Transmission of double‐stranded RNA and a disease factor in Ophiostoma ulmi

Abstract

A diseased isolate of Ophiostoma ulmi was found to contain 10 segments of double‐stranded RNA (dsRNA) with molecular weights ranging from 2.40 x 106 to 0.23 x 106. In contrast seven other healthy isolates in the same vegetative compatibility group contained either no dsRNA or up to four dsRNA segments. Transmission of the disease to healthy isolates by hyphal anastomosis was accompanied by‐transmission of the 10 dsRNA segments. In a genetic cross in which the diseased isolate acted as the female parent single‐ascospore progeny were healthy and either contained no dsRNA or only one segment of dsRNA. When elm trees were inoculated with diseased isolates, subsequent reisolations were healthy and retained only two to seven of the dsRNA segments or were diseased and retained all 10 dsRNA segments. Following conidiogenesis a diseased isolate gave rise to single‐conidial progeny which were either slow growing and diseased, like their parent, with all 10 dsRNA segments, or faster growing like healthy isolates. Some of the faster growing conidial isolates retained only two to seven of the dsRNA segments and were disease‐free. However a majority of the faster growing conidial isolates retained all 10 dsRN A segments and were shown to carry the disease in a latent form. The possibility that the disease of O. ulmi is conferred by specific segments of dsRNA and the potential of d‐factors for the control of Dutch elm disease are discussed.

Molecular and cellular aspects of Dutch elm disease

“…Although the présence of dsRNA is often associated with viral infections in other disease states (Brasier 1986), no viral particles hâve been observed in dMnfected O. ulmi (Rogers et al 1987). The d 2 -factor transcribes 10 dsRNA species of différent sizes which co-purify with mitochondria of affected isolâtes (Rogers et al 1986a). The présence of spécifie dsRNA segments of 2.4,1.0, and 0.33 kb are correlated with the disease in the fungus (Rogers et al 1987) and it may eventually be possible to assign responsibility for the symptoms to spécifie dsRNA molécules.…”

A comparison of thirteen d‐factors as potential biological control agents of Ophiostoma novo‐ulmi

“…The colony morphologies of the same isolates were assessed after a further 10 days in diffuse light, as described by Brasier (1981). see Webber & Brasier (1984); Rogers et al (1986). EUSG, European NAN vc supergroup; see Brasier (1988); GSG, Guadalajara NAN vc supergroup; see Brasier (1988) and Brasier & Kirk (1991); EANSG, EAN vc supergroup; see Brasier (1988).…”

“…For long-term storage, isolates were maintained on 2% MEA slopes at -20ЊC, and under liquid nitrogen. Isolates for dsRNA extraction were grown on 2% MEA in 9-cm Petri dishes overlain with deplasticized cellophane discs (Rogers et al, 1986;Rogers, 1987), and maintained in the dark for 14 days at 20ЊC.…”

“…Mycelium from cellophane cultures was ground to a fine powder in liquid nitrogen. The dsRNA was extracted, essentially as described by Morris & Dodds (1979) and Rogers et al (1986), but with two consecutive phenolchloroform-isoamylalcohol extractions.…”

“…D-factors are believed to be potential biological control agents for Dutch elm disease (Brasier, 1986b;Webber, 1987Webber, , 1993, and to have influenced the outcome of recent epidemic events (Brasier, 1988(Brasier, , 1990. Most d-factors are associated with multiple double-stranded RNA (dsRNA) segments (Rogers et al, 1986(Rogers et al, , 1988Rogers, 1987), and some cause abnormal mitochondrial function, probably via the induction of mitochondrial plasmids Charter et al, 1993). D-factors share certain similarities, including dsRNA, with some forms of transmissible Cryphonectria hypoviruses in the chestnut blight system (Day et al, 1977;Van Alfen, 1986) that are associated with a natural remission of the disease in parts of Europe (e.g.…”