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Red Rust of Wheat

By Al Lowe
Contributor

Did you ever see a cloud of red dust rising from the field, as a farmer harvested his wheat? Or grain stubble with long black streaks on it? Or small yellow 'cups' on the underside of a Common Barbarry leaf? These are all part of the life cycle of the infamous Wheat Rust (Puccinia graminis).
This disease has a very complicated life cycle. It is a fungus plant, and so reproduces by means of spores. But this one actually produces four totally different kinds of spores during its annual life cycle. The most noticeable part of the cycle, and the costly part, is the red rust stage.
In the spring and early summer when wheat is growing fastest, rusty red blisters appear on the young plants. These are just the spore part of the disease. The plant itself grows all through the wheat in the form of a mass of tiny microscopic threads (biologists call this mycelum). It will grow in both the stems and leaves of the plant. It also grows very quickly, and multiples rapidly. The red-coloured spores are called urdiospores, and can infect more and more wheat at the grain crop grows.
As fall approaches, the mycelium changes its pattern, and now produces teliospores. These are black, and show up as streaks on the stems and leaves of grain. The disease goes through the winter in this black-spore stage. The spores have very thick walls, enabling them to withstand the cold weather, even here in Northern Ontario. The black spores cannot infect wheat at all.
In the spring, a very odd thing happens. These teliospores start to develop two little stems. On each of these are four small round spores, called basidiospores. These cannot infect wheat either, but they can grow on the Common Barberry (not the cultivatede Japanese Barberry). These microsopic spores develop a little cup-shaped lesion on the underside of the Barberry leaf. The cup is full of small round spores, called aeciospores. And it is these which can infect the young wheat, starting the cycle again.
So this fungus disease has a very, very complicated life cycle - it involves four different kinds of spores, and two entirely different plants. To stop it, you need to break the life cycle in some way. Of course, the simplest thing to do is to get rid of the Barberry. This is exactly what has been done (or tired) for a great many years. barberry has been eliminated from great areas of Canada and the U.S.
As with everything, there is a catch or two. For one thing, the red rust spores can be carried for immense distances by the wind - from Mexico to the Canadian prairies, for instance. For another, in mild winters, the red spores may not all be killed, so in the spring, they just skip the black and barberry stages, and re-infect the wheat directly.
Agricultural scientists are continually coming up with varieties of rust resistant wheat. These are not infected very much by the Wheat Rust. But this lasts only for a few years, until the rust itself adapts so that it can infect the new crop. Then comes another rust-resistant variety, and so on. The battle between crop scientists and Wheat Rust goes on and on. This disease costs millions and millions of dollars every year. Only constant research enables us to keep ahead of it.