By Donna Fleury
Aphanomyces disease in peas and lentils is a widespread and serious problem across Western Canada. In 2017, even with dry conditions in many areas, the disease remained a significant problem in peas, with crop and yield losses in infected fields remaining high. In lentils, the incidence and severity was reduced under the drier conditions, however the inoculum is still likely present. Currently the only control option in fields with Aphanomyces is extended rotations away from peas and lentils for at least six to eight years.
“We have a collaborative group of researchers working together across Western Canada to better understand the pathogen, its distribution and impacts, and to work on developing new tools,” explains Bruce Gossen with Agriculture and Agri-Food Canada (AAFC) in Saskatoon. “Aphanomyces is a difficult disease to identify visually and typically occurs as part of a disease complex with other root diseases such as Fusarium and Pythium. Longtime traditional culturing approaches used to identify pathogens didn’t work well for Aphanomyces, making it difficult to detect. With the more recent development of a good molecular test developed by researchers in France, we can now properly confirm the identity of the pathogen.”
A large collection of pea and lentil crop samples were recently collected across Western Canada and analyzed at AAFC Lethbridge, Alta., confirming that the disease is very widespread across the region, and likely has been around for a longtime, but not recognized in the disease complex. “Aphanomyces is a soilborne disease and is known to be a pathogen in alfalfa,” Gossen says. “Therefore, it may have been here for a longtime but undetected, and possibly may even be a pathogen in native vetch populations, although never studied.”
Aphanomyces root rot is caused by a soilborne water-mould pathogen Aphanomyces euteiches, not a fungus. Therefore, most fungicide seed treatments that are still very good at controlling other root rots like Fusarium are not providing good control of Aphanomyces. Gossen compares Aphanomyces to other root rot pathogens to explain why it is a much bigger problem. He explains that Pythium pathogens for example are picky eaters, take tiny bites and are mostly a problem early in the season. Fusarium gets established in a plant, often causing a major lesion on the primary taproot, but doesn’t spread too quickly. Aphanomyces is a glutton, taking big bites and lots of them. Once it gets established, it spreads through the root system and throughout the season, often causing major crop losses at flowering. Aphanomyces develops large resting spores that are very long-lived, compounding the problem.
New management tools
Although various tools and strategies are under development, most are likely three years or more away from being available. And so far, there are no silver bullets. Aphanomyces continues to be a difficult disease to identify solutions and management options.
“We have considered many different options and approaches, but so far it has been very difficult,” Gossen says. “We don’t have really effective seed treatments to control the disease. Although there are some fungicide seed treatment products that do a good job of protecting the seedlings, Aphanomyces is quite aggressive and remains in the soil to attack the crop later in the season. When it attacks the crop at flowering, the crop can go down very quickly. Lots of research is also being done in various jurisdictions on the potential of biological seed treatments for control, but so far no commercial products have been developed or released.”
Researchers and plant breeders have made some progress in the identification of partially-resistant field pea lines, although so far there aren’t many genes showing a strong source of resistance. “Some of our research team, including Bob Conner with AAFC in Brandon, Man., and Sheau-Fang Hwang with Alberta Agriculture and Forestry have been collaborating on developing markers for some of the quantitative genes (QTLs) associated with partial resistance,” Gossen says. “They have developed a couple of lines from crosses with fairly good adapted commercial cultivar material. These lines look good in the field under moderate disease pressure, but won’t be effective in fields with high levels of disease. This work continues, along with effort from others plant breeders at the University of Saskatchewan and elsewhere, and we expect to have partially resistant commercial field pea varieties available in the next few years.”
“Decision support system” tested
Another promising tool under development is a “decision support system” to help growers assess the level of risk of Aphanomyces and to develop suitable management strategies. Syama Chatterton, a research scientist at AAFC Lethbridge is leading the research and development of this tool, which is expected to be ready to pilot in two or three years.
“The idea is to help growers make a decision on the level of risk and rotation strategies,” Chatteron says. “There is a soil test currently available for Aphanomyces, but it only confirms if the disease is present or absent from the sample. Therefore, if a soil test detects Aphanomyces in a field under current testing, it is likely a high-risk field. The future direction of the research is focused on developing a soil test that would be able to more accurately quantify the level of disease in the field and help growers determine if their field is considered low, medium or high risk of infection. This information, as part of a decision support system, will help with management decisions and rotation strategies.”
The decision support system of the future will assist growers to evaluate options and their suitability for various risk levels. For example, for low risk fields, such as those with one or two areas with yellowing patches, seed treatments may help delay the infection. “When other options, such as partially resistant pea varieties become available, the decision support system will help identify which fields will be suitable,” Chatterton adds. “For example, partially resistant varieties will be recommended for use in low risk fields, but should not be planted in high risk fields because of the risk of overcoming the resistance. The decision support tool will be an important part of selecting and implementing new control options as they become available.” Similar systems are under development in France and other jurisdictions.
For now, growers with infected fields need to implement extended rotations away from both peas and lentils for at least six to eight years. Peas and lentils are considered the same crop and risk in a rotation, so growers will need to find alternatives in those fields. Growers who have been growing peas and lentils for many years can generally expect to see some disease, but for new growers with fields without a history of peas and lentils, it may take several years of rotations before Aphanomyces appears. Soil tests are available to help growers confirm whether or not they have Aphanomyces in a field. Selecting samples from a low spot where water tends to sit is recommended. Seed treatments are still very important and are recommended for control of other root rot pathogens such as Fusarium and Pythium. Researchers hope to have new tools available for growers over the next few years to tackle this widespread and serious disease in peas and lentils.
Other research for control options
Chatterton discussed other research with delegates at the Field Crop Disease Summit in February 2017. Along with Sabine Banniza at the University of Saskatchewan (U of S), Chatterton tested a number of different legume crops to see which ones were hosts or non-hosts. Peas, lentils, and the vetches like cicer milkvetch are very susceptible to Aphanomyces. Dry bean and alfalfa show variable responses depending on the cultivar. Some of the more resistant pulse crop choices that show very little infection are chickpeas, fababeans, and soybeans.
Two seed treatments that Chatterton has been evaluating are Intego Solo (ethaboxam) and Phostrol. Intego Solo is registered for early-season suppression of Aphanomyces root rot in peas and lentils. It has to be used in combination with other seed treatments that have a seed treatment colourant. And, because they’re targeting a root rot complex, it makes sense to have that whole seed treatment package that’s targeting Pythium, Fusarium and Aphanomyces.
“The other product we’ve been evaluating is Phostrol. This is a mixture of different phosphite salts. It’s currently not registered on pulse crops, so we’re looking at whether there is any reason to expand that label to pulse crops. It is registered on potatoes against late blight,” Chatterton says.
However, these two products won’t provide full-season protection. If you get wet conditions coming on later into June or July, these products really won’t be active anymore.
Another control option currently being researched is the effect of Brassica cover crops as a green manure to reduce oospore levels. As Brassica green manure products break down, they have shown a biofumigant effect in the soil and can disrupt oospores from surviving. The benefit of these options is that they can provide long-term solutions because they can potentially reduce oospore levels in the soil rather than just targeting the early-season infection stage.
Effect of soil moisture on pea grown in Aphanomyces-infested soil
One of the unexpected outcomes of a five-year organic agronomic project initiated in 2010 comparing diversified crop rotations and tillage at the Swift Current Research and Development Centre was the discovery of Aphanomyces disease in peas in 2014. During the project, the area received much higher levels of precipitation than usual.
“We took the opportunity to initiate a project to look at the effect of soil moisture on pea grown in Aphanomyces-infested soil,” explains Myriam Fernandez, a research scientist with AAFC in Swift Current, Sask. “For organic growers, crops like peas and other pulses are very important for N-fixation and adding organic matter to the cropping system, so the appearance of Aphanomyces infection was very concerning. This was at the time when Aphanomyces was emerging as a concern.”
The objectives of the project, conducted by post-doc Olanike Aladenola, were to determine the influence of different watering regimes on pea plants growing in Aphanomyces infested soil compared to Aphanomyces disease-free soil under controlled greenhouse conditions. Both soils showed a presence of Fusarium species. There were three water treatments, including pots watered daily, pots watered every other day and pots watered weekly. Each time, the soils were watered to field capacity in all pots.
“We still have some analysis to complete, but some of the results show differences in various growth parameters between the different soils and the three watering regimes,” Fernandez says. “Regardless of the water frequency, the peas grown on Aphanomyces infested soil had higher levels of root rot, matured quicker, were shorter and had lower grain yields and above-ground biomass. On the other hand, regardless of the soil infested or not, factors such as grain yield, above-ground biomass and plant height were significantly better for pots watered daily as compared to the other watering regimes. However, there was no difference in seedling emergence in any of the treatments, the differences appeared later on.”
Although more analysis needs to be completed on root rot data, preliminary analysis suggests that root rot was less severe in the pots watered daily than in those watered once a week. This is likely a result of stress, and the reduction in performance for plants under stress. “The results also suggest that once Aphanomyces infestation is present, there will likely be infection of crops even when the soil is dry for some periods of time” she adds. “We look forward to improved soil testing and other tools when they become available to help address this disease problem.”
Fernandez and her team are taking a proactive strategy and are trialing a variety of intercrops and cover crops along with various crop rotations to see if they can reduce the risk of Aphanomyces infection moving into the areas that so far are not infested. Many of these crop combinations tend to reduce disease levels and some crops have biofumigant and bioherbicide properties, which may prove to play a role in disease management in the future. A wide variety of species, including legumes and other N-fixing plants that aren’t as susceptible to Aphanomyces disease are being trialed to see what strategies can be used to help both organic and conventional growers increase N and organic matter in their cropping systems.