Forest and Shade Tree Pathology
Bacterial and Virus Diseases
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Bacteria are microscopic, unicellular, prokaryotic organisms. They may be spherical, rod-like, spiral, mycelial, or pleomorphic.
Examples and Symptoms of Bacterial Diseases
- leaf and shoot infections: water-soaked lesions that become necrotic and macerated or slimy, mostly on agronomic crops.
- bacterial canker of cherry
- crown gall: hypertrophy of stem and root tissue
- fire blight: wilted, necrotic leaves
- wetwood (though it may not be a disease and may not be caused by bacteria! - see details below): slimy flux from branch stubs, cracks, etc.
- scorches: leaves yellowed at margin, with dead tips
- yellows: witches' brooming; small, malformed, wavy and yellow leaves; branch dieback; and stunting
- infection: Pathogen enters plant through stomates, wounds in leaves and roots, or via insect vectors
- spread: plant pathogenic bacteria have no spores, and cannot be wind-disseminated
- insect vectors are very important, especially in the propagation of scorch and yellows
- some can also be carried by splashing rain
- in host or in resting stage of vector
- some in soil
1. Wetwood can be defined as wood in living trees that:
- is non-conducting but has a high moisture content and appears watersoaked
- is somewhat darker in color than surrounding wood
- has a fetid, fermentative odor
- is occupied by bacteria
- occupies the heartwood of some tree species on a normal basis:
- conifers: firs and hemlocks primarily
- hardwoods: elms, poplars, birches, oaks
- also may form in response to wounding in wood internal to the wound
2. Wetwood has a bad reputation in the logging and wood products industries. Let's face it: it stinks, is loaded with bacteria, and pressure may build up, squirting the foul liquid on a hapless logger (leading to the appellation piss-fir in some parts). In landscape trees, the liquid may ooze from pruning wounds, cracks, etc., become colonized by a dog's breakfast of microbes, become slimy, and may kill bark that it contacts.
It is associated with a variety of problems during wood products production:
- Wood is more difficult to dry and requires more energy.
- Wood dries unevenly and may warp and twist.
- During kiln drying, acid vapors cause kiln corrosion.
- It is associated with ring shake and honeycomb, two lumber defects. Ring shake in elm leads to the term "onion elm" in the lumber trade.
3. For some time, wetwood was considered a tree disease caused by bacteria.
Later research showed that:
- Wetwood is often the normal condition of heartwood of mature trees in species in which it occurs.
- Wetwood can be formed following wounding under conditions that preclude bacterial growth.
- Wetwood appears to be wet in part because of accumulation of calcium and magnesium salts, lowering the osmotic potential. A drier transition zone with living parenchyma separates sapwood from wetwood.
- Wetwood is colonized by facultatively and obligately anaerobic bacteria that bring the oxygen content far too low for fungal growth
- The bacteria also produce volatile, low-molecular-weight organic acids (acetic, propionic and butyric acids, which are responsible for the odor and kiln corrosion) that are inhibitory to fungi.
4. This led to the conclusion that wetwood is formed by the tree itself during heartwood formation and as a response to wounding. It is a favorable environment for the growth of bacteria that create conditions inimical to the growth of root- and butt-rot fungi.
Thus, rather than being a disease, wetwood, at least in some species, appears to represent a mutualistic relationship between trees and bacteria in which the bacteria create conditions that help defend the tree from decay fungi.
The most important bacterial forest diseases are scorches and yellows.
- Pathogens have been called:
- Rickettsia-like bacteria because of their similarity to that group with their small size and rough wall.
- Fastidious, Xylem-Infecting or -Limited Bacteria (FXIB or FXLB) because they are hard to culture (fastidious) and found in the xylem only.
- Best known is Xylella fastidiosa; we don't know if the other pathogens in the group belong in the same species.
- obligate parasite, difficult to grow in culture
- limited to the plant xylem
- tree hosts are elm, oak, Norway and red maple, mulberry, pear
- vectored by "sharpshooter" leaf beetles or leafhoppers, which feed on xylem
- discoloration, wilting, scorching and curling of leaves
- possibly defoliation
This disease is often observed in trees after stress such as drought.
This is a common disease in the northeastern United States.
- pathogen: once thought to be caused by viruses, but actually phytoplasmas (formerly known as mycoplasmas or mycoplasma-like organisms, MLO's)
- phytoplasmas lack cell walls, and are thus pleomorphic
- limited to plant phloem
- vectored by phloem-feeding leafhoppers
- 150 tree diseases are caused by phytoplasmas
- often see witches' brooming
- leaves small, malformed, wavy and yellow
- branch dieback and stunting
- ash yellows
- elm yellows (=elm phloem necrosis)
- black locust witches' broom
- pecan bunch
Viruses are acellular organisms too small to be seen individually with a light microscope. They posess nucleic acids, composed of either 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
- stunted growth
- decreased photosynthesis and increased respiration
- reduction in cold tolerance
- rarely, death results
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 vectors are required for entry into plant cell
- biological vectors: aphids, leafhoppers, fungi, mites, nematodes, beetles
- others: water, soil, other plants, organic debris
Examples of viral diseases:
- tomato ringspot virus, tobacco mosaic virus:
- green ash: ringspots or overall yellowing of leaves
- tobacco necrosis virus:
- willow: brown or necrotic lesions on leaves
- Norway spruce: possible effect on growth
- tomato mosaic virus:
- willow: brown necrotic lesions on leaves
- red spruce: up to 50% growth reduction in seedling experiments
Last modified 27 May, 2007