Winter wheat is ecologically different than other crops due to the overwintering dormancy stage that kills off many diseases and weeds; therefore, winter wheat isn’t prone to much disease or weed interference. But there are a few specific things to look out for to manage in the early stages.
Wheat is the only cereal that is seriously affected by Wheat Streak Mosaic Virus (WSMV). It causes stunted growth in wheat plants and lower seed production.
WSMV is transmitted by the wheat curl mite and by leaf rubbing. Mites can be blown from field to field by the wind and can overwinter on winter wheat. Its development depends on the population of mites, virus-infected wheat plants, and sufficient moisture for good plant growth and rapid mite reproduction. A severe outbreak can occur when there is an abundance of mites in a spring wheat field and a field of winter wheat is planted early next to it.
Winter wheat will rarely show symptoms of WSMV until spring. Symptoms become more pronounced when temperatures rise above 10°C in the spring. Dashes, streaks, and yellow stripes will appear on leaves parallel to the veins and will become increasingly mottled until the leaves die. Infected plants have stunted growth from the time the infection took place. If the infection took place during the early tillering stage the plant will stop growing and produce few to no heads. If the plant gets infected in the late tillering to early jointing stages there can be head formation, but the flowers may be sterile. With a late-season infection, flowers can be fertile but kernels will be reduced in size. Fall-infected plants will not produce grain the following spring.
WSMV can be controlled by preventing transmission by eliminating the “green bridge.” The green bridge is when a maturing spring crop is close enough in proximity to allow the transfer and survival of viruliferous mites. The green bridge can be eradicated with a seven to ten day break between the drydown of the spring wheat crop and the emergence of the winter wheat crop.
A Fusarium Head Blight (FHB) infection can be a problem and cause downgrading in winter wheat. Early flowering is the best way to try and escape the prime FHB infection period, so plan on seeding early in September if you are concerned about this disease. It is also a good practice to avoid irrigation at flowering to reduce the risk of FHB.
Leaf rust is common disease in winter wheat mainly in Saskatchewan and Manitoba. With round lesions largely confined to the leaves, they should be easy to spot; however, they are much smaller than those of stem rust. Each pustule develops orange-red unrediniospores and as the plant matures the pustules will darken.
Spores can overwinter on straw and germinate in the spring. In Western Canada, leaf rust infections are usually observed in June and will peak in August. Resistant cultivars are avialble and folicur fungicides are effective in treating leaf rust. Spraying is recommended if the threshold is met in three of five spots sampled in the field.
Stem rust, like leaf rust, is also more common in Saskatchewan and Manitoba, but instead of pustules forming on the leaves, you will see them on the stem, and they will have a darker, brick red colour. As the plant matures, pustules will darken to a black colour.
Spores can be seen in southern Manitoba and Saskatchewan in mid to late July. The severity of the disease will depend on weather conditions, time of arrival, and growth stage of crops, and it will rarely overwinter. Resistant cultivars are avialble and folicur fungicides are effective in treating stem rust at the very early stage of disease development.
Stripe rust is a disease more common to Alberta that can defoliate and shrivel kernels. With this disease you will see elongated yellow pustules that develop on leaves and extend lengthwise on the leaf. This disease will affect juvenile and adult leaves as well as the kernels.
Stripe rust can overwinter in a mild winter. It is best to scout for stripe rust from emergence in fall 35 to 45 days before harvest. Resistant cultivars are avialble and folicur fungicides are effective in treating stripe rust.
Leaf spotting diseases can be caused by one or a combination of leaf spotting pothogens causing tan spot on leaves and potentially infecting wheat kernels causing red or pink smudge and black point. Severely infected kernels can result in significant downgrading of seed quality.
Managing leaf spotting diseases starts with the seed: try and ensure you are planting disease-free kernels. Proper tillage, crop rotation practices, and fungicides are also management practices for leaf spotting diseases.