## 1 Introduction  
Harmful algal blooms (HABs) are a primary concern in water management, particularly regarding recreational and drinking water resources [1; 2; 3; 4; 5]. Although many algae are capable of blooming under suitable conditions, bloom events are generally specific to locales in which eutrophication and meteorological factors (e.g., temperature) are major driving forces [6; 7]. Additionally, algal growth and development in natural fresh waters is subject to seasonal succession, depending on growth kinetics, competition, and planktonic herbivory [8]. Cyanobacteria, such as _Microystis_, _Anabena_, and _Aphanizomenon_, often proliferate during warm summers, whereas some diatoms, such as _Stephananodiscus_, are dominant during cold seasons in temperate regions [9; 10]. Abnormal outbreaks of these algae often cause serious problems by producing harmful materials, such as toxins and off-flavors, and hindering drinking water treatment processes. Thus, effective control of these bloom-forming nuisance algae is of prime importance for water use and ecosystem management.  
A variety of techniques have been developed and applied to control HABs, mostly involving physical (e.g., ultrasonic radiation, dissolved gas flotation, and nanofiltration) and chemical (e.g., copper sulphate and silver nanoparticles) methods [11; 12; 13; 14]. These techniques are effective under certain circumstances but have shortcomings in terms of cost and ecological security [15; 16]. Namely, chemical adjicides (e.g., copper, chlorine, and aluminum) have been widely used in the control of harmful algal blooms. However, most chemical algicides are nonselective, and cause general toxicity. Development of efficient, sustainable methods of controlling HABs still is challenging.  
The design of environmentally safe, selective HAB inhibitors has been a focal point of study [17; 18; 19; 20], and methods using natural compounds are of particular interest. Allelichemical control, a potential eco-friendly method of suppressing harmful algae, employs biological materials of plant origin [18; 19; 20], Barley straw and the allelichemicals extracted from rice straw and hulls, for example, have been applied to control harmful algae over the years since the 1980s. Also, diverse higher plants (e.g., _Myriophyllum spicatum_ and _Polygonatum odoratum_ var. _pluriform_) have been studied for allelopathic inhibition on algal growth. Various allelichemicals have been demonstrated to inhibit the growth of toxic cyanobacteria, particularly including that of _Microystis aeruginosa_[18; 21; 22], whereas their effects on other algal taxa, such as diatoms and green algae, are less well studied.  
Pure juglone and crude walnut hull extracts have been found to inhibit the growth of a broad range of microorganisms, including bacteria, algae, and fungi [23], and prior studies have shown that juglone has allelopathic potential against some algae [24; 25; 26; 27; 28; 29]. However, studies on juglone have not examined its specific effects on a broad spectrum of algal species, including diatoms and green algae. This study assesses the allelopathic effects of juglone on diverse species belonging to three algal phyla (Cyanophyceae, Chlorophyceae, and Bacillariophyceae). Particularly, we address the selective allelopathic potential of juglone on some harmful bloom-forming algae via both laboratory and field studies.