CROP DISEASES LURKING JUST BEYOND OUR BORDERS
anyone who has ever read a comic book knows no amount of good comes without some share of evil.
In the tale of modern agriculture, our hero the North American Farmer bursts onto the scene of exciting new markets worldwide and appears to have triumphed through globalization. But Demon Diseases are hiding in the shadows and are preparing their next attack.
A band of sidekicks are joining forces to shed new light on these ever emerging global diseases. Cooperation between agriculture researchers everywhere is helping to expose these diseases for exactly what they are and circulate crucial information to farmers battling these new menaces around the globe. Albert Tenuta, Field Crop Pathologist at the Ontario Ministry of Agriculture, Food and Rural Affairs, has identified the following three super villains as some of the greatest threats for Ontario farmers.
“All of us should be going out scouting and monitoring. I don’t think we are doing as much as we should,” says Tenuta. “These diseases are specialized and they are very good at what they do.”
ug99 the ugly
Stem rusts are no stranger to wheat and barley farmers in North America. Variations of this fungal disease caused wide spread destruction in the 1930s and 1950s in both the American mid-west and Canada’s prairies. Held at bay for roughly 50 years by resistance genes, as part of Green Revolution breeding programs, it seemed scientists had succeeded far beyond their wildest dreams. The disease seemed completely defeated in North America when combined with eradication programs that reduced the population of Barberry shrubs, alternate hosts the disease needs to reproduce and potentially develop resistant isolates of the fungus. Unfortunately, as the original plant breeders predicted, a resistance tolerant strain has developed and is rearing its ugly head.
Discovered in Uganda in 1999, the strain now known as “Ug99” has spread through wind-borne spores to Kenya, Ethiopia, Sudan, Yemen and Iran. It is characterized by large, rust coloured, diamond shaped lesions that are often accompanied by a yellow halo. The lesions are primarily found on the stem of infected plants but the disease is capable of attacking all above-ground parts, unlike leaf and stripe rusts. Infected plants produce fewer tillers, little seed and severe infections are fatal.
Experts predict Ug99 will reach North America after attacking Asia and Western Europe and anticipate regional yield losses of 30 to 70 percent. However, there is also a chance that a virulent race of stem rust could develop locally first. To prepare for either scenario, researchers are working to identify effective fungicides and determine best management practices that could decelerate any invasion. Meanwhile, plant breeders worldwide continue to search for new gene combinations that might be able to provide durable resistance once more.
the gruesome goss’s wilt
This particular pest is on the move north and was first discovered in Nebraskan corn crops in 1969. Unlike Ug99, this is a bacterial disease that cannot be successfully controlled by pesticides. It is also capable of attacking both surface tissues, particularly leaves and the entire plant systemically. Since the bacteria are water loving creatures, they enter the corn plant’s vascular system causing the plant to wilt in the same way that Stewart’s Wilt occurs. External clues such as dark freckling on the tissue surface and glossy, varnish-like residues help distinguish it from its less sinister cousin.
Goss’s Wilt can’t work on its own and relies on primary attackers such as feeding insects, sharp machinery, and hail for plant entry sites. Extreme weather is very good at transmitting the disease since bacteria can travel short distances in raindrops to other corn plants, green foxtail, shatter cane and barnyard grass. Once infected, plant residues can house the bacteria for long periods of time and will over-winter on soil surfaces. Bacteria are also capable of penetrating viable seed and seed companies have been required to be extremely vigilant in monitoring seed production sites in order to prevent spreading the disease.
While an outbreak of Goss’s Wilt has yet to be identified in Ontario, there have been some suspected cases of infection in Manitoba. In Indiana fields where this villain has been found, overall damage to the crop has been estimated between 10 to 30 percent. American farmers reduce their risk of further infections by burying crop residue and using hybrids which are better able to ward off the disease. However, at this time, resistant Canadian hybrids are not well understood or identified.
cunning charcoal rot
This pest has already invaded Ontario and reported attacks on Essex and Kent county soybean crops have been confirmed. To date, a 100 percent crop loss has yet to be experienced in Canada. Unfortunately, if the right conditions present themselves, this threat is a conceivable reality in the not-so-distant future.
While ‘the right conditions’ for most diseases usually come with warm, wet weather, Charcoal Rot thrives in a hot and dry environment. Since it attacks the soybean crop in patches, especially during July and August, this particular fiend can easily disguise itself as drought stress. Affected plants begin to wilt; leaves turn yellow and plants senesce prematurely. Unlike drought stress or sudden death syndrome however, the leaves of the deceased plant do not drop. Upon close inspection, the base of these affected plants will appear grey and a root dig will expose tiny spores, called ‘microsclerotia,’ embedded in the root.
Root digs are an important monitoring tool for farmers who are watching for this disease. Noting a lighter soil texture where a Charcoal Rot outbreak is suspected offers the first indication of the disease. If microsclerotia are discovered, then a farmer better be prepared for a drawn out battle because these spores aren’t going anywhere for three years. A strong defence, such as a good crop rotation, is the best offence against the disease. Unfortunately, Charcoal Rot has also been found to affect corn crops so, while it can’t cause the same crop loss, it will survive to attack any soybeans planted in following years.
fighting the good fight
As potentially devastating as these three fiends could be, there are actions being set in motion to help farmers everywhere fight the good fight. International communication efforts to educate field workers in identifying these and other violent diseases are assisting with the rapid detection of pathogen movements. Chemical and breeding technologies that are widely used today are providing tools to combat diseases in modern agriculture more effectively than farmers and researchers could have dreamed of 50 years ago. Monitoring, forecasting and management programs are well established according to Tenuta and may be the key in good triumphing over evil once again.
“We are being very proactive to prepare our growers, and the industry, for inevitable outbreaks of these diseases,” says Tenuta. •