Biological DED control using enemies of O. ulmi s.l.

Natural enemies of O. ulmi s.l. may provide an opportunity for the biological control of DED. Although a number of these organisms and microorganisms have been described, current research primarily focuses on the introduction of virus-like factors into populations of O. ulmi s.l. (see Enemies of O. Ulmi s.l.). These d-factors probably induce disease by generating defective mitochondria, resulting in disruption of the fungus cytochrome oxidase respiratory system. So far, thirteen d-factors that have a negative effect on O. ulmi s.l. xylem infection have been found; their effect ranges from slight to strong {[8],[643],[855]}. Typical characteristics of d-factor infection are reduction in growth rate, CU production, perithecia formation, and germ-tube vigor. In addition, the conidial viability is low and the competition ability towards other microorganisms present in the host can be lost {[8],[105],[531],[855]}. D-factors are cytoplasmatically transmitted through anastomose. Infection of O. ulmi s.l. isolates probably occurs during the saprophytic growth phase in the bark of weakened or dying elm trees (see Life cycle of O. Ulmi s.l., {[643]}).

The decline of the first European DED pandemic in the 1940s is thought to be the result of viral infection of the DED fungus {[855]}. Likewise, the decline of the current worldwide DED pandemic may be achieved by d-factor-mediated weakening of the O. ulmi s.l. population. Webber {[643]} suggested that for biological DED control, a d-factor should be introduced that exhibits a moderate effect on xylem infection of the fungus, e.g., d⁵, d⁶, or d¹³. To be effective as a biocontrol agent, the d-infected O. ulmi s.l. fungi must be highly competitive and retain their ability to establish new colonies {[592]}. The moderate effect of the d-factors on their fungal host will be sufficient to prevent healthy elms from becoming diseased. Furthermore, O. ulmi s.l. infected with the ‘moderate' d-factors will be regularly transmitted by insect vectors, enabling maintenance during the saprophytic bark-to-bark cycle. Another approach would be to introduce a mixture of d-factors and leave nature to select the fittest {[8],[643],[855]}.

A disadvantage of using virus-like factors for DED control is their restricted transfer between O. ulmi s.l. individuals belonging to different VCGs. Therefore, this control measure may be effective only in regions where O. ulmi s.l. exhibits a low VCG diversity, such as in New Zealand and the USA {[855]}. The DED fungus may lose deleterious d-factors during sexual reproduction as well as during the mycelium-yeast switch in the elm xylem. Single cell budding during this pathogenic phase results in a proportion of d-factor free cells that show a higher growth rate than d-infected ones {[643]}. Many d-factors reside in the fungal mitochondria. The release of genetically modified d-factors or O. ulmi s.l. isolates that carry a copy of the virus in their genome may help prevent loss of the viruses.

Although d-factors appear to specifically infect O. ulmi s.l., no definite data are available {[855]}. Thus, the long-term ecological effect of the introduction of d-factors cannot be accurately predicted based on the current knowledge {[592]}. As a result, at the moment the use of virus-like factors cannot be considered an environmentally safe method for DED control. Notably, the latter conclusion applies to the introduction of genetically modified viruses or fungi. Changes in the genetic constitution of O. ulmi s.l. and/or d-factors may render them pathogenic or more pathogenic to other organisms. Recombination events may create new genetic combinations with undesirable traits. Therefore, the unexpected and unpredictable effects of introducing genetically modified organisms into the environment will hamper their large-scale use in the battle against DED.