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Pathogen Biology

The Ophiostoma species that cause Dutch elm disease grow and reproduce only within elms. At times they are parasites, feeding on living tissue of the elm tree; at other times they are saprophytes, getting nourishment from dead elm tissue. Ophiostoma ulmi caused the original Dutch elm disease epidemic in Europe and North America in the mid-1900s. Ophiostoma novo-ulmi, an even more aggressive pathogen of elms, largely replaced O. ulmi during the second half of the 20th century. These fungi spread within stems and roots of living elms both by passive transport of spores and by mycelial growth of colonies initiated by spores that germinate in the xylem. The mycelium of these fungi is creamy white (Figure 5) and is composed of septate hyphae with haploid nuclei.

Figure 5

Asexual reproduction
Ophiostoma ulmi and O. novo-ulmi have two asexual forms that produce asexual spores called conidia. In the xylem vessels of living elm trees, small, white, oval conidia (Figure 6) are formed in clusters on short mycelial branches. These conidia are carried in the xylem vessels where they reproduce by budding, germinate to produce mycelium, and thus spread the disease throughout the tree.


Figure 6	Figure 7

In dying or recently dead trees, conidia (Figure 7) are produced by mycelium growing in the bark and in tunnels created by beetles just under the bark. These sticky conidia are produced at the tips of 1-2 mm tall synnemata. Each synnema consists of hyphae fused to form an erect, dark stalk with a round, nearly colorless head of sticky spores. Beetle vectors carry the sticky spores to new elm trees.

Sexual reproduction
Based on the structures produced by their sexual stage, the Dutch elm disease pathogens are placed in the ascomycete genus Ophiostoma. When two mating types come in contact, ascospores are produced in spherical, black, long-necked perithecia (Figure 8). Perithecia form in the bark, either singly or in groups. Ascospores are produced in asci that degenerate inside of the perithecia. The free ascospores are discharged at the opening of the perithecial neck where they accumulate in sticky droplets that may be disseminated by beetle vectors.

Figure 8