When we study nature, it is probably a good idea to expect the unexpected.
Certainly the richness and variety of living creatures can never fail to amaze us. One fascinating situation involves certain parasites or disease causing organisms. In several cases, a parasite passes through two entirely different kinds of host in order to continue its nasty parasitic existence. Not only are these situations fascinating, but biologists are mystified how these relationships could ever have developed.
Most fungi do not indulge in such fancy lifestyles, but there is one fungus group which does include parasites famous for their dependence on alternate hosts. The rusts include many highly specialized fungus diseases of plants. Some attack only a single kind of victim. Other rusts however require two distinct hosts in order to complete their life cycle. One of the most famous rust diseases is Cronartium ribicola or white pine blister rust. Pine (Pinus) trees, of course are conifers, non- flowering evergreens which reproduce by means of seeds borne in cones. Most of these plants are medium to tall forest trees. They are characterized by needles grouped in bundles. The white pines typically have five needles per bundle while the black pines have groups of 2-4 needles.
Pine trees are found throughout the northern hemisphere, from Alaska to Sumatra and from Europe to North America to China. The fungus was apparently originally native to white pines in Asia. At some point centuries ago, plant collectors introduced diseased material to Europe and from there it came later to North America.
The story of white pine blister rust appears uncomplicated, but looks can be deceiving. The fungus which grows on pine cannot infect another pine tree or seedling. It needs an alternate host, namely flowering shrubs of the genus Ribes (currants and gooseberries). It so happens that Ribes species are native to Europe, temperate Asia and North America. Places where suitable white pine grow also tend to have currant bushes and/or gooseberry shrubs growing on the forest floor.
During the late summer and early fall, especially when the weather is cool and moist, special fungus spores from Ribes leaves may be carried by the wind to white pine needles. These spores (called basidiospores) are delicate and survive only a couple of days. Ideally they encounter white pine within a few hundred metres. The spores germinate and grow through the needle into the bark and the fungus then overwinters in the pine victim. As the following summer dawns, the fungus produces a swollen area which breaks through the bark late in the season. Sticky drops of sweet liquid from the fungus, attract flies which inadvertently transfer special sexual cells. Following this sexual union, the fungus settles down for another winter. In the second spring, swollen white to yellow blisters appear where the sticky areas were formerly. These blisters are full of spores called aeciospores. The fungus on the pine keeps growing farther on the stem. It may kill a whole branch or a whole seedling. More and more aeciospores are produced season after season. The aeciospores however cannot infect another pine tree.
White pine aeciospores can only infect a Ribes bush. These spores are tough and can spread hundreds of kilometers, carried by the wind. Once they light on a suitable Ribes bush, they invade the leaf. Within two weeks, orange uredospores (rust) appear on the bottom of the leaf. These spores infect more currants or gooseberry leaves. In the fall, dark spores called teliospores appear instead of uredospores. When conditions are cool and moist, each teliospore produces a short club-like growth bearing four basidiospores. The basidsiospores are delicate and short lived and can only infect pine. All fungus stages on Ribes die when the leaves drop off in the fall. Only in pine trees does the fungus survive the winter.
The obvious way to control this disease is to eliminate one of the hosts. Since white pine trees are very valuable, the prudent course of action is to eliminate Ribes. In central Canada and the eastern United States, such efforts have lagged of late, but they certainly helped. In western North America however, the native shrubs are so common, and so hard to kill, that the initiative was soon abandoned and the blister rust continues to extend its toll.
There are other rusts with interesting host choices. In many grain cultivating regions, wheat rust is the most famous disease which exhibits an alternation of hosts. It all begins with thick walled teliospores which overwinter on wheat stems. In the spring, each teliospore develops a tiny club like structure on which four delicate basidiospores are produced. The basidiospores, spread by the wind, land on the leaves of a small shrub called barberry (Berberis). There are native shrubs of this genus in most wheat growing regions. The fungus firstly develops sweet sticky pustules on the barberry leaves. These pustules attract flies which inadvertently spread the sexual cells. Once sexual union is accomplished, the fungus vigorously develops aeciospores on the lower leaf surface. The aeciospores, unable to reinfect barberry, are carried by the wind to wheat stems. Subsequent growth on wheat stems and leaves results in large rust coloured areas releasing myriad uredospores. These can and do infect other wheat stems. Eventually in the fall, the fungus produces dark teliospores which again overwinter on wheat stubble.
It might seem that if barberry bushes were eliminated, the fungus would likewise be gone. This program was indeed successful in some European countries. Similar programs were undertaken in North America. During the 1930s in the United States, many jobless citizens were employed in barberry eradication programs. There were still barberry shrubs in Canadian gardens during the 1950s, but later they too were rooted out. Within the past 4-5 years however, popular new resistant cultivars of barberry (from Japanese stock) have appeared on the market.
In view of the immense importance of wheat to the Canadian economy, this country has devoted a lot of money to research on the rust fungus. Eliminating the alternate host (barberry) unfortunately did not result in control of the disease here. Non-hardy rust coloured uredospores are able to survive the winter in the southern United States. This is a repeating stage which is able to reinfect wheat. Thus the need for an alternate host is bypassed. As the Canadian spring appears, uredospores proceed northward on native grasses.
Other examples of such lifestyles include malaria (a protozoan or single celled animal) which exploits both humans and some mosquitoes, and a worm parasite which exploits aquatic snails and water birds. Swimmers’ itch develops when the worms mistake humans for suitable bird hosts. In people, the worms die under the skin, causing an allergic reaction. Other similar worms in tropical regions exploit humans and snails.
Fascinating as all these stories are, the really interesting question is how these parasites came to exploit, in alternate fashion, two highly different hosts. While few of us would be sorry to see such organisms disappear, we must admit to a respect for such an ingenious plan. Blind processes never exhibit such design skills. The ability of the parasite to find and infect each host at an appropriate time of year and at an appropriate stage in the parasite’s life cycle, is a clear illustration of purpose and planning.
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