By Carolyn King
Goss’s wilt has been in Western Canada for only a few years, but plant pathologists, agronomists and breeders are already working to learn more about this corn disease and enhance management options for Prairie growers.
Goss’s wilt is caused by the bacterium Clavibacter michiganensis subspecies nebraskensis. “The bacteria overwinter on infected stubble, so the disease is a concern in fields with shorter corn rotations. But even in fields with longer rotations, it can be a problem because corn stubble is very mobile in the fall, blowing across the roadways and carrying the disease to new fields,” Holly Derksen, field crop pathologist with Manitoba Agriculture, Food and Rural Development (MAFRD), says.
The disease usually occurs in a non-systemic form in which the pathogen infects the plant’s foliage. “The bacterium enters the plant through a wound from hail or wind or sand blasting,” Wilt Billing, DuPont Pioneer’s area agronomist for central and eastern Manitoba, explains. “The infection usually appears on the upper canopy at first. Then with high humidity and rain splash, the disease moves very rapidly throughout the plant, usually from the top down.”
The disease also has a systemic form where the bacteria infect the corn plant’s vascular tissues. However, Billing and Derksen have not seen the systemic form in commercial corn fields in Manitoba.
A relatively new disease, Goss’s wilt was first identified in Nebraska in 1969. In the 1970s and early 1980s, the disease spread through Nebraska and into some surrounding states. Then very little disease occurred until about 2006 when Goss’s wilt resurged and began spreading into new areas.
Billing notes, “Goss’s is continuing to expand. In the U.S. it has moved right across most of the Corn Belt as far south as Louisiana. It moved into the southwestern edge of Michigan, so it has moved east of the Mississippi River.”
In Western Canada, the disease was first found in Manitoba in 2009 and in Alberta in 2013.
DuPont Pioneer has been conducting Goss’s wilt surveys in Manitoba for several years, evaluating all commercially available hybrids (the surveys didn’t target other companies’ products, but if growers had products from multiple companies, then the other hybrids were also included in the surveys). “In Manitoba over the past five or six years, we’ve seen anything from an insignificant infection which doesn’t have any yield loss all the way up to the most severe fields experiencing close to 50 to 60 per cent yield loss. So it can be very impactful,” Billing says. The severity of the disease depends on weather conditions, the amount of inoculum in the field and the susceptibility of the hybrid to Goss’s wilt.
Fortunately, late summer conditions in Manitoba in 2014 didn’t favour the disease. Billing says, “In 2014, we found the disease in many fields in mid to late July. However, we had a dry spell during late July to early August, so the disease was really limited in its impact.”
In 2014, Derksen and Morgan Cott from the Manitoba Corn Growers Association (MCGA) conducted a survey for Goss’s wilt across Manitoba’s corn-growing region between September 19 and October 7. “In the past, the only surveys for Goss’s wilt in Manitoba were carried out by different companies,” Derksen says. “We wanted to conduct a third-party survey to get representation of all the corn acres.” This was their first Goss’s wilt survey; the plan is to do a provincial survey every year.
The survey was low-cost and non-intensive. Derksen explains, “We knew what percentage of the corn acres each RM [Rural Municipality] represented, and we surveyed that same percentage of corn fields in the RM. We drove around on a grid system and picked fields randomly to survey.” They surveyed the roadside edges of the fields; the disease often begins in patches along a field’s edges.
About 14 per cent of the surveyed fields had Goss’s wilt. The disease was not severe in any of those affected fields and did not cause yield losses in those fields. However Derksen says, “The disease seems to have spread to the majority of Manitoba’s grain corn growing area so it’s something that our grain corn growers definitely need to be aware of.”
In Alberta, the disease is less widespread so far. According to Mike Harding, a plant pathologist with Alberta Agriculture and Rural Development (AARD), in 2013, five out of 45 corn fields were positive for Goss’s wilt. Four of the positive fields occurred in three counties in southern Alberta south of Highway 1, and one positive was from the Edmonton area. In 2014 two positive samples were submitted from about 80 fields scouted, and both came from locations in southern Alberta south of Highway 1.
Know the enemy
Understanding more about Goss’s wilt is key to enhancing management of the disease. Fouad Daayf, a plant pathologist at the University of Manitoba, is leading a new project to “decrypt the puzzle of Goss’s wilt” by studying the strains occurring in Manitoba.
“We are going to do the background work to ‘know the enemy,’” Daayf says. He and his research team are collecting samples of plants that have or are suspected of having the disease, isolating the bacterium from the samples, and studying the isolates to see how they differ from each other.
This four-year project officially started in December 2014, but Daayf and his research team had already started to do some work on it before then. For the sample collection, Daayf is collaborating with Cott and Karin Rose at the MCGA and with Derksen. “They deal with growers directly and do surveys, and that is helpful for the project; for example, when there is a suspicion of disease occurrence, we can collect samples and we can talk to them and get more data about the occurrences,” Daayf says.
The samples are being sent to Daayf’s lab in Winnipeg and to the lab of James Tambong, a research scientist with Agriculture and Agri-Food Canada (AAFC) in Ottawa. Daayf explains, “We will be isolating the bacterium and making a collection of isolates to study using different markers here in Manitoba and in Ottawa. Dr. Tambong has already been using some molecular markers [for the Goss’s wilt bacterium], and we are going to introduce a number of biochemical markers and look at the level of pathogenicity of those isolates, at how they behave on commercial lines that we have in Manitoba.”
Tambong’s molecular marker work involves the use of PCR-based and other techniques to examine different portions of the bacterium’s genome to look for DNA sequences that differ among the different isolates as a way to identify the different strains of the pathogen.
Daayf’s work with biochemical markers will provide other information on the isolates. “For example, we might find that different isolates produce different molecules. Down the road we might look into the importance of those molecules especially if, for example, we find a group of isolates that are causing a high level of disease and that also have one or more specific molecules in common. This would be especially interesting if such molecules are produced by the most aggressive strains, but not by those that cause no damage to the plant. Then we would focus more on such molecules because that would help us in tackling the disease down the road.”
Because Goss’s wilt is a fairly new disease, this project offers great potential for gaining new insights into the pathogen and how the plant responds to it. The results will shed light on the pathogen’s different strains, how they are distributed in Manitoba, what level of damage they can cause and how they affect the various commercial corn lines grown in the province.
In the longer term, the findings from this project will lay the foundation for further studies to enhance Goss’s wilt management. For example, Daayf says, “We could conduct more specific tests on the isolates to try and understand what makes certain isolates or strains more damaging to plants, what makes some corn lines more resistant than others, and so on, and maybe provide markers to breeders that would be helpful to them for breeding higher resistance levels into corn.”
Daayf’s project is funded by the MCGA and Growing Forward 2; Monsanto has also contributed some funds.
The MCGA and Growing Forward 2 are also co-funding a new corn breeding project led by corn breeder Lana Reid, who is with AAFC in Ottawa. She says, “We’re working towards developing germplasm with Goss’s wilt resistance and early maturity for Manitoba.”
In this project, inbred lines are being evaluated for early maturity at a 500-row breeding nursery in Manitoba. For now, the Goss’s wilt screening for the project is being conducted by one of the seed companies in Manitoba. In 2014, 100 lines were screened for response to Goss’s wilt, and some of those lines were found to have very good levels of resistance.
Seed companies, such as DuPont Pioneer, are also working on providing Prairie corn growers with hybrids that have good tolerance or resistance to Goss’s wilt.
“DuPont Pioneer has a long and extensive track record of managing and breeding for Goss’s wilt resistance in corn hybrids thanks to our colleagues and partners down in Nebraska and Colorado, where the disease originated,” Billing says. “So we were able to move some of the information from there into Manitoba to select inbreds that show signs of or have strong genetic platforms for battling Goss’s wilt.”
DuPont Pioneer is using various breeding technologies to develop new inbreds and hybrids for Western Canada, and is screening them for tolerance or resistance to the disease. “At our [Goss’s wilt] nursery, inbreds and hybrids are fairly intensely screened with heavy disease pressure to identify and rank them for their tolerance. We select the best ones and build hybrids out of them, and then we screen those hybrids in our nursery and in field-scale trials,” Billing notes. “Within the five years we’ve been working with it in Western Canada, we’ve selected hybrids that have really increased the Goss’s tolerance in our overall portfolio.”
Managing Goss’s wilt
Symptoms of Goss’s wilt may sometimes be confused with problems like drought, frost damage or sunscald, or with other diseases like Stewart’s wilt or northern corn leaf blight. To identify Goss’s wilt, Billing advises, “When you’re walking through your corn field, look for greyish brown lesions with water-soaked margins. The telltale sign of Goss’s wilt is the black freckling that shows up along the lesion edges. If you scout during drier conditions, you’ll see that black freckling. If conditions are damp, like a heavy dew in the early morning, you’ll sometimes see a glossy sheen on the lesion.”
Derksen notes fungicides are not effective for controlling Goss’s wilt because it is a bacterial disease. She has two main recommendations for managing the disease: “One is to lengthen your crop rotation. However, that may not always be enough to prevent the disease if neighbouring fields have Goss’s wilt. The other key is to grow a resistant corn variety. At this time there isn’t any third-party testing to compare varieties from different companies, but most companies have a range of tolerances to Goss’s wilt, so you can check with your seed supplier for information.”
Billing recommends a multi-tactic strategy. “First and foremost is hybrid selection. There are hybrids available with very strong Goss’s tolerance, although not full resistance, so you’ll see the disease appearing in fields, but you won’t have a yield reduction.
“Next is managing your corn stubble. Remove as much stubble as possible because the bacteria will overwinter on the stubble.”
He also recommends crop rotation. “Usually with Goss’s wilt, you get a warning shot. It won’t necessarily come in and immediately take over your whole field. You’ll see a little bit of infection along field edges where an infected leaf might have blown in from the previous year. However, if you continue to put susceptible corn hybrids on those acres, the disease can expand very quickly.”
As well, good grassy weed control helps because the disease can thrive on weeds like barnyard grass and green foxtail.
Billing notes, “The growers in southern Manitoba are managing the disease very well using those four tactics; in fact, we have several corn growers who are growing their highest corn yields on fields that were once infected with Goss’s wilt.”