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A smut by any other name

Smut is one of the few diseases that’s not a hot button issue this winter. But true loose smut can cause headaches for barley growers. Here’s how to keep it out of your fields

Covered, loose, false, true, stinking, stem, common. Apparently, a smut exists for every occasion. Smut is a broad term that refers to a variety of fungal diseases that affect cereal grains on the Canadian prairies. Each smut is caused by a specific fungi which attacks a particular grain and produces slightly different effects. One thing all smuts have in common: they are as ugly as their name.

The smut that receives the most attention on the Prairies is the true loose smut (sometimes referred to only as loose smut) of barley, which is a seed-borne infection cased by the fungus Ustilago nuda. Barley is the only cereal grain in Canada that can be downgraded because of the presence of smut and the tolerances are low as the numbers from the primary grades determinants table show.

Surface-borne smuts are not commonly found. Other cereals have a much lower rate of infection than barley, for example, smut in oats is very rare and CWRS wheats are fairly resistant to smut — only 20 per cent of the samples surveyed in 2014 showed any signs of infection. Durum wheat, though, is more susceptible with 50 per cent of the samples showing levels of infection.

How TLS works

TLS presents some challenges to growers because in the first year, there are no visible signs of infection. Then, in the second year, by the time there are visible signs of smut in the field, nothing can be done to prevent that year’s losses.

tls-tolerance-levels-barleyTLS is a seed-borne disease so it does not survive in the soil or in other plant material. During flowering, spores from smutty heads fall onto the florets of healthy plants and infect the developing seeds. The fungus remains dormant inside the seed embryo for the winter. Visually, a grower cannot tell infected seeds apart from healthy seeds.

In the spring, when infected seeds are planted, the fungus also starts to grow undetected inside the seemingly healthy plant. It ultimately attacks the head and replaces the healthy grains with clumps of dusty, black spores, which spread via wind and rain to healthy plants, starting the disease cycle all over again. The grower has no visual clues of the level of disease in the field until the smutty heads actually appear.

While low levels of TLS can be found across most of the Prairies, infections tend to be higher in areas — or during years — with higher moisture and cooler temperatures in July. Cool (16 to 22 C) wet weather lengthens the flowering period and provides ideal conditions for the spores to spread thus increasing the possibility of infection. Six-row barley is more susceptible than two-row barley because it has a longer flower season.

TLS on the Prairies

Smut has been around for decades. European settlers likely introduced it to the Prairies along with the first seed they brought. Losses due to smut have been severe at times in the past but the occurrence of true loose smut on the Prairies peaked in the 1980s and 90s. Since then, levels have consistently dropped due to the introduction of effective seed treatments and resistant cultivars. Hot, dry weather in July has also played a part in bringing the levels down.

Currently, TLS in barley is not a hot-button issue. Yield losses due to diseases like common root rot or seedling blight are higher. But, like any pot that’s on the backburner, TLS levels still need to be watched. Yield losses are rarely above one per cent, but if a heavy outbreak occurs, they can be as high as 40 per cent.

Of the barley fields tested in 2014 by Dr. James Menzies, Research Scientist and Plant Pathologist with Agriculture and Agri-Food Canada, and his colleagues, about 50 per cent had TLS at extremely low levels — less than 0.1 per cent. There were more unusual incidents of fields with levels as high as 25 to 30 per cent TLS.

The percentage of TLS in the field directly translates to the percentage of yield loss, so a grower could take quite a hit with a TLS infection of 25 to 30 per cent — which would mean 25 to 30 per cent less yield as well as a reduced grade. Luckily, though the nutritional value of the grain may be compromised, TLS is not toxic, so it can still be used as feed.

tls-resistant-barley-varieties

Menzies observes that smut often emerges in barley grown for feed as well as in areas where there aren’t high returns on the crop. In these cases perhaps smut is monitored less vigilantly because the economic value of feed barley tends to be underestimated by growers. Also, seed is often used again and again when growing feed barley. If TLS levels are not monitored, over the years it slowly builds up and can result in more significant losses.

Counting in the field

Growers can take clear, simple steps to keep their smut levels in check. The first and most effective, Menzies suggests, is to monitor. By walking through their fields during flowering, growers will have an immediate sense of the level of smut. Menzies emphasizes the importance of actually counting heads and calculating a percentage rather than just estimating. Because smutty heads are so conspicuous and alarming, at first glance, farmers often assume their levels of infection are much higher than they really are. If they walk down a row and actually count, say 1,000 heads, they will discover that the percentage of smutty heads is often much lower than they first thought. For the purposes of their research, Menzies and his colleagues will count 1,000 heads in three or four different parts of the field.

If the grower calculates less than one per cent smutty heads, then the seed can be planted the following year without treatment, but, Menzies maintains, at one per cent or higher, the seed could be planted again only if it has been treated with a systemic fungicide. Then at higher levels — some recommend five per cent — the seed is not worth planting again even with treatment and the grower should invest in certified, disease-free seed. There are no hard and fast guidelines, so farmers need to monitor and weigh their decisions carefully.

Testing in the lab: another way to monitor

Post-harvest, farmers may want to send a representative sample of seed to an accredited lab and have it tested for True Loose Smut. For a cost of about $60, the lab will determine its presence as a percentage of infected seeds. As a side note: Labs can’t test for surface-borne smuts; a grower will receive results only for True Loose Smut. The rate of TLS in the field the following season is often slightly lower than the level identified in seeds in the lab because some plants can outgrow the pathogen especially if growing conditions are ideal.

Results from the lab test can be used in the same way that counting smutty heads in the field can be. Depending on the levels, a grower can decide to: 1) plant the seed again; 2) plant the seed with treatment; or, 3) buy new disease-free seed.

Producers can refer to the seed guide to choose barley varieties with good resistance to TLS. AC Oxbow, AC Metcalfe, AC Meredith, CDC Stratus, CDC Select, and AC Bountiful are some of the resistant varieties available. Growing a resistant barley also means that farmers won’t have to treat seeds for TLS.

Best practices: a quick recap

On the whole, TLS is being kept to fairly low levels across the Prairies. To maintain these low levels farmers can continue to:

  1. Pre-harvest: Monitor fields and calculate percentage of TLS present. Or,
  2. Post-harvest, send seed for testing to an accredited lab to determine TLS levels.
  3. At levels of less than 0.1 per cent, plant the seed again without treatment.
  4. At levels between one and five per cent, treat the seed with a systemic fungicide before planting. Choose the appropriate treatment in consultation with an agronomist, taking into account other disease issues you might be facing. Saskatchewan Agriculture provides a list of treatments for TLS (find it online at: http://www.agriculture.gov.sk.ca/seed-borne-diseases-cereals.)
  5. At levels over five per cent, buy new disease-free seed, preferably of a resistant variety.

Smut is not difficult to conquer. According to Dr. Jim Menzies, “As long as farmers make a point of checking for smut, they aren’t likely to have issues with it.”

About the author

Contributor

Patty Milligan is a freelance writer based at Bon Accord, Alta.

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