Bacterial and Viral Diseases

Bacterial Diseases

Bacteria are microscopic, unicellular, prokaryotic organisms. They may be spherical, rod-like, spiral, mycelial, or pleomorphic (variably shaped).  Bacterial leaf spots and blights, shoot blights, and cankers can be important on woody horticultural crops, but generally are not common on forest and landscape trees.  We will cover several subjects under bacteria:

  • Drippy Nut of Oak
  • Bacterial Cankers
  • Yellows
  • Bacterial Leaf Scorch
  • Crown Gall
  • Wetwood (separate page)

Drippy Nut of Oak

Drippy nut of oak was described in 1967 ​[12]​.  It affects Quercus agrifolia (coast live oak) and Q. wislizenii (interior live oak) in California, especially the central coast area.   The disease was caused by a new species of bacterium, Erwinia quercina.  The pathogen was later moved to Brenneria and now to Lonsdalea ​[3]​. (The disease name is awesome, and aren’t you glad they didn’t first call it “Erwinia blight” or some such? The disease name would probably be changing with the pathogen.)

Developing acorns are infected through wounds caused by insects.  They later drip a sticky syrup.  Beyond reducing seed production, the disease has little impact on the trees.  However, it can be a nuisance to people when it drips on patios, vehicles, etc. and dries as yellow spots.

For many years the curious disease seemed limited to California.  However, it was eventually found in Spain in 2003 ​[2]​.  There, the pathogen also causes cankers and a drippy bud symptom on Quercus spp.

In 2010 the pathogen was identified in urban oaks in Colorado ​[23, 25]​.  There, in addition to causing the drippy nut symptom, it is associated with a scale insect.  The bacterium infects areas colonized by the scale, and the two together appear to cause more damage than either do alone.  Trees can be heavily damaged and require removal.

It appears that this pathogen may cause different sets of symptoms depending on the insects available locally to create infection courts.

Bacterial Cankers

There are at least two bacterial cankers of Populus spp. (poplars).  One, caused by Xanthomonas populi, is a damaging disease of poplars, even in natural stands, and can also infect Salix spp. (willows).  It is found in western Europe, including the UK and Ireland.  Other regions monitor for the disease so it can be eradicated if it is found early enough.  Infection occurs through leaf scars and small wounds ​[18]​.  Cankers may begin as small, oozing blisters on small stems and branches.  Older cankers may look like irregular galls or severely roughened bark.  Foliage on cankered branches and stems may get black spots, then die.  Mechanical failure of cankers, branch dieback, and even mortality may result.  The disease is managed by breeding resistant varieties.

Relatives of Lonsdalea quercina, cause of drippy nut of oak, can cause cankers on various hardwoods ​[15, 24]​.  These include L. brittanica, L. iberica, and L. populi.  To keep things confusing, there are also some residual species of their former genus, Brennaria, that cause cankers of woody plants, including a Brennaria populi ​[14]​.

Lonsdalea populi has caused substantial damage in hybrid poplar plantations, both in Europe and in China.  In Europe, it has been identified in Spain and Hungary ​[1, 24]​, and likely exists elsewhere.    In China, it has been found in Henan and Shandong provinces ​[14]​.


Yellows diseases can cause debilitation and death in affected hosts.  They are widespread in the eastern United States.  Symptoms include leaves that are chlorotic, small, sometimes malformed with wavy edges; witches’ brooms; branch dieback; stunting; growth loss; and death.

The pathogens are phytoplasmas.  These are bacteria without cell walls, previously known as mycoplasmas or mycoplasma-like organisms (MLOs).  Since they have no walls, they are pleomorphic.  They are obligate parasites.  In contrast to the scorch pathogens, which are limited to xylem, yellows pathogens are limited to phloem.  Like the scorch pathogens, they are vectored by insects, in this case phloem-feeding insects, leafhoppers.  And like the scorch pathogens, they exist as host-specialized, genetically distinct populations.

The yellows phytoplasmas have been placed in the genus Phytoplasma, and several species have been described in it.  According to bacterial nomenclature, these are considered “candidate” taxa because they have not yet been cultured.  Therefore, the full name of the genus is “Candidatus Phytoplasma”.

Elm yellows (elm phloem necrosis)

Elm yellows is a damaging disease of elms in the United States that often occurs in well-defined epidemics.  These epidemics often kill most elms in affected areas, and have sometimes proved disastrous to elm breeding programs and those designed to control Dutch elm disease ​[7, 22]​.  It appears that the distribution of the disease and occurrence of epidemics may be related to climate and weather that determines abundance of the vectors.

Elm yellows has apparently been present in the midwestern United States since at least the latter 19th century ​[22]​.  It infects elm species native to North America, but generally not those of Eurasian origin.

Symptoms begin with rootlet mortality, then progress to necrosis of phloem in larger roots and the lower stem ​[22]​.  This is followed by leaf drooping (epinasty), yellowing, and casting.  Foliar symptoms may be limited to portions of a crown.  Inner phloem becomes discolored with darker flecks and may eventually die, leading to an alternate name of the disease, elm phloem necrosis.  Ulmus rubra may also produce small witches’ brooms in the second year of infection.  Trees may die within several years of first symptoms.

Elm yellows is widespread in Europe, where it generally causes less damage than in North America. Because Eurasian elms are more resistant or tolerant than American elms, it is suspected that the pathogen originated in Eurasia ​[17, 22]​.  Tolerance or resistance often characterizes hosts that have coevolved with a pathogen. 

The phytoplasma that usually causes elm yellows (now known as Candidatus Phytoplasma ulmi) is sensitive to certain antibiotics, but antibiotic injections at best delay progression and are not useful in practice ​[22]​.

Ash yellows

Ash yellows, caused by Ca. Phytoplasma fraxini ​[9]​, was a very difficult disease to characterize, and it was not recognized until the 1980s.  It causes slow growth and branch dieback of ash (Fraxinus) species that is usually indisinguishable from symptoms caused by adverse environmental factors such as drought, shallow soils, or chronically wet soils.

Other symptoms are more helpful in diagnosing ash yellows.  Reduced shoot growth may cause short internodes and a tufted appearance at twig tips and the overall crown.  Leaves may be small, light green, with upturned margins, and fall coloration may occur prematurely.  As dieback begins, witches’ brooms may occur at the base of the trunk.  The brooms are considered diagnostic, but they only occur on a small subset of infected trees.

Ash yellows causes considerable volume reduction.  Dominant trees may slow growth markedly, while intermediate and suppressed trees may die.

Bacterial Leaf Scorch

Bacterial scorch is a bacterial disease often characterized by a scorch symptom – marginal necrosis, often with a yellow border, blackening, and curling of leaves. Leaves may also discolor, wilt, and drop off. These are similar to the symptoms of acute drought, but the disease itself often appears in trees after stress such as drought.

Hosts are many and varied, including both horticultural crops and forest trees ​[20]​. In grapevine the disease is known as Pierce’s disease, which causes substantial losses and prevents grape production in some areas. Hosts also include pear, peach (phony peach disease) almond (almond leaf scald), plum (plum leaf scald), periwinkle (periwinkle wilt), citrus (citrus variegated chlorosis), and the respective leaf scorches of elms, maples, mulberry, oaks, and sycamore ​[10, 11]​. Each host group has host-specific variants of the pathogen, indicating a highly specialized host-pathogen relationship ​[4, 20]​.

Bacterial leaf scorches are fairly common in the hardwood forests and shade trees of the eastern United States.

The bacteria that cause these diseases are somewhat unique. They were first isolated and pathogenicity proven in grapevine only in 1978 ​[6]​. They are sometimes called fastidious, xylem-limited bacteria (FXLB) because they are hard to culture (fastidious or picky about their food) and are found in the xylem only. They are also sometimes called rickettsia-like bacteria because of their similarity to that group with their small size and rough wall. They are obligate parasites.

Taxonomy of the group is difficult. Most pathogens are put in the species Xylella fastidiosa, but there are many host-specific variants that are genetically distinct ​[4, 11, 21]​. The current practice is to divide the species into subspecies, with further host-specific variants in some of the subspecies ​[20]​.  I predict that the subspecies will be raised to species rank in the not-too-distant future.  You heard it here first.

The fact that the pathogen is xylem-limited gives a clue to the mechanism of disease. Bacteria multiply in xylem vessels, producing gummy polysaccharides and eventually preventing water flow. This is the mechanism of a wilt disease.

The pathogens are vectored by sharpshooter leaf beetles or leafhoppers. Evidence suggests that vectors can transmit the disease effectively in tree nurseries ​[19]​.

Xylella fastidiosa is a growing problem in agriculture.  A strain of it causes citrus variegated chlorosis, a serious disease of citrus, and a new epidemic of Pierce’s disease in the Temecula Valley threatens the wine industry of southern California.  Although it was long known only in the Americas, it is now killing century-old olive trees (olive quick decline syndrome) in southern Italy, where olives and olive oil are major exports ​[16]​.  It has been found in other parts of Europe and Asia and is considered a global threat.

Crown Gall

Crown gall is so named because it is characterized by tumors that often form on or near the root crown.  It is caused by the bacterium Agrobacterium tumefaciens.

Crown gall is primarily a problem in agriculture, mostly with woody plants.  Some people call just about any gall on a tree crown gall, but I am usually hesitant to accept the diagnosis.  There are other things that can cause galls in trees, and it takes a lot of specialized work to prove that galls found in a population of trees are really crown gall.

According to Kado ​[13]​:

Fresh crown galls are relatively sturdy and hard. As these galls age beyond one year, they appear convoluted with cavities and become friable. Insects such as earwigs commonly reside within these cavities. It is difficult to isolate A. tumefaciens from aged galls when most of the diseased plant tissue is dead. Aged galls are easily detached.

This doesn’t sound like the large woody galls that are often attributed to crown gall.  Although there are over 650 hosts, only a small proportion develop tumors of appreciable size ​[8]​.  Conifers are generally resistant.

The most fabulous thing about this disease is the mechanism of pathogenesis.  The bacterium, which can survive and grow in the soil associated with roots, carries extrachromosomal DNA in a circular plasmid.  Small wounds are the infection court.  When the bacterium encounters wounded cells, it attaches and injects the plasmid, which is taken up by the host cells and incorporated into host chromosomes.  The new genes cause higher production of hormones, such as auxin and cytokinins, that stimulate localized growth.  The resulting galls provide a nutritious environment for the pathogen.  Eventually the galls decompose and the bacteria return to the soil.

In some cases, the pathogen can move up into the stem and branches, initiating galls higher in the plant.

Viral Diseases

Viruses are acellular organisms too small to be seen individually with a light microscope. Their genome is single-stranded or double-stranded RNA or DNA. They also have a protein coat, and occasionally a lipid envelope.

Symptoms and effects of viral diseases

Symptoms are often confused with mineral deficiency, ozone damage, or drought. Many say that viral diseases in trees are unimportant, for the effects are often subtle.

  • leaves are mottled with necrotic and chlorotic lesions,
  • ringspots, and yellowing
  • Effects:
    • stunted growth
    • decreased photosynthesis and increased respiration
    • reduction in cold tolerance
    • rarely, death results

Disease cycle

Viruses are obligate parasites, and require living cells to replicate. Once entry into the cell is obtained, the host’s nucleic acids, amino acids, and enzymes are recruited by the virus for replication, placing additional demands on host metabolism.

  • infection: wounds and often vectors are required for entry into plant cell
  • spread:
    • biological vectors: aphids, leafhoppers, fungi, mites, nematodes, beetles
    • others: water, soil, other plants, organic debris

Examples of viral diseases of trees

  • Tomato ringspot virus and tobacco mosaic virus are known to infect green ash, causing ringspots or overall yellowing of leaves.
  • Tobacco necrosis virus can infect various hosts, including Populus spp. and conifers including Pinus sylvestris and Picea abies ​[5]​. In many hosts it is transmitted by root-infecting fungi and is confined to roots, so foliar symptoms may not be evident.  Although it has been found in deteriorating clones of Populus tremuloides, effects on growth are unclear.
  • Tomato mosaic virus, like tobacco necrosis virus, can infect willow, causing brown necrotic lesions on leaves.  It has also been shown to infect Picea rubens (red spruce).  Seedling experiments suggest that it can cause up to 50% growth reduction, but no impacts have been demonstrated in nature.

Bacteria and viruses are special and challenging groups of tree pathogens.


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