Last summer, bacterial leaf streak was found in fields in southern Alberta. For best control, avoid infected seed
Bacterial leaf streak (BLS) and black chaff occur in cereal crops around the world. In 2010, BLS in wheat was declared an emerging problem in Minnesota, North Dakota, and South Dakota. In 2013, it was found in a number of fields in southern Alberta.
BLS can cause significant yield loss on some cereal varieties, but, as is the case with other disease issues, its development is dependent on weather conditions and susceptible plant host presence.
The bacterium causing the disease is xanthomonas translucens.
“The disease can develop and become severe rapidly after the crop reaches the heading growth stage,” said Michael Harding, crop diversification centre research scientist with Alberta Agriculture and Rural Development.
Symptoms and spread
BLS symptoms appear after the crop has reached the heading growth stage. The disease can affect yield by reducing grain filling and kernel numbers.
“This bacterium can cause elongated water-soaked lesions on leaves, which later turn light brown,” said Harding. “As the disease progresses, lesions coalesce to form large stripes or streaks along leaves. The bacterium can also infect the glumes, causing black, longitudinal stripes or bands on the glumes and awns.”
The glume infections are referred to as “black chaff.”
“During periods of leaf wetness, lesions and plant tissues surrounding them feel slimy to the touch,” said Harding. “When plant tissues are dry and humidity is low, the same leaves will have a shiny appearance.
“Leaves look as if they were glazed or frosted with a thin sugary glazing. In this case, however, the glazing consists of millions of dry bacterial cells awaiting transport to another leaf or plant.”
Bacteria are transferred from one leaf to another during periods of leaf wetness.
• SEE ALSO: “Disease geometry” from Country Guide, April 2014.
Wind provides leaf movement, allowing for the localized spread of bacteria from plant to plant. “As the pathogen is spread through contact with diseased plants, fields may have initial hot spots or patterns of diseased plants running parallel with wind direction,” said Harding.
The bacteria can be spread by plant-visiting insects, and the bacteria can survive in soil organic matter for an undetermined period of time and on or within the seed.
If BLS is detected, no fungicide application can control it. “As fungicides are highly targeted to disrupt fungal metabolism and respiration, they will rarely affect bacterial metabolism and/respiration,” explained Harding.
Bacterial diseases, like BLS or black chaff are often seed borne. “Using certified, disease-free seed is an important prevention practice, as is using spring wheat varieties with bacterial stripe resistance,” said Harding.
“Bactericidal seed treatment can also add an extra level of prevention of seed borne outbreaks, but seed treatment of infected seed won’t prevent disease outbreaks, so it can’t replace the use of disease-free seed.”
Although there are a number of compounds registered in Canada for the control or suppression of bacterial diseases, Harding is not aware of anything registered for use on cereal crops to control BLS or black chaff.
Black chaff and BLS rarely cause serious yield losses. However, in extreme cases, losses of up to 40 per cent have been reported in the U.S.
BLS and black chaff are rarely seen at economic levels in Western Canada, yet, on occasion, there have been existing conditions that have led to an outbreak or epidemic.
“Some seed borne inoculum could introduce the disease to a field,” said Harding. “Then, conditions of high humidity, intermittent rains, and/overhead irrigation can compound the problem, leading to moderate or severe disease pressure.”
In 2013, there was higher than usual levels of BLS and black chaff in Alberta. “There were a handful of fields with measurable yield loss, but there will be a large amount of harvested grain infested with the bacterium,” said Harding.