Dutch Elm Disease | Cycle | Fungus | Characteristics CHARACTERISTICS OF O. ULMI S.L. Index document Fungal metabolites

    Pathogenicity of the species within O. ulmi s.l.

    Brasier et al. {[139]} analyzed the aggressiveness of the three species within O. ulmi s.l. by testing their ability to colonize live elm bark. For this, O. ulmi, O. novo-ulmi, and O. himal-ulmi were inoculated on U. minor var. vulgaris (U. procera) and incubated at 18-22˚C for eight weeks in the dark. The mean size of the necrotic lesions per cm2 of elm bark was taken as a representative of the pathogenicity of a particular species on elm bark. Virulence of the DED fungus is controlled by temperature. At temperatures higher than 24 ˚C, O. ulmi appears to be more virulent than O. novo-ulmi NAN {[308]}. A correlation between O. ulmi s.l. virulence and tolerance to host-tree-produced mansonones could not be found {[308]}. As given in Table 3, each species of the DED fungus shows a range of pathogenicity on elm bark. O. novo-ulmi is the most aggressive species within O. ulmis.l. O. himal-ulmi produces a mean lesion size range similar to that of O. ulmi, suggesting a comparable level of pathogenicity for both species. However, determination of the ability to cause wilt clearly shows that the Himalayan fungus is far more aggressive than O. ulmi. Brasier et al. {[139],[535]} studied the vascular wilt ability of the different species by determining the percentage of defoliation of elm trees (a 4-year-old, 2 m tall clonal U. minor var. vulgaris and a 4 m tall “Commelin” elm) twelve weeks after inoculation. As presented in Table 3, the percentages of defoliation on U. minor var. vulgaris and the more resistant “Commelin” elm caused by either O. himal-ulmi or the aggressive O. novo-ulmi are significantly higher than those for O. ulmi.

    Table 3:  Pathogenicity of species within O. ulmi s.l. {[139],[ 535]}

    Characteristic

    O. ulmi

    O. novo-ulmi

    O. himal-ulmi

    Pathogenicity to elm bark
    (mean lesion size cm2)1

    4.2-4.8

    6.8-9

    4.3-4.7

    Vascular wilt ability
    (% defoliation after 12 weeks)U. minor var. Vulgaris “Commelin” elm

    10-35
    0-2

    30-100
    6-50

    30-55
    16-50

    1. Analysis on infected bark billets of U. minor var. vulgaris incubated for 8 weeks in a dark cellar at 18-22˚C.

    Et-Touil et al. {[494]} recently localized the nuclear pathogenicity gene Pat1 of O. novo-ulmi EAN on a 3.5 Mb chromosome (previously designated chrom. II). Generally, five to seven chromosomes ranging in size from 1.0 to ≥ 5.7 Mb are found for O. ulmi and O. novo-ulmi.Pat1 may be introgressed from O. ulmi. The two alleles of Pat 1 – Pat1-m and Pat1-h – appear to confer moderate and high levels of pathogenicity, respectively. Although EAN strains with the Pat1-m and Pat1-h allele do show chromosome polymorphisms, no correlation has been found with pathogenicity. The karyotypes of the strains are similar. Cloning of Pat 1 and characterization of the protein product will help to elucidate the role of Pat1 in the pathogenicity of the DED fungus. Besides Pat1, additional genes may be involved in (smaller) variations in aggressiveness of the DED fungus {[494],[ 573]}.

CHARACTERISTICS OF O. ULMI S.L. CHARACTERISTICS OF O. ULMI S.L. Fungal metabolites Fungal metabolites