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Unravelling potato early dying complex

Awareness of early dying complex of potato is building in the potato industry,
but extension specialists and researchers are quick to point out that the complex
is poorly understood in western Canada. Verticillium wilt, nematodes, weather
and soil conditions may all interact to cause premature death in potatoes.

"It is called a complex because it is complex," says Tracy Shinners-Carnelley,
potato pest management specialist with Manitoba Agriculture, Food and Rural
Initiatives (MAFRI) at Carman. "There needs to be a lot of time spent on
understanding the complex before we really get a handle on how to minimize it."

In Alberta, potato specialist Michele Konschuh with Alberta Agriculture, Food
and Rural Development (AAFRD), says that symptoms of early dying complex have
been more prevalent since 2002. Whether it was really early dying complex is
still up in the air. "That depends on who you talk to. Some call it verticillium
wilt and others see it as a complex. Verticillium, fusarium, alternaria and
black dot have all been blamed," says Konschuh.

Shinners-Carnelley first started looking at early dying complex during the
summer of 2000 when growers started to take notice of patches of potatoes dying
prematurely. Over the next several years, she ran an extension program with
potato growers to look at what was happening in the field to better understand
the scope of the problem.

In 2002, Shinners-Carnelley assessed four fields that had early dying symptoms
and analyzed them for verticillium wilt infestations, nematodes and soil characteristics
such as salinity and carbonates.

"What we found was that what people were calling early dying complex could
be linked to different factors in each field. What we came away with was that
we confirmed that it was a complex problem," says Shinners-Carnelley. "When
talking about early dying complex, growers would see some part of the crop go
down early. They might call it early dying or they might say it was verticillium
wilt, and the two terms became interchangeable. That isn't necessarily so."

In the fall of 2002, Mario Tenuta, assistant professor of soil ecology in the
Department of Soil Science, and assistant professor Fouad Daayf, in the Department
of Plant Science at the University of Manitoba, collaborated with MAFRI in looking
at the complex. In 2003 and 2004, the project was expanded to include a producer
survey to help establish field cropping histories and agronomic practices used
in potato production.

"The jury is still out on whether all the fields that are 'identified'
as early dying really have early dying complex caused by verticillium and nematodes,"
says Tenuta. "In some fields they are the problem but in others it is uncertain,
and the cause is likely other agronomic factors. Because our potato acreage
is rapidly growing, we want to get a handle on the complex and want to know
what the cost to producers would be if the disease really ramped up. The belief
that our soils are just too cold for nematodes to survive is wrong," says
Tenuta. In 2003, of 24 producer fields tested, 15 had the presence of the root
lesion nematode and five of these fields had levels to be concerned about.

Symptoms of early dying complex can be difficult to distinguish from normal
senescence (maturity), other diseases, or reduced growth due to soil variability.
Symptoms are highly variable but usually show up in patches. Early foliar symptoms
may appear as uneven chlorosis of lower leaves on a few plants. Later, some
wilting of leaflets may occur, but more typical is uneven death of lower leaflets.
Leaf yellowing and death may proceed up the stems, which often remain erect.
A light brown vascular discolouration is often visible at the stem base when
sliced open. Advanced symptoms usually do not occur until after flowering and
may consist of early dying of isolated plants or patches of plants.

The symptoms may be confused with other agronomic problems, so the group is
trying to determine the extent of the complex and what effect the complex has
on yield loss. That research is still in progress in Manitoba. However, the
body of research on early dying complex shows that two pests contribute to early
dying complex, the fungus causing verticillium wilt and root lesion nematodes.
Tenuta says that the region of Manitoba with producers having greatest concern
for the disease is also where some of the highest levels of verticillium and
nematodes are found in soil. He cautions that confirmatory work is needed to
attribute early dying to these pests.

Verticillium wilt can be caused by V. dahliae or V. albo atrum.
Information from MAFRI indicates that plants infected with verticillium wilt
start to show symptoms in the middle of the growing season. Individual leaves
turn pale green or yellow. Leaves on the affected stems then wilt, and finally
the entire plant dies prematurely. Initial symptoms often develop on one side
of the plant. The lower stems of diseased plants and tubers have brown discolouration
in the vascular tissue when cut open. Verticillium species are soilborne fungi
and once established, can live for long periods in the soil, even if a potato
crop has not been planted in many years. The fungi can establish in a field
through the use of infected seed or by movement of infected soil.

Shinners-Carnelley says that other broadleaf crops are also affected by verticillium
wilt. "In sunflowers, the disease is also known as leaf mottle. Beans are
being grown on potato ground, but the effect that beans have on verticillium
levels in soil is still really unknown, and more work needs to be done on the
impact of bean and potato in rotation."

Nematodes are microscopic worms between 0.3mm to 1.0mm long and some feed on
living plants, while the majority are actually good for crop growth. Feeding
injury causes the main damage to plants on root tips and in heavily infested
soil, can damage tubers. Nematodes are also known to be vectors of some viruses
that do not occur in Manitoba. Plant injury can be increased due to the susceptibility
of the plant to pathogens like verticillium. Movement of soil, machinery, planting
materials and water may spread nematodes.

Still, Shinners-Carnelley is hesitant to point the finger too quickly at verticillium
wilt or nematodes when she sees early dying of potatoes in the field. "Early
dying doesn't always mean verticillium wilt. It could be a soil factor or something
else. We're hoping to identify some of these factors and how they are all interrelated."

Soil variability is high on the group's list of factors to consider. In some
cases, the early dying patches could be related to a problem like salinity.
In others, a soil characteristic might make the potato plant more susceptible
to verticillium wilt. "If soil drainage is altered during the growing season,
it may have a positive impact on verticillium or some other pathogen that causes
early dying," she says.

Prevention is the key
In western Canada, no magic bullet exists to control early dying complex. Tenuta
says that in the US, fumigants are widely used in soils to control the pathogens
associated with early dying complex. However, he says that in places like Idaho,
Washington, Oregon and California, the verticillium and nematode levels are
high and cause early dying losses in potatoes.

General soil fumigants, such as metam sodium and dichloropropane, or nematicides,
such as ethopop and aldicarb, are commonly used every year that potatoes are
planted. Tenuta says they can be extremely effective, albeit at a cost of upwards
to $200 per acre.

Tenuta and Daayf are both looking at natural biofungicides for control of the
complex. Daayf and a student are working on a project to identify native bacteria
that either attack the pathogens that cause early dying complex directly, or
stimulate a response within the potato plant that helps it resist the complex.

Over the last several years, they have been able to identify several bacteria
that work invitro and in the greenhouse. In 2003 and 2004, they tested several
of the bacteria in field trials and found a number of the bacterial strains
worked very well in inhibiting the pathogens that cause early dying complex.
The seed potato piece is inoculated by dipping in a bacterial solution prior
to planting. The next stages in the development of the biocontrol experiment
are to try to identify how the control works and how biocontrol might be adapted
to commercial applications.

Tenuta's background in the early dying complex includes research in Ontario,
with George Lazarovits and Ken Conn with Agriculture and Agri-food Canada, on
organic amendments such as manures and rendering wastes to control the pathogens
causing early dying complex. In Ontario, fumigants can also be used for pathogen
control, and they are generally successful in controlling nematodes but less
successful in keeping verticillium under control. Again, they are expensive.

Tenuta is interested in bringing some of that Ontario experience to western
Canada. He says organic soil amendments such as hog manure, composted waste
and sewage sludge hold potential for decreasing early dying complex pathogens.
They plan on screening these and others, such as green manure plowdown crops
of mustards and rapeseed (not canola), marigolds and sorghum-sudangrass, to
control the pathogens as well. "When these materials decompose in the soil,
they somehow inhibit the pathogens. That kind of biofumigant action is fascinating,
but we need to understand it better in order to make it practical for producers,"
says Tenuta. Another challenge is working the plowdown crop into the potato
rotation one year prior to potatoes, without costing a fortune in lost revenue.

In Alberta, Konschuh says that AAFRD and the Potato Growers Association would
like to set up a monitoring program for 2005. The goal would be to identify
the extent of the problem, and to identify some of the causes.

In the meantime, Shinners-Carnelley says that the best practices for avoiding
early dying complex include rotations of three to four years between potato
crops, and to use rotations that do not include crops that harbour verticillium.
In addition, fields should not be contaminated with soil from diseased fields,
diseased tubers or plant refuse. And proper identification is key.

"The disease is difficult to detect in the field. If a producer believes
he has early dying complex, the only real way to tell is to conduct laboratory
tests for the presence of verticillium and nematodes. And you should also look
for other agronomic factors that might be causing premature death of the potato
plants," says Shinners-Carnelley. -30-