You’re out in your wheat field, inspecting your crop. It’s been a good season, a bit wet earlier in the year, but the plants are growing strong and it looks like you’ll have a good yield come autumn. Then you spot something odd — a bunch of the plants have leaves covered in dark stripes that are leaking a translucent ooze, and the glumes have black bands running through them. They’re clearly unwell, but what’s making them sick?
You’ve likely got black chaff, a blight caused by the Xanthomonas translucens bacterium. To give you some help in identifying, controlling, and eliminating it in your fields we contacted Mike Harding, a plant pathology expert at Alberta’s Ministry of Agriculture and Rural Development.
“On the Canadian Prairies the bacterium is most likely introduced on seed,” he said. “It doesn’t survive well in soil and has a hard time surviving our winters.” While it can overwinter on crop residue and perennial grasses that is more of a problem in the U.S. with its milder winters than up here in the Great White North. Most years the sub-zero temperatures are sufficient to wipe out any bacteria that remain in the field.
Introduced on the seed, the leaf striping and black chaff don’t manifest right away. “When the conditions are humid and between 15 and 30 C the bacteria will multiply rapidly,” Harding said. On the leaves, where the infection usually starts, it causes brown stripes of bacterial leaf stripe. “Then when it rains or is irrigated it will splash up onto the head and causes the typical black chaff symptom, purple and black stripes on the glume.”
The classic sign distinguishing black chaff from other blights and diseases is the yellow- to cream-coloured ooze the leaf lesions exude during humid weather. It’s this ooze that makes the blight spread so widely. Though not as capable of being spread by the wind as fungal spores, the bacterial ooze can easily be transferred to other plants by irrigation water, driving rain, insects, people, and machinery, where it sets up new infections.
Though the disease is not considered as great a threat to wheat growing regions as rusts or other infections it can still seriously affect a farmer’s bottom line. “There’s two things that happen that take money out of a grower’s pocket,” Harding said. “The first is that the stripes reduce the photosynthetic capacity of the upper leaves, reducing yield. Number two is that when it gets up onto the glumes it’s not a serious yield problem but it can move through the glume into the seeds, discolouring them and downgrading them. If that harvested grain is then used for seed the next year it can lead to a really serious epidemic.”
In the United States farmers have reported losses of up to 40 per cent of yield to black chaff damage. Due to our climate, which doesn’t favour overwintering, you’re unlikely to see losses that high, but Canadian farmers have reported losing five to 10 per cent of their estimated yield to black chaff.
Managing black chaff
How does a farmer deal with black chaff? “The pillar of management for black chaff is clean, disease-free seed,” Harding insists. “That’s the best place to break the disease cycle. If you’re not rotating your crops away from susceptible host plants like wheat then that can fan the flames of an epidemic. It doesn’t survive that well over the winter so if there isn’t a lot of crop residue around it’s only going to be able to come in on seed.” One growing season without wheat or other cereal grains, then starting with fresh, certified disease free seed is usually enough to remove the problem, as black chaff only affects grasses and cereals.
If you have a persistent problem despite using clean seeds and rotating away from cereal grains then Harding recommends looking into black chaff resistant cultivars. The issue here is that the resistance is only partial — black chaff being less of a threat to vast swathes of agriculture than funguses, there has not been as much pressure to breed resistant strains. Check your seed guide and contact wheat specialists at your agriculture ministry to find out what varieties may be right for your needs.
Though not as devastating as some crop diseases black chaff is not something you can ignore or to which you an afford to turn a blind eye. It’s ability to reduce yields and downgrade quality can put a serious squeeze on your bottom line. Thankfully it’s rarely persistent in Alberta fields and can be eliminated with some basic measures.