Have you ever imagined what a beautiful alley lined with beautifully vibrant trees would look like without any trees? Trees have been threatened for years by pathogens. It is well known, by many Americans, that one of the biggest threats to elm trees is the Dutch elm disease (DED). Thousands of American streets have been completely devoid of elms after being infected by this fungal disease. Many even called it the real “nightmare on elm street”.
A microscopic fungus called Ophiostoma ulmi is responsible for DED. The fungus was first detected in Netherlands around 1920 and spread in Europe and North America. Breeding programs were conducted in order to find resistant elms with desirable traits for use as landscape urban trees. In Netherlands, the breeding program began in 1928 and released two resistant clone hybrids from European elms: “Commelin” released in 1960 and “Groeneveld” released in 1963. By 1970, another variant of O. ulmi, which was in fact a new species, Ophiostoma novo-ulmi, was discovered and was responsible for much larger losses as it was more aggressive than the first species. Unfortunately, the elm clone hybrid, Commelin, was not resistant to O. novo-ulmi and years of previous research went down the drain.
This race for survival between DED causing fungi and elms is applicable for many other host-pathogen interactions such as human-variants of the flu virus. Another example that had disastrous effects on the rubber tree industry in Brazil was the fungus Microcyclus ulei that attacked Brazilian rubber trees. The Ford Motor Company established a rubber plantation in Fordlândia and Belterra, Brazil and 3,651,500 rubber trees had been planted by 1941. At the start of the 20th century, Brazil was the major supplier of rubber to the world until M. ulei struck and destroyed these plantations causing the rubber tree blight. Today, a gene (M15md) that confers resistance to the rubber tree disease exists and no rubber tree blight has been detected in Asia or Africa to date, according to data from CABI.
The use of resistant clones is an efficient method for controlling tree diseases. However, even if resistant genes are present in tree clones, resistance could be broken after some years by the variability of the fungus. Indeed, this was the case of the elm clone “Commelin” with DED as previously mentioned. In addition to resistance, when producing new plant clones, it is necessary to select other desired traits which can further complicate the resistance program.
As it takes many years for crossings and tests to produce resistant clones, do we have another method to control diseases as fungi are fast-evolving? The increase of environmental and toxicity concerns also increases the demand of developing biorational and biocontrol products. In the case of DED control, the product Dutch trig is a fungus (Verticillium albo-atrum) which controls the disease by inducing resistance in healthy trees, immunizing them like a vaccine. For the rubber tree blight,the fungus Dicyma pulvinata seems to serve as an important component in systems of integrated control, but it doesn’t control the disease in plantations. More deep scientific studies are needed for biorational products. Maybe, they will be the future for controlling fungal plant diseases.
With pathogens evolving so rapidly due to their high variability, we will always have a clear need for plant pathologists, mycologists, fungal geneticists and microbiologists. If we do not know the pathogen, its variants and how it evolves, we will never be able to fight the disease.
References:
- http://www.cabi.org/isc/datasheet/33893
- http://www.dutchtrig.com/about_dutch_trig/about_dutch_trig.html
- https://www.thehenryford.org/research/rubberPlantations.aspx
- Bettiol W. 1996. Biological control of plant pathogens in Brazil; application and research. World Journal of Microbiology and Biotechnology, 12:505-510.
- Brasier CM. 1982. Genetics of pathogenicity in Ceratocystis ulmi and its significance for elm breeding. Resistance to Diseases and Pests in Forest Trees, 224-235.
- Le Guen V, Garcia D, Doaré F, Mattos CRR, Condina V, Couturier C, Chambon A, Weber C, Espéout S, Seguin M. 2011. A rubber tree’s durable resistance to Microcyclus ulei is conferred by a qualitative gene and a major quantitative resistance factor. Tree Genetics & Genomes, 7: 877-889.
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