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Seedborne bacterial blights

Researchers search for alternatives to streptomycin seed treatments.

November 19, 2007  By Bruce Barker

Dry bean growers have relied on streptomycin seed treatment for more than a
decade. It helps control seedborne bacterial blight pathogens on dry bean seed
imported into Canada from the US. In 1997, the Pest Management Regulatory Agency
(PMRA) of Health Canada indicated that it would no longer allow the importation
of streptomycin-treated seed because of unresolved questions about the potential
impact of the development of bacteria resistant to streptomycin in humans.

While PMRA has provided several extensions to the use of streptomycin, it is
not clear whether the extensions will be granted in the future. That possibility
has led to the search for alternative ways of controlling seedborne bacterial
blight in dry beans.

Dave McAndrew, an agronomy research scientist with Agriculture and Agri-Food
Canada at Morden, Manitoba, says that many compounds have been evaluated. Some
products that are effective as fungicides also show signs of effectiveness against
bacterial blights. The search, though, has only resulted in the registration
of one copper seed treatment.


For foliar treatments, Clean Crop Copper 53W (53 percent tribasic copper sulphate
formulated as a wettable powder from United Agri-Products), Kocide 101 (50 percent
copper as copper hydroxide formulated as a wettable powder distributed by United
Agri-Products) and Parasol WP (50 percent copper equivalent as copper hydroxide
formulated as a wettable powder and distributed by United Agri-Products and
Van Waters and Rogers) are registered.

In 2001 and 2002, McAndrew looked for alternative seed and foliar treatments.
He compared streptomycin and Bluestone (copper sulfate pentahydrate reagent
grade) seed treatments to two foliar application treatments including Kocide
101 (50 percent copper as copper hydroxide) and potassium chloride (0-0-60 crystals
reagent at 5kg/ha) foliar applications. Potassium chloride (0-0-60) was also
used as a fertilizer broadcast treatment to the soil at 50kg/ha.

"My impressions were that I didn't see a tremendous benefit from streptomycin.
In this trial, the results do not indicate a benefit from either streptomycin
or copper products on bacterial blight," explains McAndrew.

Potassium chloride foliar applied caused significant leaf burning and was likely
the cause of a decrease in yield with this treatment. It also did not result
in a change in the level of bacterial blight infection on the foliage compared
to the copper and streptomycin treatments including the water only check treatment.
Potassium chloride as a broadcast soil fertilizer treatment showed a slight
improvement in yield, indicating potential as a fertilizer treatment. It did
not, however, result in a healthier plant that was more able resist attack by
bacterial blight.

So what is going on here? If streptomycin and copper seed treatments are registered
products, they have to provide some measure of control to be registered with

"Streptomycin can make a noticeable difference in reducing bacterial blight
infestations on the seed itself and early in the growing season. It is less
effective, though, when infestations are heavy," says Ron Howard, a plant
pathologist with Alberta Agriculture, Food and Rural Development (AAFRD) at
the Crop Development Centre South at Brooks.

Howard says that no single seed treatment product is particularly effective
on bacterial blight, even streptomycin. He was part of a research group in the
late 1990s that put together a minor use registration research package for copper
seed treatments. Howard looked at various fungicides and micronutrient fertilizers
containing copper, zinc and manganese. These metallic ions are known for their
bactericidal properties. He compared them to streptomycin.

In general, Zineb 80WP and Bluestone performed as well as or better than agricultural
streptomycin when data on seedling emergence, disease severity, seed yield and
survival of seedborne bacteria on treated seed were compared. None of the seed
treatments tested, including streptomycin, completely controlled seedborne halo
and common blight. Rather, they suppressed disease development by reducing bacterial
populations on the seed surface.

"From the trials, copper sulphate seemed to work the best. It didn't work
all the time, but was comparable to streptomycin. In other studies, I've seen
it work okay and in others it didn't work at all," explains Howard

Howard has also seen a synergistic effect when copper sulphate is applied with
Vitaflo 280, a seed treatment fungicide containing the ingredients carbathiin
and thiram. Vitaflo 280 had a noticeable bactericidal effect on its own and,
when combined with copper sulphate, the mixture provided a greater level of
blight control than was seen when the components were applied individually.
This mixture was registered under the User Requested Minor Use Label Expansion
(URMULE) program of the PMRA in 2002.

In the absence of streptomycin, Howard says the best choice is to treat seed
with Vitaflo 280 + copper sulphate pentahydrate. But, before heading to the
farm supply store, agronomic practices also need to be considered. Use disease-free
seed and get a lab test to ensure that it is free of bacterial blight pathogens.
The seed should be treated as a precaution. Beans should not be grown on land
for at least two years previously to help break the disease cycle.

Howard also says that beans should not be over-irrigated, especially if overhead
sprinklers are used. If bean row canopies are closing up, avoid machinery operations
when the foliage is wet to prevent the spread of the disease. McAndrew adds
that the same applies to dryland production in Manitoba; if the crop canopy
is wet, avoid machinery operations, and even walking through the field should
be avoided.

Copper foliar sprays during the growing season are somewhat effective in slowing
the spread of bacterial blight diseases, but they are only effective on contact
with the bacteria and must be applied every seven to 10 days to prevent the
disease from spreading.

Ultimately, dry bean breeders are working on varieties resistant to bacterial
blights, but they have not been completely successful to-date. In the meantime,
copper seed treatments appear to be as good as any option, regardless of whether
streptomycin treated seed can continue to be imported or not. The problem that
arises, though, is that streptomycin continues to be registered in the US. Should
the streptomycin ban in Canada become fully enacted, dry bean seed treated with
streptomycin may not be allowed across the border, possibly putting Canadian
bean growers at a disadvantage.

"With the harmonization of our pesticide regulations, it will be interesting
to see how the streptomycin issue works itself out, since the US doesn't appear
to have the same health concerns as Canada does," says Howard. "I
wonder if it might be a test case." -30-



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