Features Agronomy Diseases
Powdery scab may be lurking

Plant diseases in some ways are like fires which need fuel, oxygen and conducive
conditions to ignite: diseases need a pathogen, a host and conducive conditions.
Some potato growing regions of the US, which had not previously noticed a problem
with powdery scab, have more recently recognized it as a threat. Canadian producers
may want to heed a warning that their fields might also suffer from this disease.

Powdery scab, which is caused by the pathogen Spongospora, affects tubers
and roots with tuber scab lesions and root galls. "It's more of a slime
mould than a true fungus and it produces spores that can remain viable in the
soil for up to 10 years," says plant pathologist, Dr. Jeff Miller of the
University of Idaho.

While he has been studying potential fungicide foliar and seed treatments,
Dr. Barbara Christ at Pennsylvania State University has spent years studying
the disease itself. "Powdery scab has no above-ground symptoms in the crop,
which is part of the problem: even at harvest, it may not be noticeable, but
two weeks later after exposure to air, pustules erupt on the tuber surface,"
she says.

Both agree that low levels of the pathogen may be present in most growing regions
of the US, Australia, Europe and likely Canada, too. However, favourite varieties
like Russett Burbank are fairly resistant. Now that more growers are moving
to more susceptible varieties, like Yukon Gold and Shepody, they are seeing
severe losses in marketable yield and are also increasing inoculum levels in
their soils. "If you are fortunate enough not to have this disease, make
sure you use clean seed," advises Miller.

The conditions that favour the disease are those where there are repeated periods
of saturated cool soils, typically lower than 70 degrees F (21 degrees C), says
Christ, "We don't really hear about it in a hot dry year."

Although Miller has not found suitable fungicide control treatments, he has
achieved some suppression using in-furrow and foliar applications through irrigation
systems. The most significant hope is through breeding varieties with resistance,
he suggests.

A number of researchers and advisors in Canada are watching the situation in
growing regions across the country. Khalil Al-Mughrabi, plant pathologist for
New Brunswick Agriculture, Fisheries and Aquaculture, says, "In addition
to symptoms caused by the powdery scab fungus, it also affects crop marketability.
The causal fungus transmits an important viral disease called potato mop top
virus, commonly referred to as PMTV. In 2002, the Canadian Food Inspection Agency
(CFIA) detected PMTV in several shipments of seed potatoes destined to Canada
from the US. Since then, growers in Canada have become more careful about planting
scabby seed potatoes."

Al-Mughrabi notes that it is difficult, even for experts, to distinguish the
lesions of powdery scab and common scab. "Interestingly, in some cases
we found both powdery scab and common scab in the same lesion." He adds
that the symptom-carrying seed only transmits the disease to 20 to 50 percent
of the progeny. Symptoms vary greatly, depending on potato cultivar and environmental
conditions. Usually, slightly raised lines and rings are produced on the surface
and/or brown areas and lines are produced in the flesh of tubers of sensitive
cultivars upon primary infection (in the year that they are infected from soil),
whereas secondarily infected tubers (produced from infected mother plants) can
be cracked and distorted, or have blotchy surface markings or reticulate surface
cracking with accompanying yield loss.

In Manitoba, powdery scab has been identified, according to potato pest specialist,
Dr. Tracy Shinners-Carnelley of Manitoba Agriculture, Food and Rural Initiatives.
"I am always cautious when growers tell me they have scab. I encourage
them to have a sample submitted to a diagnostics laboratory so a correct diagnosis
can be made. Powdery scab and common scab can be very difficult to distinguish
with the naked eye," she says.

Ron Howard, who is a plant pathologist with the Crop Diversification Centre
South for Alberta Agriculture, Food and Rural Development, has initiated a research
project on the disease. In Ontario, as in other provinces, the alert is out
for growers to be wary, even though powdery scab has so far only been reported
in northern regions where low soil temperatures favour development of the disease.

Across the country, a grower survey in 2003, conducted by Dr. George Lazarovits
and Jackie Hill of Agriculture and Agri-Food Canada in London, Ontario, found
that common scab was most prevalent in the eastern part of the country and that
losses were in the $36 to $40 per acre range, the highest losses being in Quebec.
Total losses estimated in Canada are in the region of $16 million per year.

Obviously powdery scab is a disease to avoid if at all possible. Though there
is, as yet, no curative product, there are management tactics that can be employed.
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