Late seeding across a lot of the Prairies due to wet spring conditions last year meant a fair amount of leaf, stem and stripe rust occurred in wheat crops, and Alberta reported incidences of stripe rust which appeared to have overwintered.
Rust diseases generally blow in as spores from the southern U.S. in early summer, but it is possible for them to overwinter. If late-maturing wheat is still in the field when winter wheat is planted, rust spores from the spring wheat can infect the newly emerging winter wheat crop, which serves as a “green bridge.” With adequate snow cover the disease can survive throughout the winter in the winter wheat and subsequently infect spring wheat in the following year.
Although it is theoretically possible for leaf and stem rust to overwinter under certain conditions, it is extremely unlikely as the fungal spores are localized on the leaf and stem surfaces and are usually killed off once winter comes.
It’s easier for stripe rust to overwinter because it grows in a systemic fashion and can travel down into the leaf tissue itself. “You can get quite a bit more infection with stripe rust,” says Tom Fetch, a research scientist at Agriculture and Agri-Food Canada’s Cereal Research Centre in Winnipeg. “And it can overwinter as urediomycelium inside the leaf. As long as you get some snow cover to keep the leaf temperature above -4 C, the fungus will remain alive inside that leaf tissue. Stripe rust can then just grow and re-sporulate when air temperatures get above 5 C and you get an earlier infection in the spring time.”
Using different crop rotations, especially in areas where farmers plan to grow winter wheat, may help to reduce the risk of large stripe rust disease populations overwintering.
Although the overwintering of stripe rust is something new to the Prairies, the general rust situation in Western Canada appears to be unchanged, especially for leaf and stem rust, neither of which seem to be showing up any earlier or in areas where they have not been seen before.
Choose resistant varieties
The first line of defence against rust is to choose resistant varieties. There are still plenty of varieties that are resistant or moderately resistant to stripe, leaf and stem rust, and the resistance genes currently in use in most North American varieties are holding up fairly well. Fetch recommends that growers check provincial seed guides, however, to make sure the varieties they choose are resistant, especially in the case of leaf rust, as there are some varieties which are still highly susceptible to this strain. As well there is greater variability in the population of leaf rust, which seem to change a lot more than stem rust. “With stem rust it has been one strain that has dominated in North America for about half a dozen years now,” says Fetch.
There are plenty of foliar fungicides which give effective control of leaf, stem and stripe rust in wheat, and provincial crop protection guides provide information about which ones are registered for use on wheat rust. Fungicides are best applied early, around the flag leaf stage, and before the disease becomes too established.
The earlier that farmers can seed their wheat the better the likelihood of avoiding the rust spore inoculums that first arrive typically in late June. Under ideal conditions, rust diseases cycle to create new spores in eight to 10 days. By planting early and getting healthy plants that are at a more advanced growth stage before the spores arrive, there are fewer cycles of rust and less inoculum is produced.
“Farmers always want to plant as early as they can,” says Fetch. “But if you have a choice between different crops you may want to plant your wheat first rather than a crop like canola, which you plan to spray anyway. The wheat can mature first and avoid the rust, so you won’t have to spray that wheat as well.”
To detect overwintering of stripe rust, early and regular scouting is recommended, beginning in late May or early June. Stripe rust requires moisture on leaves for several hours and temperatures around 10 C to infect plants, whereas leaf and stem rust will develop better in overnight temperatures ranging from 15 C to 20 C. Generally hot (above 15 C) and dry conditions discourage stripe rust development. The earlier farmers can detect rust diseases, the more time they will have to evaluate the risk and make decisions about whether it is economical to spray based on yield potential
Ug99: an update
It was recently announced that two new varieties of wheat that are resistant to Ug99, a stem rust that was first discovered in Uganda in 1999, have been released in Kenya. Ug99 is a highly virulent strain that has spread quickly across eastern Africa and into Yemen and Iran. Much is still unknown about these new varieties, however, including which genes are giving resistance. “The biggest factor in longevity of resistance is the number of genes in that line,” says Tom Fetch. Ug99 has already overcome a gene used in resistant wheat varieties in Ethiopia.
Fetch is involved in a huge research collaboration called the Borlaug Global Rust Initiative, which brings together scientists from many countries in a common objective of reducing the world’s vulnerability to stem, yellow, and leaf rusts of wheat.
Ug99 has recently been confirmed in Tanzania, Mozambique, Zimbabwe and South Africa, but its spread to South Africa is of most concern for some other wheat growing areas. From South Africa there is a possibility that the Ug99 spores could arrive in both Australia and South America. Based on recent meteorological data, rust spores would most likely arrive in Brazil on wind events that regularly occur between South Africa and South America, and could then spread into the US and Canada. “That’s how soybean rust got introduced into North America,” says Fetch.
Monitoring is currently being done in Brazil, but to date no Ug99 inoculum has been detected in susceptible trap plots there. Meanwhile research continues to find genes that are resistant to the strain and which can be incorporated into future wheat varieties. †