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Stemphylium blight of lentil on the radar screen

Stemphylium blight showed up as a blip on the radar screen again in
2009, causing yellowing of lentils in several areas of Saskatchewan.

Much is yet to be learned about stemphylium blight on lentil. (Photo courtesy of Saskatchewan Ministry of Agriculture.)

However, researchers really do not know if stemphylium blight is
causing significant yield loss, or if it might be helpful to growers by
aiding drydown. “The disease did seem to be a larger issue in 2009,
judging by the number of reports we received, but we are still a long
way from understanding the impact the disease has on lentil
production,” says Faye Dokken, provincial specialist in plant disease
with the Saskatchewan Ministry of Agriculture in Regina. 

Dokken explains that it is generally believed that stemphylium blight
is on the rise in Saskatchewan. In 2007, stemphylium blight was
relatively severe and widespread after a dry but relatively humid
growing season. However, in 2008, the disease was observed in less than
a quarter of the lentil crops surveyed in Saskatchewan, with trace
average incidence overall.

On lentil, stemphylium blight initially appears as small, light beige
lesions on the leaves or leaflets, says Dokken. While the disease is
most readily apparent when blighted leaves are noticed at the top of
the canopy, it is likely present under the canopy as well. Eventually,
smaller lesions merge to produce larger, irregularly shaped lesions
that can kill entire leaflets and branches.

Additionally, Dokken says that prolonged moist periods promote further
infections and give the upper canopy a grey-brown appearance. Infected
leaflets may fall to the ground, and serve as a source of spores for
future infections of a wide range of plants. When examined through a
hand lens or microscope, older lesions will appear dark brown and fuzzy
(due to fungal spore production), and leaves will be twisted and
rolled, as they have been desiccated by the pathogen.

Stemphylium spp. are common saprophytes, but under the right conditions
and in the presence of a susceptible host, these species will take
advantage of the opportunity to behave as a pathogen on a wide range of
crops. Often, but not always, Stemphylium. spp. will exploit a
predisposing factor, such as frost, heat or chemical damage, or even
another pathogen that causes damage initially. Stemphylium botryosum is
generally considered to be the culprit in lentil infections; however,
little is known about the host specialization. “As growers become more
aware of stemphylium blight, it is more likely to be recognized, but we
still don’t know if the disease is causing significant yield losses, or
simply occurring at a time when leaf-drop encourages the crop to dry
down and expedite harvest,” says Dokken. “While the fungus may be
damaging the plant, it can be difficult to provide a definitive answer
to the common question, “why did this happen this year?”

She says that the trend for occurrence seems to be a dryish spring
followed up by a wet and humid July. Preliminary research at the
University of Saskatchewan has found that Stemphylium botryosum prefers
warm temperatures (above 25 degrees C), and a minimum of eight hours of
leaf wetness for optimal disease development on lentils. However, the
pathogen can remain infectious even if the wet conditions are
interrupted by six- to 24-hour dry periods. High relative humidity
greater than 85 per cent is also favourable for this disease.

Dokken says it is interesting that under moist conditions, a proportion
of stemphylium spores will germinate at temperatures as low as five
degrees C, and the spores are capable of producing multiple germ tubes,
which may enhance their potential for infection. Upon infection through
leaves, the fungus will spread through the plant cells, feeding on the
available water and nutrients, and leading to death of the cells and
the appearance of symptoms on the plant.

Because of the on-going infections over the past several years,
Stemphylium spp. appears to have found a way to survive cold
Saskatchewan winters, spreading and initiating infections through the
release of wind-blown and rain-splashed spores from the crop debris in
the summer. While infection may lead to seed staining, a decrease in
seed size and low germination rates and the pathogen has been isolated
in seed testing laboratories, the significance of seed transmission of
stemphylium blight in Saskatchewan is unknown.

Control methods unknown
At this time, there are no fungicides registered for control of
stemphylium blight on lentil. Dokken explains that while the value of
crop rotation should never be downplayed, its effect in this cause may
be limited because Stemphylium spp. can survive in the absence of a
host. Generally, however, when crop residue has been buried or broken
down, inoculum levels will be reduced, she says. “The best-case
scenario for avoiding stemphylium blight will likely be to establish an
even, healthy crop that gets off to a good start in the spring, and
moves rapidly to harvest with adequate heat and dry weather between
rain showers,” says Dokken. “It will be interesting to see where the
research takes us. We need to find out if the disease is really a bad
thing. Is it causing yield loss, or is it beneficial to crop drydown?
Hopefully it isn’t a developing problem.”