Figure 8. Assessment of disease incidence in American elm seedlings infected with the fungus and treated with defense elicitors.

(a) Images showing the disease symptoms of Dutch elm disease as brown streaking on the wood of infected American elm seedlings (left) versus tissues without apparent disease symptoms (right). (b) The rate of disease incidence was evaluated based on the occurrence of brown streaking on wood tissues (80 cm above the inoculation point) two months after inoculation with O. novo-ulmi (10⁷spores/ml). Four-year-old American elm seedlings were treated with 0.01% ethanol (Control), SA (2 mM) at the 1^st day of inoculation, MeJA (100 μM) at 1^st day of inoculation or SA (2 mM) at 1^st day followed by MeJA (100 μM) at the 4^th day of inoculation. (c) Stem tissues (80 cm above the inoculation point) were collected from control and treated seedlings at 60 days post-inoculation, frozen in liquid nitrogen and then used for quantifying the expression of fungal actin (On-Actin). The expression of On-Actin was normalized to that of three reference genes (EIF 5a, vacuolar ATP synthase and splicing factor 3B) and was calculated relative to the control sample. Values marked with an asterisk (*) are significantly lower (P < 0.05) than control.