Features Agronomy Diseases
Fungicide-resistant pink rot

Pink rot is an economically important potato disease usually detected late in the season or in storage. It is caused by a soil-borne, fungus-like pathogen called Phytophthora erythroseptica. Wet conditions favour the disease.

The pathogen can infect all the underground parts of the potato plant, causing infected tissues to turn brown or black. Tubers are most commonly infected via the stolons, but in very wet autumns the pathogen can also enter through the eyes or lenticels. Above-ground symptoms, such as wilting, stunting and defoliation, tend to appear late in the season. The infection can spread to other tubers during potato harvesting, handling and storage.

When an infected tuber is cut open, the infected area gradually turns pink, then brown and finally black, over the course of about an hour of exposure to air.

Resistance is a growing concern
“Ridomil Gold’s active ingredient is metalaxyl-m. It is commonly used as an in-furrow treatment, but it’s also used as a foliar because it has some systemic activity. It has been a standard go-to product for pink rot,” says Dr. Rick Peters, a vegetable pathologist with Agriculture and Agri-Food Canada (AAFC) in Charlottetown.

Over the years, resistance to metalaxyl has developed in many species of Phytophthora. For instance, by the mid-1990s metalaxyl-resistant strains of Phytophthora infestans, the late blight pathogen, had become so common in the U.S. and Canada that metalaxyl was no longer an effective control option for that disease.

For the pink rot pathogen, metalaxyl-resistant populations began to be found in North America almost two decades ago, initially in Maine, New York, New Brunswick, Idaho and Minnesota, and then spreading to other American states.

Peters has been on the lookout for the further spread of metalaxyl-resistant pink rot in Canadian potato-growing areas.

His most recent national survey was from 2005 to 2007. “At that time we found some evidence of resistance to Ridomil creeping up in pink rot populations in New Brunswick, but we didn’t find any evidence outside of that province in our testing,” he says.

In 2012, Peters and his research team carried out a preliminary survey in P.E.I. because Island growers were having some issues with pink rot that year. The researchers found metalaxyl-resistant isolates of the pathogen in some fields.

Knowing the problem had spread to P.E.I., Peters wanted to determine the current national status of metalaxyl resistance in the pathogen. So, with funding from Syngenta Canada, the Prince Edward Island Potato Board and the Potato Growers of Alberta, he initiated a two-year project, starting in the fall of 2013. The project has two components: a national survey, and field trials of alternative control measures.

“This project will help to give growers information to use fungicide products efficiently,” explains Mary Kay Sonier, seed co-ordinator with the P.E.I. Potato Board. “For example, if the researchers discover that a high percentage of our isolates here are resistant to Ridomil, then there would not be much point in people continuing to use it. It would either further select for resistance – so increase our resistant population – or it would be a waste of product and a waste of money for the growers. If, on the other hand, the number of resistant isolates is low, it would indicate that the product is still an effective control measure.”

She adds, “This project will also help increase grower awareness of the importance of rotating control options to avoid the build-up of resistance in all pathogens, not just pink rot.”

Cross-Canada survey
The national survey is being conducted in partnership with many agencies. “Over the years, we’ve developed quite a good collaborative base across the country through many different studies of other pathogens including Fusarium, Helminthosporium (silver scurf) and so on,” Peters says. “So we have good collaborations in each province with provincial staff, industry reps from the different chemical companies, grower groups and so forth who will sample for us and then courier the samples to us in Charlottetown for our analysis.”

If growers find pink rot-infected tubers at harvest time or during storage, they send samples to their provincial co-ordinator, who sends the samples to Peters’ lab. The survey’s first season was in the fall and winter of 2013-14; it will be completed in the fall and winter of 2014-15.

For the 2013 crop, the researchers tested a total of 195 isolates from 47 fields and storages. They found resistant isolates in samples from New Brunswick, P.E.I., Nova Scotia, Ontario and Manitoba. No resistant isolates were found west of Manitoba.

Peters cautions that these are preliminary results. “A lot of this also depends on the number of samples, and every year is different that way. If you get a wetter season, it’s more conducive to pink rot, and a drier season is less so. Depending on how many samples we get this year from the western part of the country, we might have a better idea of the resistance spectrum out there.”

Peters thinks the resistance is likely spreading in two main ways: through transport of infested soil and infected seed; and through development of resistance within a production area due to heavy use of metalaxyl.

He explains, “There are always some resistant strains at a very low level in any pathogen population. So when a fungicide is used very heavily, those strains start to become more prominent in the population.”

Evaluating alternatives
“Considering what we’re seeing, especially in states like Maine, North Dakota, Idaho and so on where the Ridomil-resistant populations of the pink rot pathogen are quite pronounced, there’s definitely a need for alternative products. So we have been looking at that as well,” Peters explains.

In the current project, Peters and his research team are conducting field trials in 2014 and 2015 to evaluate in-furrow and foliar fungicide applications. They are comparing phosphite fungicides and several other products that are registered or being considered for registration for pink rot, as well as some experimental products. The trials are being conducted at AAFC’s Harrington Research Farm in P.E.I. The researchers are collecting data on factors like emergence, disease levels and yields in the potato crop.

Peters and his colleagues conducted several previous studies of phosphite fungicides – not to be confused with phosphate fertilizers. Phosphite fungicides have a lower environmental risk than some other fungicides. They are applied as a preventative treatment. They can directly inhibit pathogen growth and reproduction. As well, they can stimulate the plant’s natural defences to fight various pathogens, which can improve plant health and crop yield.

The researchers assessed the effectiveness of phosphites on various diseases. For pink rot and late blight, the studies showed foliar applications of phosphites worked as well as, or better than, metalaxyl-m applications. In addition, the tubers that received phosphites in the field were less susceptible to disease after harvest.

Peters notes that two phosphite products, Confine Extra and Phostrol, are now registered in Canada for in-crop treatment of pink rot.

The current project involves field applications of the fungicides because managing pink rot in the field is critical. “Post-harvest fungicides for pink rot and late blight are mainly phosphite-based products like Phostrol, Confine and Rampart, which work really well at killing pathogen spores on the surface of tubers. But the problem is that a lot of our infections happen in the field. And if tubers are infected in the field, the post-harvest treatment [can’t cure them],” Peters explains.

He adds, “We often consider pink rot as a late-season or harvest disease, but infection of the plant’s underground tissues can occur fairly early in summer. Then those infections progress through the stolons into the tubers as you get closer to harvest-time. So, early management can be really important.”

What you can do now
“[In the survey], we are seeing more metalaxyl-resistant pink rot in the eastern provinces. That is definitely a warning bell not to rely on one fungicide product,” Peters emphasizes.

So, a key practice for growers is to rotate fungicide products to slow the development of resistance. Peters notes that several products are available for a fungicide rotation to manage pink rot.
“Phosphites are excellent products for pink rot,” he says. “They are very systemic. And particularly when applied in the field season, which we often do for a late blight program, they give really good tuber rot control for both late blight and pink rot. In some of those U.S. states where Ridomil resistance has become really common, phosphites have become a solid alternative to Ridomil.” (Note that phosphite fungicides should not be applied to seed potatoes as a foliar or post-harvest treatment until further research has been done.)

Another fungicide option is Bayer CropScience Canada’s Serenade. “This is a biological control product, a Bacillus bacterium, which now has pink rot on the label as a soil-applied product at planting,” Peters explains. As well, Valent Canada and Nufarm Agriculture Inc. are planning to submit an application this fall to Canada’s Pest Management Regulatory Agency to add pink rot to the label of Presidio.

Peters also reminds growers to make use of the other tools in their disease management toolbox.
“Using good-quality seed to start things off is always very important, so you’re dealing with a seed piece that is healthy and able to establish a vigorous plant.

“Since moisture is a large factor in how pink rot establishes itself, areas that can irrigate might be able to control moisture levels [to help reduce the disease].

“And eliminating wounding at harvest can be important because any time you have a wound on a tuber it’s more conducive to storage infections,” he says. Other disease management practices include removing diseased potatoes before storage, and maintaining proper moisture, temperature and air circulation during storage.

Looking ahead, Peters is working on another angle to help growers manage pink rot. “This fungus produces an oospore that lasts in the soil for many years and most of our soils would have oospores in them,” he says. “We are working on molecular tests to try to quantify how much inoculum is in soils. So we might be able to have a better predictor of which fields are particularly prone to pink rot because they have so much inoculum in the soil.”