The most important bacterial forest pathogens are scorches and yellows.

Bacterial Scorch

• 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.
• Features:
  • 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
• Symptoms:
  • discoloration, wilting, scorching and curling of leaves
  • possibly defoliation

This disease is often observed in trees after stress such as drought.

Yellows

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
• symptoms:
  • often see witches' brooming
  • leaves small, malformed, wavy and yellow
  • branch dieback and stunting
• examples
  • ash yellows
  • elm yellows (=elm phloem necrosis)
  • black locust witches' broom
  • pecan bunch

Viral Diseases

Viruses are microscopic, acellular organisms. They posess nucleic acids, composed of either single-stranded or double-stranded RNA or DNA. They also have a protien 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 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:

• 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