Features Fungicides Seed & Chemical
Beating blight

Early and late blight can cause great stress and losses to a potato crop and
growers. While there are some basic guidelines to help growers manage blight
infestations, none are definitive. Knowing whether late or early blight is the
problem depends largely on climate conditions, not necessarily the time of year.
Once the blight is identified, choosing a control strategy can also be confusing,
as not all products are equally effective on both blights and under all situations.

"There is a critical need to be accurate in your blight identification
because the symptomatology and disease development can be quite distinct in
both diseases," cautions Dr. H. W. (Bud) Platt of Agriculture and Agri-Food
Canada in Charlottetown, Prince Edward Island. "Early and late blight do
not always occur simultaneously because they each require different conditions.
These diseases are linked to climate conditions, so management is dependent
on a proper diagnosis."

In most areas, early blight is an annual minor problem, but can be a major
problem in a few specific potato growing areas of Canada and the US. Early blight
occurrence and development is enhanced by hot dry weather conditions, which
increase disease risk and spread as the plants are stressed and weakened. Late
blight, however, is favoured by cool, wet conditions and the pathogen can successfully
infect healthy plants. Confusion occurs when a hot, dry period is immediately
followed by cool, wet weather because then both blights can exist together since
their conditions for infection have been met. However, in most cases, both blights
will not begin to infect simultaneously if the conditions are only present for
one type of blight.

"Unfortunately, we don't have a reliable prediction model for both early
and late blight, so growers have to begin a spray program in the spring as part
of their production plan," says Graham Kempton, an agrologist with Bayer
CropScience in Prince Edward Island. "Growers require a regular spray program
and will spray every seven to 10 days. They may shorten the interval to five
to seven days if there is heavy late blight pressure in the area." His
company recently released a new product that they believe will fit into any
spray program because it is effective on both early and late blight, reducing
the pressure on growers to accurately identify the blight that is infecting
their fields. He says the new product, Reason, is only registered as a tank-mix
so there are two modes of action working to reduce both early and late blight
infection.

Platt says it is important to manage blight to reduce infection, and economic
thresholds information would assist growers in knowing when blight will be a
yield reducer. However, he says, researchers have had difficulty coming up with
good recommendations. "So much depends on the cultivar that is planted,
the growth stage of the crop, the disease inoculum levels in a particular growing
area, the opportunities for successful application of fungicides, and the climate
conditions," he explains. "All can be a factor in how serious a problem
blight can be and how well the disease or diseases can be managed."

Various aspects of crop production, crop protection, water management, current
and forecasted disease information need to be determined by specialists in each
potato growing area, according to Platt. As an example, he says, an early maturing
variety may not be affected adversely if early blight occurs while the plant
is dying naturally and so fungicides may not be needed. However, if the blight
occurs earlier in the season, it may reduce yield so growers would need to begin
a spray program. He adds that it is most important to realize that late blight
can have more effect on yield than early blight, and can also cause further
losses by infecting potato tubers. In addition, late blight will attack healthy
plants from emergence to senescence if the conditions are right and almost always
require fungicides for disease prevention and/or disease management.

Growers need to rely on accurate information about disease risk and management
in their area to effectively use fungicides to protect their potato crop. However,
as with all chemical products, there is always the danger of the diseases developing
resistance to the products. Therefore, careful management of the products that
are available will help to preserve their effectiveness. Resistance to some
commercially available fungicides has been identified throughout the potato
growing world, but Platt says use of the new chemistry that is often site specific
in combination with older, broad spectrum fungicides helps to minimize the development
of resistance.

"We need to use products responsibly to ensure their continued efficacy
and availability," Platt says. "Using tank-mixes is a good resistance
strategy."

Kempton agrees and adds this is why Reason is registered as a tank-mix only.
"It is a local systemic product that will control spores and move into
new tissue, which makes it different than older products because they don't
protect new growth. Because it controls both blights, it gives growers peace
of mind because they don't need to worry about the timing of spraying certain
products." The tank-mix options include Bravo, dithane and mancozeb, which
gives growers some options on their choice of products, particularly if they
have one they prefer over another. Kempton recommends management of all products
to reduce the chance of resistance development, suggesting that Reason be used
in the second or third spray of the season and then alternating it with other
products afterwards. While it is new chemistry, Reason has similar action to
the strobilurin products and Kempton recommends limiting use of these products
if Reason is being used.

The introduction of new products will always help growers with their disease
and chemical resistance management strategies, but understanding the differences
between early and late blight can also be a valuable tool to controlling infection
in a crop. Platt says growers need to consider all control strategies for both
diseases and, if possible, mix them up to ensure maximum control and prevention
of resistance. -30-