Potato Pointers: Understanding Super Pathogens
May 10, 2010 I had a frightening conversation with a colleague of mine a few days ago. Like me, she is a plant pathologist, only she works with cereal crops. She mentioned that there is a new strain of cereal stem rust, first reported from Uganda, currently sweeping through the Middle East. The Ugandan point of origin has lent the new “super pathogen” a portion of its name: UG-99.
This new threat got me thinking about what it takes to make a pathogen more aggressive and more dangerous. It also occurred to me that those of us who work with potatoes have had to deal with our own super pathogen: Phytophthora infestans, the causal agent of potato late blight, on a regular basis.
New strains of late blight began to appear in the U.S. and Canada in the early 1990s. These new strains were not only very destructive, but they had other new and ominous capabilities. One characteristic of the new strains was resistance to metalaxyl, the most effective fungicide that had been developed up to that time for management of late blight.
This was a very serious development, but these strains had some other characteristics that made them even more of a challenge to manage. It seemed that they were, well, more “pathogenic.”
The Aggressive Organisms
There are a number of ways in which a pathogen can become more of a problem. One way is that a new pathogen strain could simply be “more aggressive” than the normal strains. One of the ways the new strains of late blight are more aggressive than those they have almost replaced is that they produce a greater number of reproductive structures. (For late blight, these are called “sporangia.”)
Larger numbers of sporangia increase the likelihood of greater spread and therefore add to the organism’s “aggressiveness.” It was also determined that the new strains of late blight produced sporangia more rapidly. More rapid reproduction is a perfect recipe for increased aggressiveness.
More aggressive might also mean the lesions produced by the pathogen get larger faster and, as an obvious result, the pathogen becomes more destructive. The new strains of blight have placed a different twist on this strategy. It seems they have a greater ability to produce lesions on the stems of infected plants.
This characteristic makes the new strains more destructive because a single infection site can readily take out an entire stem of a potato plant and all of the foliage above the lesion. Many multiple lesions in the leaves would be required to give the same result.
The stem lesions have also proved to be more difficult to protect than the leaves of the plant. Often the stems are shaded by the foliage, and getting fungicide on the stems to protect them has proven to be significantly more difficult than the more straightforward task of protecting leaves.
Destructive Strains Emerge
Another way for a pathogen to become more pathogenic would be the appearance of new strains that are capable of infecting and reproducing at slightly higher or slightly lower temperatures than the “normal” strains. Using the same line of reasoning, if a production area were to experience temperatures higher or lower than normal, the types of disease and the seriousness of the diseases could change dramatically.
A recent example of a potato pathogen with an expanded geographical range due to changes in how it responds to temperature is the colder temperature-tolerant strain of the brown rot/bacterial wilt pathogen (designated as race 3 biovar 2) of Ralstonia solanacearum reported from Europe a few years ago.
Working To Stay Ahead
So, what does this mean for the future? The more aggressive, metalaxyl-resistant late blight strains seemed to show up overnight and they had all but taken over within a year or two. In the case of the new stem rust strain, UG-99, the organism is able to overcome several of the bred-in resistance genes that have protected many of the most popular wheat varieties currently grown worldwide.
We will no doubt be plagued by other super pathogens in the years to come. It’ll take our best efforts to stay ahead in this game.
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