Dutch Elm Disease | Cycle | Fungus | Life Cycle The saprophytic phase Index document Life Cycle

    The pathogenic phase

    Ophiostoma ulmi s.l. starts its pathogenic phase by invading the living elm xylem. Although the pathogen may gain access to the host tree through root grafts and wounds made by beetles for breeding or emergence (adult H. rufipes), transmission of O. ulmi s.l. to the healthy elms primarily occurs via wounds made during twig crotch feeding of the DED vector {[230],[536]}

    .

    Photo 28: Penetration of a pit membrane by O. ulmi (Courtesy of W.L. MacDonald and H.S. McNabb, Jr., Iowa State University of Science and Technology, Ames, Iowa, USA)

     

    Photo 29: Xylem vessel of an elm infected with O. ulmi s.l. The fungus (stained blue) penetrates the pits. Picture produced with a light microscope.

     

    The beetle wounds may give the DED fungus direct access to the host's water-conducting vessels (primary transmission). Inoculation of an elm tree with O. ulmi s.l. mycelium is less effective compared to inoculation with fungal spores {[324]}. The location and depth of the (feeding) wounds – which are reported to vary among beetle species – influence the rate of direct transfer of the fungus to the elm xylem. In this respect, the timing of beetle feeding activities appears to play an important role. In the northern parts of the US, most large water-conducting vessels are formed from mid-May to early July {[331]}. These new early-wood vessels located just beneath the bark can be reached easily by chewing beetles. Later in the season, access to the new vessels is hampered, since additional latewood tissue will have been laid down outside the layer of early wood {[230]}. Following mycelial colonization of the feeding groove, the DED fungus may grow secondarily into the functional xylem. The humid environment and high nutrient content found in feeding wounds provide excellent opportunities for growth and sporulation of O. ulmi s.l., particularly in dry, hot climates {[333],[371]}. Webber {[371]} considers secondary xylem infections to be more likely to occur than primary infections. O. ulmi s.l. genotypes isolated from xylem beneath the feeding wound frequently appear to be the same as those present in the groove itself.

    Within the feeding groove, competition between different O. ulmi s.l. may occur. Clumps of Sporothrix- and Graphium-type spores brought into the wound are likely to be of the same genotype and can be expected to cooperate during host-tree colonization. However, ascospores will represent individuals that differ genetically. In particular, genotypes of distinct VCGs may antagonize and compete during colonization. The growth rate and pathogenic abilities of a certain isolate will determine whether the individual colonizes a major or a minor part of the elm tree.

    Once inside the xylem vessels, O. ulmi s.l. can be transported vertically by the host's transpiration stream. All conidia of O. ulmi s.l. are able to bud in a yeast-like fashion. In submerged laboratory cultures and in xylem vessels, the yeast-like form of the O. ulmi s.l. spores dominates {[230],[536],[606],[608]}.

    Upward movement of the DED fungus to stem parts above the point of xylem entry may occur a few minutes after inoculation {[230]}. Banfield {[327],[610],[611],[612]} observed a rapid stem invasion due to transport of fungal spores in the large xylem vessels of the youngest annual growth ring. Pomerleau et al. {[607]} reported a very rapid transport of 32P-labeled O. ulmi spores to elm shoots and leaves. Accumulation of the radioactive material was mainly detected in leaf veins, nodes, and points of petiole attachment. Elgersma et al. {[742]} showed that the rapidity of O. ulmi and O. novo-ulmi spread in DED-susceptible U. hollandica c. “Belgica” was comparable. However, survival of the aggressive O. novo-ulmi in the latter trees was proven to be better than that of the non-aggressive O. ulmi. O. ulmi s.l. strains impaired in their ability to sporulate within the xylem vessels appear to be avirulent {[604],[605],[606]}.

    Although much slower than upward movement, downward transport of O. ulmi s.l. spores does occur {[11],[613]}. Usually, it takes days to weeks after the appearance of the first external DED symptoms for the fungus to move 3 m downward. In comparison to O. ulmi, O. novo-ulmi shows a much faster downward spread {[11]}. It has been suggested that gravity and reversal of the sap flow as a result of saturation deficits in the basal part of the tree (especially in the blocked vessels) play a role in downward spore transport {[327],[612]}.

    The velocity of spore dissemination shows seasonal variation. While in dormant trees it can take the spores up to several weeks to move a few inches from the point of inoculation, in the leafy season the conidia can spread throughout the trunk and crown within just a few hours {[327],[610],[611],[612],[613]}.


    Photo 30: Scanning electron micrographs of mycelial growth of O. ulmi s.l. in elm xylem vessels.

    O. ulmi s.l. causes disintegration of the end walls of newly formed vessels. Subsequently, the spores can move freely to the end of the vessels {[230]}. By mycelial growth and penetration of secondary cell walls and intertracheary pit membranes, the fungus can reach adjacent xylem vessels (Photo 27 to 31). Sporulation of the fungus will follow. MacDonalds et al. {[616]} reported that when conidia closely appressed to a vessel wall germinated, their germ tubes usually directly penetrated cell walls. In addition, germ tube penetration of border pit walls was observed. Penetration of intertracheary pit membranes and cell walls by hyphae and germ tubes occurs within five days after inoculation. The diameter of the germ tubes is smaller than the diameter of the spores from which they arose. The O. ulmi hyphae expand following penetration. Krause et al. {[85]} reported that O. novo ulmi is able to penetrate susceptible American elm callus to a depth of more than 75 cell layers in 60 hours. Penetration of the callus of resistant U. americana or U. pumila was limited to 30 and 18 layers, respectively. In comparison to secondary cell walls, penetration of the intertracheary pit membranes occurs more frequently {[616]}. The fungus is able to extend its hyphae with a relative ease across/around the periphery of the lumen of an occluded vessel. The infection proceeds around the ring vessels of the earlywood {[230]}. Radial colonization of the host tree – which can be detected as early as one day after infection of the xylem – requires fungal growth through living parenchyma and the penetration of vessel and fiber cells {[230],[609],[614],[615],[616],[617]}.

    Host resistance mechanisms (e.g., thickening of the vessel walls) restrict radial growth of the DED fungus in living trees {[230]}. In roots, large xylem vessels (>50 mm in diameter) are more diffusely distributed in the growth rings than in stem tissue. In addition, relatively small clusters of thick-walled fibers separate the vessels. As a result, movement of the DED fungus from one vessel to another will be relatively easy {[230]}. Recently, Scala et al. {[11]} showed the absence of O. ulmi and O. novo-ulmi in symptomatic or symptomless leaves of stem-inoculated elm trees. In the experiments, only the stem xylem vessels and branches were colonized by the two fungi.

    Photo 31: The perforation plate at the end of a xylem vessel hampers fast movement of O. ulmi s.l. Fungal spores have to germinate and penetrate the barrier.

    After death of the elm, O. ulmi s.l. rapidly grows through the ray parenchyma cells, resulting in a saprophytic colonization of the inner as well as of the outer sapwood {[609]}. Highly pathogenic and fast-growing isolates will probably dominate the outward movement of the DED fungus from the xylem to feedback into the O. ulmi s.l. gene pool and contribute to the saprophytic phase.

     

The saprophytic phase The saprophytic phase Life Cycle Life Cycle