EFFECTS OF EXCESSIVE ANNUAL APPLICATIONS OF TERBACIL, DIURON, SIMAZINE AND DICHLOBENIL ON VIGOR, YIELD, AND CATION NUTRITION OF YOUNG APPLE TREES

Abstract

A field trial was conducted to determine the effects of repeated applications of residual herbicides on young apple trees on dwarfing rootstock planted in coarse soil and under full growing season irrigation. Bisbee Delicious/M.26 apple trees (Malus domestica Borkh.) planted in 1978 were subjected to various, but continuous soil management treatments from 1980 to 1985. These treatments included full ground cover, glyphosate [N-(phosphonomethyl)glycine] herbicide applied at 1.5 kg a.i. ha−1 yr−1 and recommended (X), twice recommended (2X) and four times recommended (4X) rates of terbacil (3-tert-butyl-5-chloro-6-methyluracil), diuron [3-(3,4-dichlorophenyl)-1,1-dimethyl urea], simazine [2-chloro-4, 6-bis(ethylamino)-s-triazine] and dichlobenil (2,6-dichlorobenzonitrile). Herbicide application at X was 1.5 kg a.i. ha−1 yr−1 for terbacil, 2.5 kg a.i. ha−1 yr−1 for diuron and simazine and 4.0 kg a.i. ha−1 yr−1 for dichlobenil. Applications of terbacil, diuron and simazine above X resulted in linear decreases in tree vigor and yield. Cumulative initial yield, 1982–85, was reduced at 4X by 46.8% for simazine and 60.8% for terbacil compared to their respective yields at X, while 4X diuron began causing tree mortality after the second application. High rates of dichlobenil did not significantly affect tree vigor or yield. Although simazine applications directly increased leaf Mg and Zn concentrations in 2 of 3 yr, the results had no practical significance, due to the toxicity of simazine. Other herbicides did not consistently influence tree cation nutrition.Key words: Apple, yield, vigor, herbicide toxicity, leaf nutrition