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Problem
(factsheet)
Plant NameTypeDescription 
Anthracnose (Blotch)WalnutDiseaseAnthracnose or blotch of walnut is caused by a fungus. The disease affects leaves, young twigs, and nuts. Leaves develop irregularly circular reddish-brown to grayish-brown spots up to 3/4" across. Severe infections on sensitive cultivars can result in significant leaf drop. On twigs of the current season's growth, anthracnose causes sunken grayish lesions with reddish-brown margins. Fruiting bodies may be visible in the lesion. Nuts develop sunken spots on the husks. These spots are typically smaller than the spots on the leaves. Infections that occur early in nut development often cause premature drop and reduction in yield. The fungus overwinters in infected plant debris and in twig lesions.
BlacklineWalnutDiseaseBlackline is a viral disease of English walnut (Juglans regia). It causes death of water-conducting tissues at the graft union. Initially, infected trees show poor growth, yellowing and drooping leaves, early leaf loss, and other signs of inhibited water and nutrient uptake. As the disease progresses, branches die back and trees decline and die over a period of years. Bark at the graft union may have small cracks or pits. Cuts made through the bark at the graft site show an intermittent or continuous thin black line of dead tissue between the rootstock and the scion wood. Black cankers may also be present beneath the bark on the rootstock 'Paradox'. The virus can be transmitted to healthy trees via infected pollen and can also be spread by grafting and seed transmission. The disease is primarily found in mature trees (15-40 years old).
BlightWalnutDiseaseWalnut blight is a bacterial disease affecting leaves, nuts, and other new growth including flowers. Bacteria are spread to new growth in the spring by rainy weather during the bloom season. Leaves develop reddish-brown spots with a yellow halo. Leaves affected during expansion are typically distorted due to these lesions. Twigs develop small black cankers and may be girdled and killed. Young nuts show a dark lesion at the blossom end and drop from the tree. Nuts infected later in development have small, water-soaked spots on the husk which develop into sunken, cracked, black lesions. Nut kernels may be shriveled or of low quality. The bacteria overwinter in buds and twig cankers and are spread by water, infecting through natural openings in plant tissues.
Thousand cankers diseaseWalnutDiseaseThousand cankers disease (TCD) of walnut is a fungal disease associated with and spread by walnut twig beetles (WTB). Beetles feed on both healthy and damaged trees, but injured or weakened trees are more susceptible to rapid decline. The beetles attack branches about 3/4" and larger, feeding in the phloem (living bark tissue). Initial damage consists of numerous small, diffuse cankers that appear as brown to black stained areas associated with beetle tunnels. These small cankers are seldom visible on the surface of the twigs and branches. The earliest visible foliar symptoms may be yellowing leaves, thinning or flagging foliage in the crown, poor leaf-out in the spring, or limb dieback, but by this time the disease is well-advanced in the tree. Over time, the small cankers coalesce to form large, girdling cankers that kill twigs, branches, and eventually the entire tree. Large cankers (up to 6 feet in length) may develop on the trunk and can show visible brown or black staining. TCD also weakens trees and makes them more susceptible to other problems, including Armillaria root rot. Small cankers are most easily detected on smooth-barked branches about 3/4" in diameter. Shaving the bark on these twigs should expose the galleries and staining associated with the beetles and the disease. Exit holes made by adult beetles may be observed, as well. Since the beetles and the fungus are both found on all infected trees, the presence of either one is considered diagnostic for TCD. The disease is eventually fatal to black walnuts (Juglans nigra). Black walnut hybrids appear to be somewhat resistant. English walnuts (J. regia) are considered fairly resistant, though not immune. See Walnut: Walnut twig beetle for more information.
European fruit lecaniumWalnutInsectThe European fruit lecanium is a shiny brown scale insect found on twigs. Adults are 1/8"-1/4" in diameter and roughly turtle-shaped. They may have light markings or appear somewhat waxy. Crawlers (immature scales) are flatter and may be found on leaves. Scales produce large amounts of honeydew, a sweet, sticky material which may attract ants or become covered with a growth of dark sooty mold. Heavily infested branches may be wilted, yellowish, or show other signs of stress. New growth may be stunted or lacking. This lecanium scale closely resembles frosted scale, but does not produce the waxy coating found on frosted scale in the spring. Lecanium scales are common pests in the landscape, infesting many plants including maple, rhododendron, and fruit trees.
Walnut aphidWalnutInsectWalnut aphids are small, pale yellow, pear-shaped insects. They are typically found on the undersides of leaves. A second aphid species common on walnuts is the dusky-veined aphid, a grayish aphid found on upper leaf surfaces, usually in clusters along the midvein. Both species produce honeydew, a sweet, sticky material which may attract ants or become covered with a growth of dark sooty mold. Honeydew causes the husks to turn black on some walnut cultivars. Heavy infestations of aphids can cause wilting and leaf drop and weaken trees. Severe leaf loss also increases chances of sunburn damage to nuts.
Walnut blister miteWalnutInsectThe walnut blister mite is an eriophyid mite. These tiny, whitish, sausage-shaped mites cause raised green to yellowish blisters on the upper leaf surfaces. Beneath the blister, on the underside of the leaf, is a yellow to orange felt-like mass, in which the mites can be found. Later in the season, the velvety areas turn brown. The blisters can be large and unsightly, but damage caused by the walnut blister mite is primarily aesthetic.
Walnut husk flyWalnutInsectAdult walnut husk flies are slightly smaller than a house fly. They are brown with a yellow mark on the back, and the otherwise clear wings are marked with dark bands. Females lay eggs on walnut husks in mid- to late summer. The white to yellow maggots are up to 3/16" long and feed in the husks, causing soft, blackened areas. Husk tissue is destroyed and the nut shell and kernel may be stained, reducing quality. Early infestations may also cause kernels to shrivel. Mature larvae drop to the ground and overwinter. Husk fly damage may be confused with blight, but blighted areas on husks are usually hard, sunken, and cracked. Most English walnuts are considered to be very susceptible, but black walnuts are the favored hosts.
Walnut twig beetleWalnutInsectThe walnut twig beetle (WTB) is a small bark-boring beetle (Pityophthorus juglandis) believed to be native to the southwestern US. Not a great deal is known about the beetle's life cycle, but it is believed that two or more generations may develop per growing season in warmer areas. Adults are tiny yellow-brown beetles (around 1/16" long) with lines of puncture marks along the wings. They breed and feed in the bark tissues of various species of walnut trees, including black (Juglans nigra), Arizona (J. major), California (J. hindsii and J. californica), and English (J. regia) walnuts. Adults bore into the bark of branches (they prefer branches around 3/4" in diameter and larger) and the trunk, where they feed and lay eggs. Larvae develop and pupate in the living tissues of the bark. Emerging adults produce a tiny exit hole in the bark, often on the underside of branches. Adults from late summer and fall generations are believed to overwinter in chambers they bore in the thick outer bark of trunks. Upon emergence in spring, overwintering adults fly to new sites, often in the upper limbs of host trees. Some of the larvae may also overwinter. Beetle entry/exit wounds may show a small amount of staining, particularly on English walnut, but this is not consistent. WTB may be confused with other boring beetles, but damage by WTB is always limited to the live bark tissues. Localized staining and discoloration around the beetle tunnels may be seen in trees infected with Thousand Cankers Disease (TCD). See Walnut: Thousand cankers disease for more information.