Modulating Apple Vegetative Growth and the Systemic Movement of Erwinia amylovora with Cultural and Chemical Management Practices

Annie Vogel, PhD Defense Seminar
Under the direction of Dr. Tom Kon and Dr. Gina Fernandez, Co-Chairs
Department of Horticultural Science, NCSU

Tuesday, October 22, 2024, 9:00 am
Mountain Horticultural Crops Research & Extension Center / Hybrid

Zoom link: https://ncsu.zoom.us/j/99763041682?pwd=ZaiODZz09FEsPnymhWeoAdqa3yjky9.1
Meeting ID: 997 6304 1682
Passcode: 845993

 Vogel.Abstr.pdf

Abstract:

Cultural, chemical, and environmental factors impact vegetative vigor, orchard establishment, and systemic pathogen movement in young, high-density apple (Malus x domestica L. Borkh) orchards. Rapid orchard establishment due to unbridled growth leads to earlier productivity and profits, but potentially increases disease susceptibility. An economically critical bacterial disease, fire blight, caused by Erwinia amylovora, results in bloom and shoot dieback and ultimately tree death if the bacteria reaches the central leader. Through the use of cultural and chemical management practices, we attempted to modulate vegetative vigor and investigate the relationship between vigor and fireblight infection. Three studies were initiated in a newly planted ‘Gala’ orchard to investigate the horticultural and pathological impacts of prohexadione calcium (P-Ca) and acibenzolar-S-methyl (ASM) rates and interaction, rootstocks, and nitrogen rates in a fertility program. Tree size, relative growth rate (RGR), linear bearing surface (LBS), and yield were measured to quantify differences in vegetative vigor and tree productivity. Disease incidence and severity and bacterial population density and movement over time were recorded to assess the susceptibility of trees and ability to slow or resist infection. Results of these studies will dictate decisions in young orchard systems to maximize orchard establishment while protecting trees from fire blight infection.

Vegetative growth and tree height were reduced by P-Ca in a negative curvilinear fashion. However, linear bearing surface and canopy infill were maintained regardless of P-Ca or ASM rate. As P-Ca rate increased there was an increase in fruit set, but the addition of ASM lessened the effect, resulting in better crop load management. There was a negative curvilinear relationship between P-Ca rate and disease incidence and severity on multiple dates. In 2022, ASM reduced disease incidence and severity, but was not as impactful in 2023. Bacterial density in the scion was decreased by P-Ca. These results indicate that repeated applications of P-Ca in young orchards will limit growth, but not orchard establishment and will help improve disease management. Fruit set may be increased by P-Ca, but the addition of ASM could help manage crop load and, in some years, improve disease management.

Five dwarfing rootstocks of varying vigor and level of fire blight resistance were evaluated for orchard establishment and scion-susceptibility to fire blight. Tree size, RGR, LBS, and yield were significantly impacted by rootstock selection. Trunk-cross sectional area was a good predictor of shoot lengths, LBS, and pruning weights. Rootstocks had limited effects on disease incidence and severity and no impact on bacterial systemic movement. Of the rootstocks tested, Geneva® 935 and 41 had rapid canopy infill and could therefore be selected for efficient orchard establishment.

Four rates of nitrogen fertilizer were tested to evaluate the relationship between nitrogen rate and tree growth and fire blight susceptibility and movement. Nitrogen was applied at 0, 22.4, 44.8, or 89.7 kg N∙ha-1. Chlorophyll content increased as the rate of nitrogen increased. There was no relationship between nitrogen rate and tree size, RGR, LBS, disease incidence and severity, or bacterial population density.