Pest Notes, Publication 74171 Revised January 2020 Armillaria Root Rot Integrated Pest Management for Home Gardeners and Landscape Professionals rmillaria root rot is a disease of trees and woody Aplants, although it also affects palms, succulents, ferns and other herbaceous plants. This disease is caused by fungi in the genus Armillaria, also known as “oak root fungus,” although the fungus has no specificity for oaks. Several species of Armillaria are known to occur in California, but Armillaria mellea is the species commonly found in home gardens, parks, vineyards, tree crops, and natural landscapes. This pathogen has a broad host range, potentially affect- ing thousands of ornamental and crop plants. Other species of Armillaria occur in forests, causing heart rot and acting as saprophytes. The oak root fungus attacks and kills the vascular cambium Figure 1. Armillaria mushrooms grow in clusters, (the tissue that generates bark and wood) in woody roots, then have a ring (annulus) around their stem, and are spreads laterally to the main stem, which can girdle the base of tan to honey colored. the trunk and kill the entire tree. Armillaria is also a white rot wood decay fungus which destroys the strength of wood in roots and at the base of infected tree trunks, thereby increasing the likelihood of tree failure. This dual nature of Armillaria, both as a pathogen (killing the living tissues in a tree) and a saprobe (living on dead or non-functional wood after the infected host dies), presents a challenge to management because its inoculum (infec- tive tissue or propagules) can persist for decades below ground as mycelium (vegetative fungal tissue) living in partially-decayed woody roots (residual roots) long after the infected host plants have died or were removed. IDENTIFICATION Armillaria root rot is commonly recognized by the presence of light brown mushrooms (known as “honey mushrooms” due to their color; not their taste!), which typically appear in a cluster Figure 2. Mycelial fans of Armillaria under the of several to dozens of mushrooms at the base of infected trees bark of an infected tree. or shrubs (Figure 1). Sometimes the mush- Mushrooms may not always appear, and some trees Authors: room cluster will will succumb to the disease without ever giving rise arise from a buried Arthur James Downer, UC to mushrooms. Therefore, it is better to look for other, root in a turfgrass or Cooperative Extension, Ventura more consistent, signs of the fungus. County. groundcover area away from the infect- One diagnostic sign of Armillaria root rot is the white Igor Lac´an, UC Cooperative ed tree (or where an fan-shaped mycelium, thin, flat sheets of fungal tissue Extension, San Mateo and San infected tree once (mycelial fans) that grow just below the bark (Figure 2). Francisco Counties. stood) or will form To find these mycelial fans, select a dead or dying plant around an old stump. to examine. Dig into the soil at the base of the trunk and Pest Notes: Armillaria Root Rot Page 2 of 5 down to approximately 12 to 18 inches beneath soil line. From the trunk and any large roots branching off the trunk, peel or cut away the dead bark with a knife to reveal mycelial fans and rotted wood, a common symptom of Armillaria root rot. Presence of black strings of fungal tissue (rhizomorphs) is a third sign of infection. Rhizomorphs may be found under the bark alongside the white mycelium, on infected roots, or in soil immediately adjacent to the infected tree. Rhizomorphs can be difficult to distinguish from fine roots, but gen- erally are darker, smoother, and lack the woody inner cortex of plant roots. Rhizomorphs of Armillaria mellea are smaller than those of other Armillaria Figure 3. Peruvian pepper dying of Armillaria root rot. Rapid death of the whole tree is often the first symptom noticed. species, ranging in diameter from 1/32” to 1/8”. Since rhizomorphs and mush- rooms are sometimes absent, and the mycelial fans are hidden under the DAMAGE bark, the presence of this fungus is of- The first and most obvious symptom ten first revealed by the host symptoms of Armillaria root rot may be decline on the trunk or in the canopy. or death of a tree or woody plant (Figure 3). Often, death occurs so The genus Armillaria is in the division rapidly that the leaves turn brown, but Basidiomycota and is part of the family don’t yet fall off the tree. Sometimes Physalacriaceae. Though Armillaria this decline may be slower, charac- mellea was once considered the only terized by dieback of shoot tips over species in North America, we now the course of months or even years. know of more than a dozen species. Armillaria infects woody plants and However, only 6 Armillaria species are destroys their vascular cambium thought to be present in western North and the underlying root wood. The America, and only 2 species are wide- pathogen can also attack non-woody spread in California: Armillaria gallica, monocots and some herbaceous plant common in forests of the Coast Ranges tissues. The infection typically begins and the Sierra Nevada but also found when plant roots grow into direct in urban landscapes, and Armillaria contact with infected residual roots. mellea, which is widespread in both Although the mushrooms release natural and ornamental landscapes wind-dispersed spores, these are not a throughout California. Armillaria Figure 4. Bleeding is often a common means of infection. mellea is far more virulent than A. symptom of early attack by gallica. Thus, once a fungus affecting After the cambium has been killed and Armillaria. an ornamental landscape in California the underlying root wood has been has been identified as Armillaria, invaded, a mycelial fan usually grows to especially where dead and dying hosts colonize the main stem, where it may places to look for mycelial fans under are present, it is reasonable to presume continue to invade other roots or move the bark, especially on trees that have that the species is Armillaria mellea. up and around the root collar. Where canopy symptoms. the cambium of the main stem or large Other visible symptoms at the surface For information on other fungi that roots has been destroyed by the fungus, include flat cankers on the trunk or decay trees, see Pest Notes: Wood Decay the formation of new wood and bark main stem near the ground, flat surfac- Fungi in Landscape Trees. ceases, and a flat lesion on the sur- face of the bark (visible as an external es on an otherwise round stem, crack- canker) develops—these are the best ing bark, and bleeding sap (gummosis, Pest Notes: Armillaria Root Rot Page 3 of 5 common in Prunus species) (Figure 4). Infection is thought to proceed primar- Avoid physical damage to roots, soil With continued destruction of its roots, ily by direct Armillaria-to-host contact, compaction in the root zone, and a tree loses its ability to conduct water, either when healthy roots grow into addition of soil on top of the existing and symptoms of decline begin to ap- contact with residual roots or when rhi- grade (especially during construction). pear in the canopy. Canopy symptoms zomorphs grow out from infected roots Most importantly, prevent the wa- include leaf wilting, defoliation, twig and contact susceptible roots. terlogging of soils around trees from and branch dieback, and thinning out. excessive or inappropriate irrigation, MANAGEMENT especially in summer. Do not irrigate Development of canopy symptoms trees that are mature, established, and may be gradual, over the course of have never been irrigated. Excessive years. The course of symptom devel- Prevention irrigation can be especially common opment likely depends on the progress There are no known cultivars or vari- for drought-adapted trees planted in and pace of infection. When disease eties of plants that are completely im- lawns. In such cases, it is preferable to progress is slow, initial symptoms may mune to Armillaria root rot, and some maintain a mulched turf-free zone out be subtle: defoliation at the top of the plant plants are very susceptible to the to the dripline of each tree, if possible, tree or plant may be the sole indication, pathogen. For example, peach (Prunus and allow the soil under the tree to dry sometimes accompanied by increas- persica and related Prunus hybrids) out between irrigations. ing proportion of dead twigs (thinning and Peruvian pepper (Schinus molle) out). In some cases, early leaf loss are highly susceptible. Susceptible or dormancy and leaf color changes plants should not be planted in land- Cultural and Biological Control are also possible. By the time canopy scapes where trees have died from In a landscape with plants known or symptoms appear, it is very likely that Armillaria root rot, especially when suspected to be infected, the prin- mycelial fans are well developed under large dead roots may remain in the soil. cipal cultural management strategy the bark low on the trunk, on the large involves carefully regulating irrigation. roots, or both. Reducing tree stress and ensuring Immediately correct any over-watering good tree condition are the principal and poor soil drainage, and, during the LIFE CYCLE AND beneficial practices in preventing rapid warm summer months, ensure that the decline. Although A. mellea is consid- root collar of trees stays dry (i.e., do BIOLOGY ered highly virulent, most deaths of not set up the sprinkler to hit the base Armillaria root rot has a complex life infected trees in California landscapes of the trunk). This is especially import- history that involves vegetative spread can be attributed to excessive irrigation ant for native California oaks, which and sexual reproduction via spore dis- or other stresses.