Researchers are working on innovative ways for growers to detect late blight before symptoms appear in their potato crops.
Late blight is a deadly disease that requires integrated management, including foliar fungicides as a key tool. Usually potato growers make repeated preventive applications of broad-spectrum fungicides starting before row closure, and then use late blight-specific fungicides if the disease is found in their fields. Now studies in Ontario and the Maritimes are working on easy-to-use technologies for early detection of late blight that could allow more targeted fungicide strategies.
Late blight is caused by Phytophthora infestans, a fungus-like organism. It can spread by wind-blown spores, infected seed tubers and infected tomato transplants. The pathogen has the potential to infect a crop as early as emergence and throughout the rest of the season. Under conditions that favour the disease, late blight can spread rapidly, going from a single infected plant to an epidemic. It can kill an entire potato plant within a few days of the appearance of visible symptoms, and it can destroy tubers in storage.
The early detection technologies being developed in these studies have the potential to help growers fine-tune their fungicide applications, reduce their fungicide costs, decrease the risk of fungicide resistance developing in the pathogen, and enhance control of this devastating disease.
Early Warning with Spore Trapping
Eugenia Banks, a potato specialist for the Ontario Potato Board, has been developing and evaluating a simple-to-use spore trapping system. The idea of this approach is to detect the spores before the disease begins attacking the crop.
In 2016 and 2017, Banks evaluated this spore trapping system in two Ontario potato production regions: the Alliston and Shelburne-Melancton areas. In 2018, she added the Simcoe-Delhi and Leamington areas to the three-year project. Also, potato growers in the Sudbury area installed two spore traps in their region. For the project’s first two years, the Ontario Potato Board obtained partial funding from the Growing Forward program. In 2018, Syngenta, Bayer CropScience, Holmes Agro and Alliance Agri-Turf provided the funding needed for the project.
Various kinds of spore traps are available on the market. For her project, Banks chose a Sporometrics’ Spornado trap for several reasons.
“They are passive spore traps; they do not require an external source of energy,” she explains. “They are activated by wind. They require little maintenance. They use filters rather than sticky slides to trap spores, and the filters are changed easily.
“The spores caught in the filters are identified in the laboratory by PCR, a very accurate molecular technology test. This was unique, and it was the main reason I decided to evaluate these traps.” With traps that use sticky slides, the spores need to be identified using a microscope, which may result in misidentification.
Banks used two traps per field and placed the traps in windy areas of the field. The filters were changed twice a week and were sent immediately to the lab. The lab provided the results by email to Ontario potato and tomato grower organizations on the same day it received the filters. Growers were alerted if late blight spores were detected in their areas.
Banks evaluated such factors as the location and height placement of the traps; biweekly retrieval of filters to be analyzed in the lab; intensive field scouting to double-check for the presence of a late blight outbreak; and the response of potato growers to detection of late blight spores in their region.
According to Banks, the Spornado traps had previously been evaluated in another province with mixed results. However, in that earlier study, the traps had been placed about 2.5 metres above the ground, which she thought was too high. “The spores that eventually infect the potato plants are carried by air currents close to the crop and deposited on the leaves or stems,” she explains. So she tried placing the traps at 90 centimetres above ground, which worked well.
The project also showed the importance of scouting at least twice a week in isolated fields with limited air movement, to complement the spore trap data. “[In one instance], the detection of spores was four days after a scout had found late blight in a field surrounded by trees. The trees delayed the spread of spores by the wind, resulting in a four-day delay in spore detection. The lesson learned was that potato growers should have intensive scouting on fields with poor air movement.”
The passive spore traps have proved to be accurate predictors of the presence of late blight spores in a potato production area, Banks summarizes. “This allows growers to time the application of late blight-specific fungicides. These fungicides are more expensive and more susceptible to development of resistance than broad-spectrum products,” she says.
“Growers were pleased with the spore trapping technology and the quick communication of results via email.”
If growers want to do spore trapping on their own farms, their costs would include the traps, the filters and the PCR tests. Banks says time will tell whether growers adopt this technology. “Potato growers are very good at selecting and adopting new integrated pest management tools that contribute to their farm’s sustainability.
“Spore traps are another component of an effective late blight management strategy that includes planting healthy seed, early spraying, destroying volunteers and cull piles, field scouting, and timely applications of late blight-specific fungicides.”
Growers should make sure that new growth is always protected, she adds. “After rainfall, water accumulates there, and new growth is an easy target for late blight. Their fields should be scouted twice a week. Destruction of cull piles and volunteers is a must. If a field cannot be sprayed due to very wet soil, growers should consider aerial spraying.”
Banks says if spores are detected in their area and the weather is conducive to late blight, she recommends growers tighten spray intervals using late blight-specific fungicides tank mixed with a broad-spectrum fungicide.
The Ontario Potato Board is currently collaborating on a new two-year study to evaluate an in-field spore identification method. “This year, Sporometrics is validating loop-mediated isothermal amplification (LAMP),” Banks says. “LAMP performs PCR analysis in the field, which would be an advantage for early pathogen detection. It is also cheaper than traditional PCR, but less sensitive.”
The study will compare LAMP and conventional PCR methods when used with filters from the spore traps.
Banks says Phillip Wharton, a plant pathologist at the University of Idaho, has used LAMP to identify late blight and some other diseases in the field.
“Dr. Wharton envisions LAMP could be used as a first line of defence in detecting pathogens in the field, with PCR testing done in a laboratory to confirm any positive results.”
Some other provinces already have or are working on late blight forecasting systems that include spore trapping. For example, Alberta’s system uses data from a provincial network of spore traps along with weather data to forecast late blight risks.
This type of approach would also make sense for Ontario, says Banks.
“I am sure a provincial network of spore traps in Ontario’s potato-growing areas would be welcome not only by growers but potato industry personnel as well,” she says. “Weather data is also an important component of late blight management. In my opinion, it would be a very useful system to have available for Ontario potato growers.”
Would spore trapping for other important potato diseases make sense? “Spore trapping works well for airborne, explosive diseases like late blight,” explains Banks.
“Some other potato diseases are endemic; their inoculum overwinters in the soil and will develop each season in most fields. Early blight is a good example. It has a well-known development pattern. The symptoms develop first on the lower leaves, and disease incidence is high if the crop is under stress. Growers need to apply the first fungicide spray before the vines close to protect the lower leaves. Sporometrics evaluated air samplers for early blight spores with limited success.”
White mould and Botrytis grey mould overwinter in the soil as sclerotia, Banks explains. Spores produced during the season are blown to neighbouring fields, but they are not explosive diseases on potatoes. Regular field scouting should provide timely information on their occurrence, and air samplers may be of some value.
Brigitte Leblon, a professor at the University of New Brunswick (UNB), is leading a three-year study to use drone imagery for late blight detection. The idea is to detect crop infection before any symptoms are visible in the plants.
This study is one part of Leblon’s major project to develop various precision agriculture technologies that can be mounted on drones or ground-based equipment. She and her UNB group are collaborating on the project with A&L Canada Laboratories Inc. and Western University, while one of her PhD students conducts the project’s field and greenhouse experiments. A&L, the Natural Sciences and Engineering Research Council of Canada (NSERC), and the New Brunswick Innovation Foundation are funding the project.
The project team is developing a multispectral camera with the collaboration of Jayshri Sabarinathan from Western University. The camera captures different wavelengths of light, including wavelengths beyond the spectrum human eyes can see.
The team is also creating algorithms (sets of calculations) that can rapidly analyze the multispectral imagery for use in four precision agriculture applications. Those applications are detection of late blight and early blight in potato crops; detection of diseases in greenhouse cucumber plants; mapping crop damage in various crops for more efficient data collection by crop insurance agencies; and mapping crop nitrogen status in various crops to help with nitrogen fertilizer management.
“Late blight and early blight are major diseases for potatoes particularly in Eastern Canada but also in Western Canada,” says Leblon. “Late blight is a catastrophic disease, and it spreads like crazy. Early blight is not so aggressive, but growers currently have a greater chance of getting early blight than late blight because they do so much prevention for late blight.”
Early blight (Alternaria solani) is a widespread fungal disease that can impact tuber yields and quality. Applications of protective broad-spectrum products can be used for early blight management.
Leblon explains multispectral imagery can be a great tool for plant disease detection. “As soon as the plants become sick, they express differences in their chemical content and so on. These changes affect the type and amount of radiation they reflect, and the image captures this reflected radiation. So if the plants are not as healthy as they should be, we can see the illness on the image of the field.”
Some of the changes caused by an infection occur before the plants develop symptoms visible to human eyes. So multispectral imagery enables very early detection of crop infection.
Mounting a multispectral camera on a drone offers a quick and easy way to map plant health and disease across an entire field.
“With a drone, we can get an image of a whole potato field in about 10 minutes. And if we have a powerful algorithm to interpret the image, then in 10 minutes the grower will know if there is disease in the field or not, so if there is a need to spray or not,” states Leblon.
“The images that we are using have a very high spatial resolution, between five and 10 centimetres, so you can see the individual plants. And if the UAV [unmanned aerial vehicle] flies very low, we can get between three and five centimetres with the resolution.”
The exciting potential advantage of this drone imagery approach is growers would not need to make preventive applications of broad-spectrum fungicides.
“Because the drone takes only 10 minutes to get the image, the grower could fly the field every day or every two days [to closely monitor the disease situation],” says Leblon. “So instead of doing preventive spraying, he can spend his time flying the drone. And he would only spray when the disease is actually present.”
By eliminating preventive applications while still controlling the disease, growers would save money and time, reduce the potential for negative environmental impacts from fungicides, and lower the risk of fungicide resistance in the pathogens.
Algorithms for P.E.I.
Leblon’s potato disease study, which started in 2018, involves fieldwork and controlled environment experiments. The controlled environment work is being carried out in collaboration with Jinfei Wang from Western’s Department of Geography, who has access to Western’s biotron. The biotron is a unique facility with specialized environmental chambers, labs and equipment ideal for this type of research.
For the study’s first step, the project team conducted a large experiment in the biotron. They inoculated Russet Burbank and Shepody plants with each disease. Then they tested their camera and sensors to determine which particular wavelengths are the most effective for early detection of late blight and early blight.
In 2019, they will be conducting fieldwork in five to 10 potato fields. Most of the fields will be in Prince Edward Island and some in New Brunswick. The team will be selecting the fields to encompass a wide range of variation in factors that could influence the wavelengths reflected from a potato field, such as differences in soil properties, nutrient levels, plant health, and potato varieties.
“The idea is to get enough data to produce algorithms that would be applicable for all of P.E.I.,” Leblon explains.
Leblon’s team will map each field with the drone and also collect a lot of data on the ground, such as conducting ground-level spectral measurements and identifying potato plants with visible disease symptoms.
In 2020, the team will do some further biotron experiments, and they will likely begin to produce some scientific papers about their results.
For a grower who already has a drone, using imagery for early disease detection would require obtaining the algorithm software specific to the grower’s area and perhaps purchasing a specialized camera, depending on the capabilities of the drone’s existing camera.
Additional research would be needed to develop algorithms for Canada’s other potato-growing regions.
Leblon sees two ways the technology might be applied by growers.
“You could put the camera on any kind of drone,” says Leblon. “Then the farmer can capture the image of his potato fields and apply our algorithm to see if he needs to spray. Another way is to put the camera directly on a sprayer and link it to the sprayer for real-time spraying. The system would automatically detect the disease and spray only where the disease was found.”
Whether a grower prefers using drone imagery or spore trapping, early detection of late blight has the potential to provide valuable information for making fungicide application decisions.