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Control late blight early

After a summer plagued by late blight, growers need to plan for 2007's potential late blight problems.

November 14, 2007  By Rosalie I. Tennison

The summer of 2006 was not good if you were a potato grower in the Atlantic
provinces and Maine. Late blight hit early and stayed late and, despite persistent
spraying, many growers could not keep the pathogen in check. At its worst, government
and industry agronomists recommended a five day spray schedule. Even though
the 2006 crop is in the bin, a late blight threat looms for the 2007 growing
season unless the weather is hot and dry and growers begin now to reduce problems
in the field.

"Late blight has been and will continue to be a major disease worldwide,"
says Dr. Robert Coffin, a crop specialist with Cavendish Farms on Prince Edward
Island. He says Cavendish Farms put on workshops in 2006 to help growers manage
storage better and to help them learn some field strategies for late blight
control. "Late blight is not going away and it's no longer a question of
whether you have it, the potential is always there," adds Coffin.

Only one infected plant can cause devastation in a field if it is not controlled.
Dr. Coffin says five-to-six billion spores are released in a 200 square foot
plot containing numerous infected plants, which means one infected leaf can
produce thousands of spores per day.


Dr. Yves Leclerc at McCain Foods in New Brunswick recommends controlling late
blight early by eradicating infected patches. "Remove small patches early,"
he says. "Controlling infection early in the year makes a huge difference.
I saw growers disc down an entire acre of crop and that saved the field."

Both Coffin and Leclerc recommend removing cull piles early as well. Mid June
is the recommended date for disposal of cull piles, but both experts suggest
the earlier the better. Leclerc says as long as there is infected living tissue
anywhere near the new crop, there is potential for late blight.

They also agree there is no eradicant type of product to ensure late blight
is controlled. The only options growers have are products that protect against
infection or reduce the spread of the fungus. "We only have protectant,
not cures, so we can only control the spread," explains Leclerc. "Protectant
fungicides remain the basis of effective late blight control, along with an
adequate spray schedule."

Part of the issue with late blight is the pathogen's ability to create new
strains. When the A1 and A2 strains cross and re-cross, there is potential for
resistance to products to develop and some of the mated strains are more virulent.

Again, Leclerc and Coffin refer to late blight as a "community disease."
If late blight is causing you problems, you want to prevent it spreading to
your neighbours' fields. "You have to protect your neighbours," Coffin

They also recommend planting seed that is free of infection. Leclerc suggests
examining seed and removing any suspicious tubers. He adds that seed piece treatments
containing mancozeb may help, but, again, this is preventive and not a cure.

Finally, when the potatoes go into storage keep checking your bins. Leclerc
says that whatever was bad in storage in 2006, was already bad prior to going
into the bin. Coffin says Cavendish Farms tried to help growers who had losses
in storage by arranging an earlier delivery date, which enabled part of the
crop to be saved.

Some positive news from the US on the storage front has Coffin excited. He
says that research on using phosphorus acid to treat tubers going into storage
has been successful south of the border, but the process has not been approved
for Canada. Coffin adds that he and several other plant pathologists in Canada
have repeated the experiments done by US pathologists with similar results showing
that phosphorus acid does help to prevent tuber rot in storage caused by late
blight. Currently, the treatment is approved for commercial use in the US and
Coffin hopes Canadian growers will get the green light as well.

While many people might not like the idea, a cold winter may help minimize
late blight infections in 2007, according to Leclerc. "If there is frost,
volunteer tubers will be killed," he says. It remains to be seen what the
late blight situation will be in 2007 and much of the problem can be aided or
reduced by the weather conditions.

Coffin says there are many scientific challenges that need to be addressed
in regards to the late blight issue. "Without a doubt, we have to approach
this problem scientifically and diligently," he concludes.

Certainly growers who managed to minimize their late blight infections in the
Atlantic region in 2006 are feeling pleased, however, 2007 is a new year and,
perhaps, there will be a whole new late blight challenge that cannot be predicted.
But, following the advice of Leclerc and Coffin to remove cull piles and patches
of infection, will go a long way to minimizing the problem. -30-

What if you get caught out?
When the best laid plans and spraying schedule is hampered by inclement
weather or some other circumstance and conditions are such that late blight
infection is a high probability, what are the options? Eugenia Banks, potato
specialist for the Ontario Ministry of Agriculture, Food and Rural Affairs,
recommends using two back-to-back applications of Curzate.

Curzate is not considered to be an eradicant because it will not stop existing
lesions, she advises. "In our experience, Curzate is the best choice to
apply in the field once late blight has been found. Curzate is not applied alone
but tank-mixed with a broad spectrum fungicide. The two back-to-back applications
should be made when late blight is detected in the field or in neighbouring
fields." She adds, "The product has a kick-back effect of 36 hours.
In other words, any infection that has taken place about 36 hours before the
spray will be inhibited by the fungicide."

DuPont's Alex Crouse goes further: he says the product can reach back to control
late blight started 72 hours before application. "It is a locally systemic
fungicide that rapidly penetrates into stem and leaf surfaces for post-infection
control, suppression of sporulation and inhibition of sporangium and zoospore
germination," he says. -30-



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