exclusion netting represents a potential alternative to chemical or mechanical thinning for managing crop load, particularly in organically managed systems.”