Relationship between the application dosage of PP333 and the incidence of twig dieback in Chinese bayberry

Abstract

In this study, trees in production and saplings of Chinese main bayberry cultivars ‘Dongkui’ (Myrica rubra Sieb.&Zucc.) and ‘Biqi’ (Myrica rubra) were used to investigate the relationships of the different dosages and methods of 15% paclobutrazol (PP333) application and the incidence of twig dieback. The results showed that higher concentration of PP333 application caused greater PP333 residues in the plant body, and the residue of PP333 was positively correlated with the incidence of twig dieback, which means greater PP333 residues in the plant body leads to higher incidence of twig dieback. Moreover, the content of PP333 residues in the plant body after soil application was more than that after foliar spraying. The detection of pathogens in field plants showed that in addition to the two pathogens Pestalotiopsis mangiferae and P. vismiae, the presence of pathogen P. versicolor was found among different bayberry cultivars. The incidence of twig dieback of ‘Dongkui’ cultivar and ‘Biqi’ cultivar were ranked in the first two of all bayberry varieties, and the incidence of this disease in the ‘Dongkui’ cultivar was higher than ‘Biqi’ cultivar. The results of investigations conducted in 5 key provinces and 22 key counties suggested that low concentration of PP333 used as a plant growth inhibitor with 1 or 2 times intermittently throughout the entire life cycle was feasible to regulate the growth of Chinese bayberry. However, continuous excessive application could lead to extreme discordance between reproductive and vegetative growth and severely inhibit conducting tissues and tree vigor. In addition, the shrinking leaves and dead branches may increase sharply, promoting the infection and transmission of Pestalotiopsis spp.