Fungal Vascular Wilts of Trees  
 

Water use by trees

• Water is absorbed by roots, carried by tracheids and vessels, used in leaves in photosynthesis, and large amounts are transpired (up to 100 gal/day).
• Disruption of flow quickly results in loss of turgor (wilting), and death of parts without water.

 

Mechanisms of water flow

• Cohesive tension of water molecules allows water to form a continuous column of water hundreds of feet tall.
• Forces that move water
• Root pressure - due to active accumulation of solutes across the casparian strip.  Is a relatively small force.
• Transpiration - plants literally suck.  Water moves from a region of high Q (water potential) to a region of low Q.  Air is dry relative to the tree, which is dry relative to the soil.
• Cavitation - Pockets of intense vacuum that form due to a break in the water column.

Wilt

• A symptom - can be caused by any agent that interrupts the flow of water to plant parts (drought, mechanical girdling, canker fungi, certain bacteria)
• A category of diseases
• Specifically those caused by fungal invasion of xylem vessels and tracheids
• Caused by Ascomycetes
• Typically require a wound
• May be insect vectored, soilborne, or move through root grafts

Wood anatomy

Sapwood is a living and conducting organ which is located in the most recent ring (or rings) of wood growth.

 

• Conifer

 

Longitudinal tracheids have pits which allow lateral movement of water and solutes. Longitudinal and radial parenchyma also has pits.

 

Materials move from cell to cell through opposing pits.  All cell types are interconnected.

 

• Hardwoods

Hardwood anatomy is more complex than that of conifers.  They have vessels which are analogous to tracheids, but are more complex.  Vessels have sieve plates on the ends for longitudinal transport; and pits in the sides for radial transport.

 

Material moves from vessel to vessel through sieve plates or from one cell type to another through opposing pits.

 

• Diffuse- vs. Ring-porous
 

Diffuse-porous - little or no variation in size of vessels during the growing season.  Several year's growth can carry water.	Ring-porous - large vessels are formed in the early wood.  Most transport occurrs in current-year's vessels.  As a result, these are very susceptible to wilt diseases.

 

 

• Tyloses (Tylosis is singular) - only occur in angiosperms (hardwoods)

 

Tyloses form when membrane-bound cytoplasm from  neighboring parenchyma cells protrude through a pit pair and expand into the vessel element.  Cell wall material is then laid down

 

Permanently clogs vessel cells. Some species normally form them in the process of forming heartwood.

 

• Form in the sapwood of some species in response to injuries:  felling, pathogen invasion, etc.
• White Oak abundantly forms tyloses.  This is why this wood is favored for making whisky barrels.
• Root grafts

• Hardwoods and softwoods both form root grafts which are most common between members of the same species.
• Grafts permit water, minerals and photsynthate to move from tree to tree.
• Also permits the movement of wilt pathogens from tree to tree.

Modes of action of fungal wilt pathogens

• Induction of tyloses.
• Plugging of vessel sieve plates by fungal tissues.
• Increased sap viscosity due to degradation of cell wall materials.
• Toxin production that disrupts water flow mechanisms.
• Parasitism of parenchyma cells to cause associated vessel dysfunction.
• Cavitation.

Symptoms and diagnosis of wilt diseases.

• Symptoms
• Foliage becomes chlorotic, and later necrotic.  Individual branches will die (called flagging)
• Vascular system becomes discolored.
• Diagnosis - must observe fruiting bodies.
• Isolate the fungus in pure culture and observe the production of fruiting bodies.
• Fruiting bodies may also form on host material.
 
 

Important fungal wilt diseases of trees:

• Oak Wilt
• Dutch Elm Disease
• Verticillium Wilt