The Kingdom FungiD. Andrew White M.Sc. 11/08/2011
The Mycota, or the Fungi, are usually conisidered to be a separate taxonomic Kingdom from either plants or animals. True Fungi are eucaryotes with chitin lined cell walls, that live by digesting food substances. They are not directly photosynthetic. Cells tend to be syncytial, the cells run-together into long tube-like multi-nucleate 'hyphae'. This continous cytoplasm is most pronounced in the lower fungi. In the ascomycetes and basidiomycetes, there are partial septa dividing up the hyphal cytoplasm into segments. Most fungi are isogametic, meaning that the gamete mating-types are very similar in size and shape. Fungi often live in soil, but some are parasites on other organisms, including plants. Fungi have many physiological and genetic similarities to animals. The chytrid moulds even produce flagellated gametes, like animal spermatozoa. To find out more about fungi, visit White's Mycology Page .
Cedar-Apple Rust - 'Orange Jelly'
Sexual spores from the juniper host spread via the wind to apple trees in the spring. From these sexual spores develop spermatia and receptive strains of hyphae. (Hyphae strains are somewhat analogous to the male and female sexes in higher plants.) If the two hyphae 'strains' encounter one another they undergo sexual fusion to become the aeciospore generation. The 'fruiting bodies' appear as a typical leafspot fungus. From these leaf spots protrude aecia. These aecia bear the asexual aeciospores. A rim of pale yellow, or red, area of leaf tissue may surround each cluster of spore-bodies. The asexual spores can be carried long distance in the wind, and thus infect nearby junipers.
On red cedar junipers, the rust causes the development of swollen twigs. These swellings are teliospore masses also called telial galls. Telial galls swell up to a few centimetres wide. Teliospores produce the sexual spores on a basidium. (A basidium is a spore bearing structure typical of the Basidiomycotina fungi.) These teliospore masses eventually ooze an orange gelatinous substance. This ooze projects in long translucent strands up to four centimetres long. These jellies look bizarre enough to cause concern.
Telial galls can be pruned off junipers. Rusts are harder to treat on apples, as systemic fungicides need to be applied in advance. In apple orchards this is often done on a scheduled regimen anyway. Since the cedar-apple rust does little damage to the juniper, it can be tolerated to some degree. Indeed, some people find the orange jellies interesting.
Cedar-apple rust can be avoided by planting juniper and apple trees fairly far apart. Since the spores can travel for kilometres in the wind, the distance between host species needs to be great. This is not always practical, as some one else may own the other host tree(s).
Pear Trellis Rust
Pear trellis rust Gymnosporangium sabinae is similar to apple-cedar rust, but it infects pears (Pyrus spp. ) instead! This rust fungus even uses juniper as its alternate host! The orange jellies on the junipers are somewhat similar. But, the orange spots on the pear leaves are a little more colourful.
The rust fungus was originally native to Europe. It spread to western North America in the 1960s. From British Columbia it then spread by the late 2000s to Ontario! However, the comestible pear is a native to Eurasia anyway. So in a sense the fungus simply has found its old host in a new land.
Pear trellis rust is controlled in almost exactly the same manner as apple-cedar rust. Though like other rusts if the life cycle can be checked at any point, it reduces the contagion in the following year. This includes all of the standard hygienic measures such as disposing of old leaves and minimising the number of junipers in the neighbourhood.
Yet Other Rusts
Hawthorn rust (G. globosum) and quince rust (G. clavipes) are yet other related rusts. They afflict hawthorns, and the later fungus attacks quince as well. All of these fungi are very similar in that juniper is the alternate host. If anything, hawthorns are even more likely to suffer from rust than are pears or apples. As with these other fruits, these rusts seldom kill the trees outright. They do however make the fruit unappetising and unsightly.
Anthracnose is a general name for a number of fungi, including: Discula spp. , Glomerella spp. , Gnomonia spp. , Kabatiella spp. , Marssoniella spp. , Marssonia spp. , and Monostichella spp. . Although caused by diverse types of fungi, all anthracnose diseases are typified by dying leaf margins, that look like frost damage. From the leaf margins, the zones of necrosis can expand to the petiole, and even onto the twig, causing cankers on the bark.
Avoiding moist conditions under, and around, a plant can decrease the spread of the disease. For leaf infecting anthracnose produces spores on fallen dead leaves. Wet weather tends to favour the development of spores, and increase their chance of taking hold. The case is generally different for those anthracnose fungi that cause cankers on twigs. These species do not generally grow on fallen leaves. However, these species also spread more rapidly in rainy weather. The spores, produced in the bark cankers, can be splashed around by rain drops.
Dogwood anthracnose is best controlled by cutting off the branches which have cankers and leaves that exhibit dying margins. Anthracnose can be diminished by applying fungicides in the spring just before the buds break. Another application should be applied when the leaves are half open, about two weeks later. Like most fungicide treatments, one must presuppose that a given plant is susceptible in advance of the actual outbreak. Fungicide application after the fact is too late.
Wood Rot Fungi
Most of these wood rot fungi have both asexual and sexual sporocarps. The asexual anamorphs can differ greatly from sexual sporocarps in form and colour. The chlamydospore and conidiospore bodies are often small and hidden inside the wood or in hollows. The asci or basidia bearing bodies are larger and more exposed.
If the 'fruiting bodies' of wood-rotting fungi appear on trees, this means two possible things: (1) fungi are digesting dead tissue in the tree, or (2) fungi are killing and then digesting living tissues in the tree. The dead tissue is usually heartwood, and the living tissue is usually sapwood.
The vast majority of wood-feeding fungi live off dead wood. These can still be a problem, if they make the tree structurally less sound. Hollow trees are often hollow because fungi have digested away the dead heartwood in the core. Such hollow trees are prone to breaking in windstorms, ice storms and other stresses.
In temperate climates very few wood-rotting fungi are significant threats to tree health: (1) honey fungi (Armillaria spp.), (2) white pocket rot (Heterobasidion spp.), and (3) artist's fungus (Ganoderma spp.).
Honey Mushroom / Honey Fungus
The toadstools are pale and tan coloured. They appear as clusters of gilled toadstools near the base of a tree, usually where some dead wood is exposed. As with all toadstools these are merely the 'fruiting bodies', the vast majority of the fungal tissue being hidden, in this case inside the tree's wood. Honey fungi toadstools are usually visible in the autumn. Inside the wood they cause damage throughout the year. Cream-white mycelial fibres grow outward from the dead wood into living, but now dying, wood in the lower trunk and roots. When actively growing, this mycelium can be bioluminescent. Hence, sometimes one can see the mycelial fibres of the fungus glowing on a dark night.
The honey mushrooms are very versatile parasites. Honey mushrooms also are saprobes, they digest deadwood and can stretch their mycelium through soil to infect neighbouring trees. These rhizomorph runners are dark, and thick like shoe-strings. These runners contrast sharply with the pale mouldy mycelial fans that spread out under dead bark. In total the mycelial mat of a honey mushroom can be very large. In fact, if an ‘individual’ is defined in genetic terms, a honey mushroom can be one of the larger organisms on Earth. Mycelium from one ‘genetic individual’ can weigh many tonnes. Although, in reality the links within a mycelium mass tend to break apart as a fungus grows.
Honey mushrooms often grow in association with the hunter's heart mushroom. There are still questions about which organism parasitises which. But it is almost certainly the honey mushroom which, in this case, is the victim.
Most mushrooms in the Armillaria genus are edible, if they are cooked. However, if they are eaten raw, mild symptoms of poisoning may occur. Mediterrenian and Slavic Europeans, as a whole, have a stronger tradition of eating the mushroom than do the British. But this stereotype is slowly changing.
White Pocket Rot
Generally speaking, once the fruiting body is visible, this indicates that the fungus has become firmly established. In which case, significant wood rot has already occurred. The fungal disease is mostly a problem in conifer monocultures. Mycologists still debate the seriousness of this pathogen on broadleaf species. Broadleaf trees with pocket rot can sometimes live for many years. Its seriousness for conifers is quite evident. Spores from the fungus most easily infect freshly exposed stumps. The mycelia enter the roots of these stumps, then they multiply feeding on the root xylem. From the stumps they spread-out to living pine roots. In this manner the fungi-infected stumps in a clearcut can infect nearby pine stands.
In the 1960s Dr John Rishbeth, at the Cambridge Botany School, developed a novel method of controlling pocket rot. Fresh stumps can be inoculated with the spores of Phlebiopsis gigantea, a type of crust fungus. This competitor fungus pushes out most of the pocket rot mycelia which may attempt to grow in the stump wood. The technique can be used on clearcuts in regions prone to pocket rot infestations.
Ganoderma / Artist's Fungus
Ganoderma is also known as “artist’s fungus” or polypore des artistes. This is because the pale spore bearing layer on the underside of the bracket can be used as a surface to engrave images upon. Drawing pictures on bracket fungi was once a common rural folk craft. The craft is now somewhat passé.
Assorted Bracket Fungi
Many species of bracket fungi are wood rot agents. However, most do not actually kill living tissue. The birch polypore (Piptopous betulinus) has a pale and smooth upper surface. The tinder fungus (Fomes fomentarius) is grey with a down-curved ‘hoof-shape’. Brackets in the Fomitopsis genus often have strongly coloured bands of red and brown. The Phellinus genus tends to form hard grey conks with tiny pores. There are also several members of the Ganoderma genus which do not kill living wood. All of these genera cause wound and heartwood rot. But most of these brackets do weaken stem wood, and thereby can contribute to tree failure.
Jack-o-lantern is not an edible mushroom. It is quite poisonous to humans, if eaten. They are not poisonous to mere touch, very few toadstools are that dangerous.
Sulphur-shelf brackets are edible, if they are cooked. However, some people experience digestive upset after they eat cooked sulphur-self fungi. It is probably best to consider the fungus to be mildly poisonous.
Stereum Disc Fungi
Red rot (Stereum sanquinolentum) tends to occur on conifer wounds. The rot is fairly isolated, and does not spread to living wood. Oak pipe rot (S. gausapatum) occurs on oak tree wounds, but its mycelial spread is usually less extensive than the sulphur-shelf rot. Relatively benign species of Stereum occur on magnolia bark. False turkey-tail (S. ostrea) is peculiar in that it is a tiny bracket fungus when it is on a vertical surface, but it is crust-like on the underside of deadwood.
Chestnut Tongue Brackets
Inonotus Heart Rots
Research suggests that the scaly pholiota is a rather weak pathogen. It is usually some other non-pholiota fungus which is the main cause of the white rot. Fairly often the other species is a honey mushroom. Therefore, the toadstool is more of a symptom than a cause of tree decline.
This polypore forms a large soft off-centre toadstool, or quasi-bracket, which can grow to several decimetres wide. The pore surface has closely packed small pore, and is pale to slightly yellowish. The upper surface eventually forms scale-like scabers. When it grows on a horizontal surface it forms a toadstool-like body similar to the winter polypore. On a vertical surface the stipe connects off-centre, with a thin wisp of a ridge of a pileus (cap) on one side.
Dryads’ polypore can look something like a saddle. (Dryads are mythical wood-fairies.) The conk/toadstool is rather pleasant tasting. Young specimens are juicy and quite comestible. It is one of the first fungi to attract the attention of children and other novice naturalists. It is especially fun to watch the toadstool expand over the course of a few days. The fruiting-bodies sprout during the spring and autumn in Ontario.
European foresters seem to have more complaints about the fungus than do North Americans. Possibly this is another example of a subspecies difference between the American and Eurasian mycoflora. Perhaps the dryads’ saddle is a similar case in point. But for now it is considered to be a single species that occurs from India, to Europe, to the Americas and even Australia. (It may have been introduced in some cases.) In the Americas the fungal conks are considered a sign of wood-rot, and not a disease per se. Although, the distinction is academic as the appearance of these conks are a sign of a decay problem.
Burnt Crust Fungus
White, Brown & Soft Rots
If a fungus digests mostly the brown lignin, and not the pale cellulose, it is a white rot. Cellulose is located mostly on the inner layers of the tracheid and vessel cell walls. Cellulose tends to give wood its tensile strength. If a fungus digests mostly cellulose, leaving behind the lignin, it is a brown rot. Lignin is concentrated largely in the outer layers of tracheid or vessel cell walls. Lignin tends to lend compressive strength to wood. If a fungus digests both cellulose and lignin it can produce a soft rot. Soft rot tends to weaken both the tensile and compresive strength of the wood.
White Rot: White rot can occur in conifer wood, but it is most common in broadleaf xylem. The plump orange brackets of the Inonotus species are often a sign of white rot. Ganoderma fungi are also white rot agents. The white pocket rot (Heterobasidion annosum) gets its English name from the colour of its rot.
Brown Rot: Brown rots are almost entirely due to basidiomycetes. The majority of brown rots occur in conifers. Some Stereum fungi can cause brown rots. The Fomitopsis brackets is brown rot which occurs in conifer wood. The chestnut tongue bracket (Fistulina hepatica) can cause a kind of brown rot in broadleaf trees such as oak and chestnut. Although, the rot caused by chestnut tongue is a mixed form of rot.
Soft Rot: Soft rots are caused by fungi that digest both cellulose and lignin. Often these fungi have very branched hyphae which enzymatically bore into the plant's cell walls. Kretzschmaria deusta is one very important type of soft rot fungus. The soft tan brackets of Meripilus giganteus can cause soft rot. The Inonotus, Chaetomium and Rigidoporus fungi can also cause soft rot in some of their hosts.
Both white, brown and soft rots can weaken a tree's heartwood. Usually, white rot is more flexible, because the cellulose fibres remain fused together. Brown rot is often very brittle. Hence, when lignin is left behind, the woodrot is often dark and broken into cubical sections. White rot tends to be stringy, and less brittle. All types of heartwood rot can weaken a tree such that it breaks during a windstorm. Soft rot is often the most dangerous of the rots. Ustulina soft rot tends to weaken the base of the tree's trunk. When an entire tree falls, soft rot is often to blame.
Fungicides are not very effective in killing well established wood-rotting fungi. For both honey fungus and pocket rot it is recommended that seriously infected trees be removed. After removal it is advised that the stump be removed. The more thorough the stump removal the better. Removal is most important for pocket rot infestations. White pocket rot can live on stumps that still have some living root tissue in them. The spores from these stumps can infect standing trees. Most fungi should be given the 'wait-and-see' approach. For jack-o-lanterns one should monitor the situation. Most fungi, such as the sulphur-shelf, should not be considered a great concern. One should always assume that very old trees have some hollowing. Visible fruiting bodies are just one of the signs of heart rot.
Otrosina, William J. and Garbelotto, Matteo. 2010. Heterobasidion occidentale sp. nov. and Heterobasidion irregulare nom. nov.: a disposition of North American Heterobasidion biological species Fungal Biology. 114: 16-25.
Schwarze, F.W.M.R., Engels, J. and Mattheck, C. 2004. Fungal Strategies of Wood Decay in Trees. Springer. Berlin.
Phytophthora spp. are subsoil oomycete fungoids that are parasitic on roots and tubers. (They are related to the blight that caused the 'potato famine' in Ireland in the 1840s.) These fungoids have little manifestation above ground. However, many species of trees are prone to root death caused by phytophthora. Symptoms of such infection are general poor growth, leaf browning, and eventually branch dieback. Visible necrotic wood may appear near the root collar. But generally, phytophthora is most easily detectable by what it is not. There tends to be no obvious signs of disease above the root collar. Branches that are cut do not show obvious signs of sapwood discoloration. If the tree is cut at ground level, discoloured sapwood is visible. If the roots are examined, the source of the malaise becomes clearly visible: root necrosis.
Phytophthporas of sundry species and strains have become a worldwide problem. The P. cinnamomi has devastated Australia's eucalypts. The P. lateralis keeps killing off stands of Port-Orford Cedar in the American west. Then there is the P. ramorum that threatens the live-oaks and tanoaks of the American west. In all cases, these alien fungoids have been spread, or introduced, through human agency. Almost invariably, the spores were originally brought in, or spread about, by accident.
Phytophthora sensitive trees often develop the disease if the soil is too moist. Since subsoil fungicides are not very effective, phytophthora is not easily treated. Site sanitation, the complete removal of diseased plants is necessary. This helps to avoid further spread of the disease. Reducing the soil moisture is another option. But this is not always practical. Generally , the best method of reducing phytophthora is to plant species or cultivars that are more resistant to the fungoid.
Sudden Oak Death
Sudden oak death (Phytophthora ramorum) is a particularly troublesome form of phytophthora. Certain strains of this oomycete fungoid have begun to seriously afflict coast live oaks in California. This epidemic may have begun when a strain of phytophthora from Europe, or Asia, was accidentally introduced to the Americas in the 1990s.
The symptoms of 'sudden oak death' are like those of other phytophthora diseases. However, in coast live oak (Quercus agrifolia) the disease tends to be fatal. In addition, cankers can appear on the lower boles of infected trees. Lesions of dead cambium under the bark can issue a dark exude though cracks in the bark. As is also too typical of phytophthora species, the fungoid can live in more than one host species. Rhododendrons, laurels, and tanoaks (Lithocarpus spp.) can also host the fungoid. Tanoak and a number of oak species are very sensitive to the fungoid.
It now seems probable that the P. ramorum fungoid came from Europe on imported rhododendrons. Furthermore, contrary to early reports, redwood trees are not very sensitive to the fungoid. However, laboratory studies indicate that the fungoid could seriously harm many eastern oak species. Luckily, it has not yet established itself in the east.
Sudden oak death is most prevalent where rhododendrons have been planted in the vicinity. Rhododendrons can act as carriers, and are themselves only mildly affected by the fungus. The nursery trade of rhododendrons has been in part responsible for the spread of this disease. It is possible that eastern oaks could become endangered by this epidemic. Diligence is required to ensure that this phytophthora strain spreads no further than it has already.
Recently a treatment for sudden oak (SOD) death has been found. This control is a pre-emptive treatment for SOD infection. It is not a very good post-infection ‘cure’ per se. So far it has been tested and proven efficacious for treating oaks and tanoaks. Applications of phosphites (PO33-) or salts of phosphonic acid can incur a degree of resistance to SOD and some other oomycetes. Injections of the solution into the active xylem (sapwood) are fairly effective. Spraying the bark with the phosphite solution is somewhat less effective - but cheaper. With spray applications, a surfactant is required to keep the solution in place until it is adsorbed into the tree’s tissues. The application should be during the active growing season, and the concentration and dose must be very carefully controlled (Garbelotto et al 2007).
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Garbelotto, M., Schmidt, D.J. and Harnik, T.Y. 2007. Phosphite Injections and Bark Application of Phosphite + PentrabarkTM Control Sudden Oak Death in Coast Live Oak. Arboriculture & Urban Forestry. 33(5): 309-317.
Schwarze, F.W.M.R., Engels, J. and Mattheck, C. 2004. Fungal Strategies of Wood Decay in Trees. Springer. Berlin.
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Verticillium spp. are common wilt causing fungal agents. V. dahliae and V. albo-atrum are the two most widely recognised tree infecting verticillium fungi. These fungi can clog the xylem of branches causing a disruption in sap flow, and consequently the death and wilting of foliage. Unlike most other wilt causes, verticillium wilt usually affects older leaves first. (Wilting caused by simple water stress usually causes young leaves to wilt before the older ones.) Generally the xylem necrosis is localised to a few branches. A crude diagnosed of this wilt can be made by cutting an afflicted twig. If verticillium fungi are the cause, the xylem usually has dark brown to greenish streaking in it.
Verticillium fungi can inhabit the soil for years. It then enters the root system of plants, if they are ill-equipped with natural defences, and takes up residence in the xylem (sapwood). In general the wilt fungus afflicts susceptible trees only when they are water stressed. That is, droughts can increase the chance of verticillium infection. In addition, verticillium wilt can develop in trees and shrubs planted in outside their normal range in regions that are too dry. Maples (Acer spp. ), red bud (Cercis spp. ), ash (Fraxinus spp.), Catalpa spp. and tuliptrees (Lirodendron spp. ) planted in dry places are prone to verticillium wilt.
‘Verticillium’ is an anamorphic name, that is the name of the asexual form. Most verticillium moulds are only known by their asexual forms. Genetic studies suggest that many of the wilts are members of the genus Torrubiella. But there are other verticillium diseases, such as those of the cultivated mushrooms. Many of these verticillium moulds turn out to be members of the Hypomyces genus.
Verticillium is difficult to totally eradicate, nearly impossible in fact. It can be starved out of a soil by removing an infected tree. To ensure that it does not return, the replacement tree should be of a verticillium resistant species. However, verticillium is not always fatal. Sometimes it only kills a few branches for a few years, then the plants recover. It is not always necessary to remove a tree at the first sign of the wilt.
Leaf Spots & Acer Tar Spot
Acer tar spot (Rhytisma spp.) are fungus that cause coin-sized black spots on maple leaves. Rhytisma americanum is a New World species which causes large dark spots on leaves. R. acerinum, the species originally from Europe, also causes large black spots. This European species is now in the Americas, where it mostly afflicts the introduced Norway maple. Tar spots are a few centimetres wide, at most, and darkest on the upper surface. The disease is more prevalent in some years than others. Luckily, tar spot does not greatly harm maple trees. However, it is widely considered unsightly. In Ontario the Norway maple (Acer platanoides) is often infected with tar spot. Usually in a 'bad year' there is about one black spot per leaf. In 2003 in Toronto many Norway maples had three or four tar spots per leaf!
Tar spot's ascospores mature and are ready for diaspora in the Autumn. Strangely, these spores seem to be most contagious after leaf fall. The spores apparently alight on leaf-buds and develop into the black blotchy fungus in the following season.
There are various species of ascomycete, basidiomycete and fungi imperfecti which cause very similar ‘leaf spots’ as the acer tar spot fungus. Generally they produce irregular small brown spots spread-out over the leaf. Many of the most common ones are ascomycetes, which occur in the asexual fungi imperfecti form. The Diplocarpon spp. mostly afflict roses and hawthorns, Blumeriella spp. cause spots on cherries. Guignardia spp. cause the common horsechestnut spot disease. Cristulariella spp. make pale spots on a wide range of broadleaf trees. The common Phyllosticta spp. form spots on many broadleaf species, especially maples. Control of these fungi is basically the same as applies to acer tar spot.
Some of the names for leaf spot fungi are the old fungi imperfecti or anamorphs names. For example, Phyllosticta is actually the asexual form of the ascomycete Guignardia. Asexual and sexual forms of a fungus can infect different hosts, or cause different symptoms. Sometimes the two forms are slightly different genetically. The name-doublets are widely used as common names, one name for one disease, and the other name for another disease.
Fallen leaves carry the contagion of tar spot and many other leaf spot fungi. Therefore, cleaning up the leaves early after they fall in the autumn minimises spore dissemination. Although, for the sycamore maple (Acer pseudoplatanus) even this hygiene does not seem to be very effective. Nevertheless, removing autumn leaves is effective for Norway maple. However, even for Norway maple, leaf removal does not totally eliminate tar spot. Even in a 'good year' there tend to be a few spotted leaves. Tar spot is a disease we can learn to live with.
Agrobacterial -Crown Galls
Crown galls are growths instigated by Agrobacterium tumefaciens, a soil bacteria. These are the burls or 'tumours' that are appear near the base of tree trunks. Sometimes the galls form higher on the trunk, or on branches. The Agrobacterium stimulates the plant host into making the gall. The uncontrolled cell division in the cambium is instigated by genetic material from the bacterium. Strangely, this genetic material becomes incorporated into the genome of the host plant's cells. (Virus are not the only lifeforms that can pass genes to other organisms.) Crown galls can be up to 100 cm wide, but mostly they are much smaller. The bacteria can live for long time periods in the soil. It is suspected that the bacteria enter the trees through small wounds. The bacters, seemingly, can be carried on air-borne dirt in the wind. Once inside a tree they insert the gall forming genes into the host, and then multiply as the gall grows.
Note: Not all burls are caused by Agrobacterium. And not all strains of Agrobacterium tumefaciens stimulate the formation of visible galls.
There is not much evidence that crown galls harm trees. Removal of infected trees is seldom necessary. In fact, removal can actually cause the disease to spread. These galls only spread from direct contact between the bacteria and living wood. Breaking up the galls with chain saws, et cetera, could releases bacteria to the air. It is also possible to transport these agrobacteria with pruning tools that have not been cleaned between uses.
There are cultivars and species that are known to be resistant to crown gall. If agrobacteria are known to be common in an area, resistant plant varieties should be selected for planting.
Fire blight (Erwinia amylovora) is a bacterial disease. The blight afflicts plants in the apple-rose family (Rosaceae). This blight is typified by a sudden death of leaves and stems. These parts die, turn black, and the woody parts develop cankers. The dead tissue are usually near branch tips. Twigs often looks like they have been scorched with fire.
Fire blight bacteria infections enter through tiny wounds. Newly developed flowers are a common entry point. The bacteria, if en masse, appear like an orange ooze. The tissue they kill, as stated before, tends to turn black. The bacters over-winter in the cankers on twigs. The bacters are then spread by rain splashing, birds or other agents that contact the bacterial ooze. The blight can cause significant damage to the plants.
Pruning out the cankered branches is an effective method of slowing the spread of the blight. Copper compounds and other bactericides can be used to kill off fire blight bacteria. Spraying is best done when the flowers are opening. Generally, this spraying is reserved for nurseries and orchards.
Elm yellows, also called elm phloem necrosis, is a serious elm disease. Elm yellows is a disease caused by a phytoplasma that infects the inner bark (phloem) of elm trees ( Ulmus spp.). Candidatus Phytoplasma ulmi is one of the newer proposed names for this bacterium. Not long ago this disease was thought to be viral. Elm yellows kill twigs by multiplying in the phloem, causing tissue death, and thus blocking the conduction of nutrients from the leaves. This in turn causes necrosis in the inner wood, xylem, starving the leaves. A consequence of this is an early yellowing of the leaves followed by branch death. Leaves turn yellow early, as if undergoing autumn shedding, and fall. A few branches, or an entire tree may display yellowing by the late summer. By the following spring these infected branches are often dead.
It is believed that this phytoplasma is usually present by the winter preceding the symptoms. After over wintering the phytoplasma spreads throughout the summer, the detrimental effects being manifest late in the season. The bacter is almost certainly spread by phloem-feeding insects such as leafhoppers.
Because leaf yellowing can be a symptom of several diseases, the best diagnosis of elm yellows is made by examining the inner bark. The surest method of diagnosis is made by examining the inner bark in a laboratory, with electron microscopy and stains et cetera. Since this is seldom feasible, there is a short cut: an olfactory test. Elm yellows infected inner bark has a distinct odour of methyl salicylate (ie oil of wintergreen). This odour is absent in normal elm wood. The inner bark should also be discoloured, with dark streaks. Although, other diseases also cause streaks, and visible streaks are a late symptom.
Unfortunately, there is no tried and tested cure for elm yellows. The Siberian elm ( Ulmus pumila) appears to be immune to the disease. Thus planting Siberian elm instead of native species may help eliminate the problem in an urban forestry context. However, considered along with Dutch elm disease, one could conclude that planting many elms together is a risky practice. Elms seems are prone to mutually infecting one another when planted together in large numbers.. Therefore, this arborist suggests, do not plant rows of elms.
Elms are sensitive to diseases of the phloem and outer xylem in large part because of their thin sapwood layer. Elm is said to be 'ring porous', it has large vessels produced in the current year that are the major conductive elements in the xylem (wood). Most of the annual rings before the current year are conductively inactive. Consequently, any thing that blocks or kills the outer annual ring can stop all growth in that year. This same weakness explains why other ring porous trees, such as sweet chestnut, are sensitive to xylem and phloem diseases.
Read about Phytoplasmas or MLOs
Ash yellows is a phytoplasma disease that afflicts both ash trees ( Fraxinus spp.) and lilacs ( Syringa spp. ). Candidatus Phytoplasma fraxini is one of the newer names for the bacter. The symptoms on both ash and lilacs are similar, in that witches' brooms commonly form. Stunting and yellowing are also symptoms.
To diagnose ash yellows with certainty, laboratory tests with staining and fluorescence microscopy are necessary. The use of in-the-field arborist skills are often good enough for practical diagnosis. Ash trees when heavily infected tend to have a loss of apical dominance, meaning that lateral branches grow equally as long as the 'leaders'. Witches brooms are an exaggerated manifestation of loss of apical dominance, along with multiple stunted sprouts. Witches' brooms do not always form on infected ash trees. 'Yellowing' is a more universal symptom. Yellowing is the early change of the leaves to autumn like colours. Even before yellowing, summer leaves are often smaller and paler than is normal for ash.
Ash yellows are spread by phloem-feeding insects, such as leafhopper. The spread of the phytoplasma along branches can be fairly gradual. Trees tend to die by progressive branch dieback. As with elm yellows, ash yellows are not yet curable. However, the spread of ash yellows can be slowed by not planting too many ash trees together. Severely infected trees should be removed.
Read about Phytoplasmas or MLOs
Grierson, D. and Covey, S.N. 1988. Plant Molecular Biology. 2nd Edition. Blackie. London. 178-181.
Buszacki, Stefan and Harris, Keith. 1998. Pests, Diseases & Disorders of Garden Plants. Harper Collins Publishers. London. 547-588.
Scagel, R.F. , Bandoni, R.J. , Maze, J.R. , Rouse, G.E. , Schonfield, W.B. and Stein, J.R. 1982. Nonvascular Plants - an evolutionary survey. Wadsworth Publishing Co. Belmont.
Thomas, William S. 2003. Field Guide to Mushrooms. Sterling Publishing Co., Inc. New York.
Adelgids are small sap-feeding bugs in the Adelgidae family. They are allied to aphids, and resemble them in many ways. Some adelgids exude a white stringy waxy covering that makes them unappetising to predators. These are the source of the woolly fuzz often seen on conifer twigs. Some adelgids species have one host, others alternate hosts. Adelgid populations usually build up via a wingless asexual female generation. This generation reproduces by parthenogenesis, ie virgin birth. In late summer or autumn a winged sexual generation is born. These winged males and females disperse in search of new host plants.
Some adelgid species stimulate gall formation. Of the most interesting gall making adelgids is the spruce gall adelgid (Adelges abietis). These insects create pineapple shaped galls on spruce (Picea spp.). These galls in folklore are said to be spruce that have 'changed their mind', halting cone growth to make twigs instead. Indeed, the galls do look like a cross between cones and needled twigs.
Spruce gall adelgid nymphs over-winter on spruce trees. In the early spring females lay their eggs, these eggs are protected by a cover of woolly wax. When these eggs hatch in early spring into mobile nymphs. These take up residence at the bases of developing leaves (needles) on the tips of young shoots. These colonies then stimulate the growing needle bases to enlarge. Since adelgids feed close together, the enlarged needle bases swell together creating a pineapple gall. In late summer the galls break up and the winged adults are released, these search for new spruce trees.
If these galls form at the leading end of a twig, they halt the elongation of that leader. Consequently, these galls can deform the shape of a growing spruce. If there are only a few galls, one can cut them out by hand in the early summer. On trees known to be prone to adelgid attacks, one can be proactive. Insecticides can be applied after dormancy in the autumn, or before flush in the spring. This can eliminate many of the over-wintering nymphs.
Ash Flower Galls
Ash trees, especially green ash ( Fraxinus excelsior ), can develop odd witches' broom type galls on their male flowers. These growths look like a cluster of miniscule deformed leaves crowded around branch tips. Theses growths are instigated by a mite (Eriophyes fraxiniflora ). In the autumn they look like fist sized balls of tuff dangling from the branches. Such galls are fairly common in urban landscape settings. Indeed, they can be so common that some people come to believe that they are normal.
Eriophytes mites over winter in bud scales. The adult feeds on the flower buds in the spring. These mites disrupts the growth of the flowers and stimulate the development of a fringe of disfigured flowers and leaves around the bud. The mites move on to new buds when the current gall dies and dries up.
Eriophytes mites are somewhat sensitive to dormant oils. If these oils are applied just prior to bud break they can be effective in controlling the mites. However, indiscriminate use of dormant oils can kill natural predators, and make the situation worse. This exacerbating of a pest problem with pesticide application is especially prevalent for mites. Natural predators of mites are often more sensitive to pesticides than are the mites themselves.
Scale insects are small Coccoidea bugs. These create the small pale clusters of scales that often infest the stems of plants. These scales are hard waxy shields that protect the insects from predators. The insects spend most of their life stationary, feeding off plant juices which they obtain by piercing the cell walls with their stylet-like mouths. Females have no wings, and adult males are winged in some species, but not in all species. Many species rely on females that reproduce parthenogenetically. In either case, the females lay eggs under their waxy shields. When a nymph hatches it is, for a short while, without its own waxy shield. In most species the younger nymphs have some ability to crawl about. This crawling is one of the main methods by which scale insects spread. When a nymph finds a new feeding local, it settles down and excretes its own scale (waxy shield).
Luckily scale insects have many natural enemies, such as ladybird-beetles. Unfortunately, the proliferation of scales can overwhelm natural controls. Large numbers of scale insects can weaken a plant. The honey dew waste of these insects can accumulate on plants, and this can become mouldy. Because of their waxy shield the adults are little affected by contact insecticides. (Systemic insecticides, that are absorbed into the plant, often become too diluted in the plant's sap to be effective.) Therefore, the best time for chemical control is when the crawlers are moving. This crawling can be confined to a short time period, a window of vulnerability. This time window varies from one species of scale insect to another. Thus, one should watch the scales with a magnifier. When crawlers are first detected, apply the insecticide. This should kill some of the new generation of bugs, and reduce their overall numbers.
Buszacki, Stefan and Harris, Keith. 1998. Pests, Diseases & Disorders of Garden Plants. Harper Collins Publishers. London.
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