Spreading fungus, not cheer

Not everyone wants a real Christmas tree—after all, they only last one season and make a mess with all that dropped foliage—but many do like to have the choice. Unfortunately, levels of Dothistroma pini, a toxic fungus that causes red banded lesions on pine needles and premature defoliation, could affect such decisions.A number of countries in the Northern hemisphere, including the United Kingdom, France, and Canada are reporting an upsurge in the severity and distribution of D. pini infection. Milder and wetter weather conditions are partly to blame, as the spread of this pathogen is favored under moist, warm, light, and sheltered conditions.Anna Brown, who heads the Dothistroma Project at Forest Research, UK, told The Scientist: "This disease has become particularly noticeable over the past 2 years and has apparently been increasing in frequency for 5 to 6 years. In the 2002–2003 period, we found over a quarter of the trees were suffering serious defoliation in the East Anglia forest district, where large numbers of the highly susceptible Corsican pine are planted."New Zealand and Australia have been coping with widespread D. pini infection for decades and have been quite successful in protecting their pine forests. The fungus rarely kills the tree, but the percentage of pine needles infected is directly proportional to loss of wood yield, so D. pini can have a severe economic impact.One new approach that could be fruitful is to genetically engineer trees that inactivate, remove, or prevent synthesis of toxin by the fungus. The toxin, dothistromin, is thought to be the primary cause of red band needle blight and is structurally related to the potent carcinogen aflatoxin, produced by some fungi from the genus Aspergillus. This similarity has enabled scientists to identify and isolate key genes involved in the synthesis pathway.Rosie Bradshaw, from Massey University in New Zealand, is one of a small number of researchers worldwide who is focusing her efforts on taming the feisty fungus. Her team has knocked out some of the genes that are essential for synthesizing dothistromin. "We now have the means to determine if dothistromin is responsible for pathogenicity and are currently trying to infect whole pine needles, before trying seedlings in the lab. We also have GFP reporter strains, so we'll be able to track where and when the dothistromin genes are expressed. If dothistromin production is essential, it will be a good target for disease control," she told The Scientist. One option would be to use non–toxin-producing strains as biocontrol competitors with the toxigenic fungi in the field. Others include engineering trees that express dothistromin-specific antibodies in the needles or targeting destruction of the toxin in the plant tissue by enzymatic means. Another approach is the deployment of toxin transporter genes into host plants. These transporters are normally used by the fungus to allow secretion into plant tissue and to provide a self-protection mechanism against the effects of its own toxin. Bradshaw has already isolated a candidate dothistromin transporter from D. pini that could be used in this way.Bradshaw acknowledged that genetic modification of trees is not necessarily the best option for disease management. "But in the long term, it may well be our detailed knowledge of the molecular biology of the pathogen and its toxin that will provide the basis for sustainable control," she said.So, where does that leave the Christmas shoppers who want nothing more than a glass of mulled wine and a pine tree to decorate? "I would think people probably wouldn't buy a tree with Dothistroma simply because they aren't very pretty—unless you're a pathologist," said Alex Woods, a regional forest pathologist in British Columbia, one of the areas worst affected. "That said, I did notice an advertisement for Wal-Mart of people picking out pine Christmas trees that all looked pretty badly defoliated by a foliar disease, and they only had the current years growth on them... Seriously! But I guess only a pathologist would have seen that."

Spreading fungus, not cheer

Increase of red band needle blight means that Christmas tree may not be quite so pretty

Dec 22, 2004

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