including (Monellia spp. and Monelliopsis spp.), which suck the juices from leaves and often deposit a sticky substance called "honey-dew" on the leaf surface that may turn black and prevent photosynthesis; and the walnut lace bug (Corythucha juglandis), which causes damage when the adults and nymphs suck the sap from the lower surfaces of walnut leaflets. A disease complex known as thousand cankers disease has been threatening black walnut in several western states. This disease has recently been discovered in Tennessee, and could potentially have devastating effects on the species in the eastern United States. Vectored by the walnut twig beetle (Pityophthorus juglandis), a fungus, Geosmithia morbida, spreads into the wood around the galleries carved by the small beetles. The fungus causes cankers that inhibit the movement of nutrients in black walnut, leading to crown and branch dieback, and ultimately death. Allelopathy While black walnut is considered allelopathic, meaning it excretes chemicals into its environment that harm competition, research over the past decade has questioned whether this long-held belief holds up to scientifically rigorous examination. Many publications that have repeated claims of black walnut allelopathy cite a very limited set of dated research literature, which has not held up to scientific scrutiny. Anecdotally, records of walnut toxicity to other plants have been observed as far back as the first century when Pliny the Elder wrote: "The shadow of walnut trees is poison to all plants within its compass." Like other walnuts, the roots, inner bark, nut husks, and leaves contain a nontoxic chemical called hydrojuglone, when exposed to air or soil compounds it is oxidized into juglone that is biologically active and acts as a respiratory inhibitor to some plants. Juglone is poorly soluble in water and does not move far in the soil and will stay most concentrated in the soil directly beneath the tree. Even after a tree is removed the soil where the roots once were will still contain juglone for several years after the tree is removed as more juglone will be released as the roots decay. Well drained and aerated soils will host a healthy community of soil microbes and these microbes will help to break down the juglone. Symptoms of juglone poisoning include foliar yellowing and wilting. A number of plants are particularly sensitive. Apples, tomatoes, pines, and birch are poisoned by juglone, and as a precaution, should not be planted in proximity to