Date of Award
Master of Science (MS)
Botany and Plant Pathology
Committee Member 1
Committee Member 2
The management of common apple diseases such as apple scab ( Venturia inaequalis) and bitter rot (Colletotrichum spp.) relies heavily on effective fungicide applications. However, the development of fungicide resistance to newer fungicides has resulted in management failures and significant economic losses. This has led to a greater reliance on captan, an older fungicide, because there is a low risk of pathogens developing resistance. Label restrictions limit growers to 18 kg of captan per season, which may not provide sufficient control of both apple scab and bitter rot in wet years. Consequently, apple growers are faced with two equally difficult scenarios, inadequate management of diseases due to resistant pathogen strains from the use of newer fungicides or insufficient management due to restrictions on captan.
The goal of this research was to identify new approaches to reduce the amount of captan needed throughout the growing season without decreasing disease control. One potential tactic is to incorporate adjuvants into management strategies. Adjuvants are tank additives that increase the coverage and retention of sprays and correct issues with the tank water by affecting the pH. The incorporation of adjuvants into current apple disease management strategies has the potential to improve disease control by increasing the efficacy of captan sprays at reduced rates and reducing initial inoculum by enhancing urea-driven leaf litter decomposition.
To assess the improvement of captan sprays, adjuvants were combined with the lowest rate of captan and applied to apple trees every 10-14 days from bloom to harvest. Disease and phytotoxicity incidence and severity were observed on apple fruit to measure the effectiveness of the treatments. Results showed that Li700 plus captan and Bond Max plus captan consistently reduced disease incidence in high-pressure years by increasing the coverage and retention of captan and lowering the pH of the tank water. Based on the data found in this study, a grower could potentially save up to $3,481-$4,667 ha -1 due to reduction of disease incidence.
In order to examine if adjuvants improved urea-driven decomposition of scab-infected leaves, adjuvants were combined with urea and applied to infected leaves. These leaves were then left to overwinter on the orchard floor. Leaf area decomposition and pseudothecia and ascospore reduction were observed to measure the effectiveness of the treatments. Results showed that Li700 plus urea and Wet Betty plus urea improved urea-driven leaf decomposition and pseudothecia and ascospore reduction by increasing the nitrogen content in the leaves. Based on this study, the addition of these adjuvants to urea could delay an apple scab epidemic, saving fungicide applications and postponing initial infection past the point when apples are most susceptible to V. inaequalis.
The addition of adjuvants to captan or urea has the potential to improve disease management by reducing fungicide rates and reducing overwintering inoculum. Together these factors may reduce the number and dose of fungicide sprays required for apple scab and bitter rot management throughout the growing season and ultimately increase a grower’s net return in apple production.
Abbott, Chelsi Patricia, "The Effect of Adjuvants on Apple Disease Management" (2016). Open Access Theses. 739.