By: Mary H. Dyer, Credentialed Garden Writer
What diseases affect a eucalyptus tree? Eucalyptus is a sturdy, fairly disease-resistant tree, and attempting to troubleshoot dying eucalyptus trees is a difficult and disheartening endeavor. Read on for more information about eucalyptus tree diseases, and tips on treating disease in eucalyptus.
Diseases of Eucalyptus Trees
When it comes to diseases of eucalyptus, wet weather, poor drainage, or damp conditions that prevent air circulation from reaching the center of the tree are often the culprits.
- Anthracnose – This group of fungal diseases affects primarily branches, twigs and leaves, and is recognized by curled, distorted growth and small black, tan or brown lesions. Younger trees are most susceptible. Anthracnose is related to excessive moisture and often follows moist springtime weather. Control the disease by pruning affected trees in fall and winter, but avoid severe pruning, which creates watersprouts – vigorous, unsightly growth that is more susceptible to disease. Application of fungicide in early spring may help staunch the disease.
- Phytophthora – Often identified as root, crown, foot or collar rot, Phytophthora is a fungal disease that affects a huge number of woody plants, including eucalyptus. It can attack all parts of the tree and is often evidenced by wilted, yellowing foliage, stunted growth, and reddish, orange or brown cankers on the trunk and stems or under the bark. The tree may ooze a reddish or dark sap that stains the trunk. Fungicides are sometimes useful if applied early, especially when combined with improved cultural practices.
- Heart rot – Often known as sap rot, heart rot is a group of several types of fungi that causes decay in the centers of limbs and trunks. Although the disease isn’t always easy to spot on the surface of the tree, damage can travel relatively quickly. Old, weak trees are more susceptible and trees that fall in rain or wind can be hazardous. Regular, careful pruning that allows rainwater to drain helps prevent the disease and safe removal of dead or disease growth helps keep the disease in check. Badly affected trees should be severely trimmed or removed.
- Powdery mildew – This common fungal disease is easy to recognize by a powdery white growth on leaves and stems. Horticultural sprays are often affective, and sulfur may help when applied before the disease is noticeable. Fungicides may be of some effectiveness in the early stages of the disease. Avoid high nitrogen fertilizers, which produce flushes of highly susceptible new growth.
Proper pruning of eucalyptus is critical. Disinfect cutting tools between each cut, and dispose of infected plant parts properly. Irrigate eucalyptus trees in morning so the leaves have time to dry. If you are planting new eucalyptus, look for disease-resistant varieties.
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Center for Invasive Species Research
The Eucalyptus Longhorned Borer, Phoracantha semipunctata andPhorocantha recurva
The Situation: Approximately 90 of the more than 700 species in the tree genus Eucalyptus have been introduced into North America over the last 150 years. Eucalyptus spp. are native to Australia and New Guinea. Many residents of California find the growth form, evergreen foliage, floral show, and other horticultural qualities such as drought tolerance highly desirable attributes of eucalyptus trees. However, exfoliating bark and dropped leaves can rapidly create flammable fuel loads under trees or on house roofs, sudden limb drop can damage property and injure people, and invasive growth habits of some species are problems associated with these common urban forest trees in California. The most widespread use of eucalyptus in California has been as plantings in residential areas to form shady urban forests. Low water requirements, tolerance of low quality soils, and, until recently, the absence of insect pests and diseases have made eucalyptus particularly valuable in residential areas.
In California, eucalyptus trees were first propagated from seed brought from Australia. Consequently, insect pests and diseases associated with growing plants were not introduced with living trees. Eucalyptus that were grown in California from seeds were relatively free of insect pests until the 1980's, when the eucalyptus longhorned borer, Phoracantha semipunctata, was discovered in Orange County in 1984. In 1995, approximately ten years after the introduction of P. semipunctata, a second cerambycid species, Phoracantha recurva, was found in southern California (Los Angeles, Riverside, and San Bernardino Counties) attacking eucalyptus.
Phoracantha beetles are large, often more than an inch in length. They are characterized by prominent long antennae, an obvious feature that is characteristic of cerambycid or long horn beetles. The two species are easily separated by the patterns on the wing covers (elytra) that lie across the backs of the beetles.
P. semipunctata has been accidentally introduced into virtually all Eucalyptus-growing regions of the world (e.g., Brazil, Canary Islands, Chile, Egypt, France, Israel, Italy, Morocco, Reunion Island, Spain, and South Africa) and is causing significant tree mortality in many of the areas it has invaded. P. recurva is demonstrating a similar high level of invasion throughout areas of the world with significant eucalyptus production.
The Problem: Phorocantha beetles are attracted to freshly cut eucalyptus wood, dying limbs, and trees suffering from stress, especially lack of water. Drought stress is exacerbated in many areas of California because many eucalyptus trees are growing in unmanaged or minimally managed environments with no supplemental irrigation. Eucalyptus species that naturally grow in wetter areas of Australia have been planted in California. When these trees experience prolonged dry periods, they undergo drought stress and this makes them susceptible to attack by these beetles.
The major damage these beetles cause to eucalyptus is done by feeding beetle larvae. Female beetles lay their eggs under loose bark, and when the eggs hatch, the young larvae burrow through the bark layer and into the cambial layer which lies just below the bark. Once the cambial layer of the tree is reached the majority of the feeding occurs here. The cambial layer is the only "living" part of the tree. The cambial layer contains the phloem (food conducting tissues old phloem becomes bark) and xylem (water conducting tissues old xylem becomes wood). As the tree grows it constantly renews phloem and xylem tissues, and only the new xylem and phloem transport water and nutrients. Beetles suffer high mortality in trees with high internal moisture levels because water floods feeding tunnels and drowns larvae.
Extensive damage to the cambial layer "ring barks" or girdles trees and the destruction of phloem and xylem means infested trees can no longer move water and nutrients from the roots to the leaves. Trees with this type of extensive feeding damage often die. When trees are heavily infested with beetle larvae, the chewing and scraping made by feeding larvae is clearly audible at some considerable distance from trees.
Over the spring and summer it may take around four months for beetles to develop from eggs to adults, while beetles developing over fall and winter may take up to nine months to complete development. Phoracantha beetles have around 2-3 generations per year in California.
Research: University of California scientists have conducted a very successful biological control program against P. semipunctatawith parasitic wasps that are very specific to this pest beetle. The most important of the parasites released against P. semipunctatais Avetianella longoi. This encyrtid egg parasitoid from Australia, was imported into California in 1992, and was established in this state by 1993. Female parasitoids lay eggs inside the eggs of P. semipunctata and parasitoid larvae consume the contents of beetle eggs killing them. A single female parasitoid can lay up to 200 eggs in her life time and up to five parasitoids can sometimes emerge from a single P. semipunctata egg.
The parasitoid can complete around six generations for every beetle generation, and by preventing the hatching of eggs, damage to eucalyptus trees by boring beetle larvae has been greatly reduced. It has been estimated that up to 90% of P. semipunctata eggs that are laid under bark on eucalyptus trees are killed by A. longoi.
This parasitoid is unable to kill eggs of P. recurva as effectively as it has killed eggs of P. semipunctata even though these two beetles are very closely related. The reason for this lack of efficacy is due to a defensive response of P. recurva eggs to larvae and eggs of A. longoi which effectively kills the parasite before it can damage the host.
Interestingly, the very effective control of P. semipunctata by A. longoi has reduced competition with P. recurva and this lack of competition and ineffective attacks on eggs has allowed P. recurva to emerge as a more serious pest of eucalyptus than it previously was. Field surveys have revealed that the proportion of P. recurva in samples of both eucalyptus longhorn borers taken in California increased from 0.1% in 1995, to 4.7%, the following year, and had reached 74% in 1997.
It would appear that another parasitoid species from Australia that is specific to the eggs of P. recurva is needed to provide effective biological control of this pest.
Four species of parasitoids from Australia that attack larvae of P. semipunctata and P. recurva have been introduced into California. One of these, Syngaster lepidus has been become established and parasitizes around 27% of Phorocantha larvae. Parasitoids locate beetle larvae by following vibrations emitted feeding within tunnels. Female parasitoids have long ovipositors, or egg laying tubes, to drill through the wood to beetle larvae feeding inside tunnels. The ovipositor is moved around in the tunnel until the beetle larvae is detected. At this time, the parasitoid injects a paralyzing venom that immobilizes the beetle larvae. A parasitoid egg is then laid next to the paralyzed host and upon hatching the parasitoid larva feeds on the eucalyptus longhorn borer beetle larva eventually killing it. The parasitoids pupate inside the host tunnels and upon completion of development they emerge from host trees to commence mating and host searching.
‘Pistachio Canker’ Tree Fungus May Be Killing Acres Of Eucalyptus Trees In East Bay Hills
OAKLAND (KPIX) – Wildfire fuel is increasing at an alarming rate in one part of Alameda County. This weekend there is an increased concern of fire danger in the East Bay hills and as if that wasn’t enough, scientists are examining eucalyptus trees to find out why acres of them are dying.
It started with Acacia trees seen dying last summer. Scientists suspect a new fungal pathogen called the Pistachio Canker which may have originated in Italy. Now Eucalyptus trees are suddenly dying and scientist are asking if the two events related.
Susan Frankel is a U.S. Forest Service researcher.
“We’re trying to make sense it, really trying to get information and get a sense of distribution and intensity of the die back,” said Frankel.
A healthy green eucalyptus is full of leaves but the infected trees are dead and dying. John Brega is an Oakland Adopt-A-Spot Volunteer for Joaquim Miller Park. He says the problem is enormous.
“There’s probably a hundred acres of eucalyptus in this park alone that are sick and declining very fast,” said Brega.
Combined with an unusually dry fall and a windy weekend, the next few days has residents on the alert for fire danger and it’s not just about sick trees.
“The biggest danger, I think, is from the shrubs, the Coyote Brush and the French Broom”, says Brega.
Researchers say they are not sure if the problem is fungus, lack of water. or both.
Frankel says, “At first, our attention was for the acacia but now, the problem does seem to be more widespread.”
Samples collected by scientists are currently being examined at University of California at Davis and UC Berkeley.
Symptoms of Armilaria root rot include leaf drop, clusters of mushrooms at the base of the tree and shoestring-like structures (called rhizomorphs) on the surface of roots. Silver dollar eucalyptus may develop root rot if soil is not properly prepared before planting. Do not plant silver dollar eucalyptus in soils prone to flooding or soils where root rot has been witnessed before instead, save those areas for resistant tree species. Remove any leftover tree roots from the soil before planting, as they could potentially harbor the disease.
It is evident that the harsh weather we encountered during December 2010 has caused considerable damages to our Eucalyptus trees, unfortunately, our British weather is fairly unpredictable and although there isn’t anything we can do to stop the cold and harsh frosts from killing our trees, there are ways to prevent. Eucalyptus trees are known sun lovers and most species despise shade, so plant your Eucalyptus tree in the sunniest part of your garden. Also, choose a species like the ones we have suggested that is tolerant of our British climate.
In the past couple of years, we have seen some of the coldest winters and so, as a result, we have seen some of our plants and trees affected by the weather. We hope this article has given you some helpful insight into how the cold weather has affected Eucalyptus trees. If you would like more information on the Eucalyptus trees or if you have any questions on a tree or plant that has been affected by the cold then please contact us.
Heart or Sap Rot
Heart or sap rot is a fungal disease that causes the wood in the centre of the trunk or limbs of the eucalyptus to decay. The length of time it takes this disease to decay the wood varies from a few months to several years. Heart or sap rot causes the wood to loose its strength, killing the sapwood storage and conductive tissues in the tree. No visible damage is usually seen unless the bark has been cut or damaged to reveal the cavity inside. To prevent this from happening, cut out any dead or diseased limbs. When you prune, make cuts so the rain will drain off that section, not collect there.