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When a tuber with pink rot is cut open, the infected tissue turns pink within about half an hour, then brown and then black. (Photo: Bennett Crane and Rick Peters, AAFC)

Tackling Resistant Pink Rot

Although Ridomil resistance is spreading, the list of alternative fungicides is growing.

Ridomil has been the standard fungicide for managing pink rot for many years. However, a national survey shows Ridomil-resistant pink rot is becoming more common.

Fortunately, Canadian research has identified effective alternatives for managing resistant populations.

“The issue of pink rot resistance came to a head in the early 2000s, when different potato production regions in the U.S. were finding some resistance to Ridomil, which has the active ingredient mefenoxam as they say in the U.S. or metalaxyl-m as we say in Canada,” explains Rick Peters, a research scientist with Agriculture and Agri-Food Canada (AAFC) in Charlottetown, P.E.I.

Pink rot is an important disease resulting in tuber breakdown and yield loss so Peters began watching for resistance in Eastern Canada. The disease is caused by Phytophthora erythroseptica, a soil-borne microbe called an oomycete. It thrives in wet soils with warm soil temperatures. The pathogen infects the potato plant’s underground tissues, turning the tissues black and spreading into the daughter tubers. When a diseased tuber is cut open, the infected tissue turns pink within about half an hour, then brown and then black. The disease can spread to other tubers during harvesting and storage.

Pink rot samples collected by Peters and his research team in P.E.I. and New Brunswick from 1999 to 2001 were all found to be sensitive to metalaxyl-m. But a few years later, they found a small amount of resistance in New Brunswick. So from 2005 to 2007, they did some “snapshot” surveys in different parts of Canada to get a broader view of the situation. Again they found resistance only in New Brunswick and it was still at low levels.

“Then in 2012, we had quite a bit of pink rot in P.E.I. so we took a more comprehensive look and found a fair amount of resistance to Ridomil here on the Island,” says Peters.

This serious situation in P.E.I. sparked Peters and his team to initiate a project to conduct a three-year national pink rot survey and to evaluate other fungicides for managing resistant pink rot. Bennett Crane, who was then a master’s student at the University of Prince Edward Island, worked on the project with Peters at AAFC Charlottetown. Funding for this research came from Syngenta, the Keystone Potato Producers Association in Manitoba, the Potato Growers of Alberta and the Prince Edward Island Potato Board.

“This research is important to all Canadian potato growers and it’s particularly important here in P.E.I. because Ridomil resistance has been found here in the past,” says Ryan Barrett, Research Coordinator with the P.E.I. Potato Board. “If growers are trying to manage for pink rot, they need to know that the product they are purchasing is going to work. So they need to know what the populations of the pink rot pathogen are looking like in their area and if they need to adjust their use of products accordingly.”

Expanding Range of Resistance

The national survey was conducted in the fall and winter from 2013-2014 to 2015-2016. Pink rot-infected tubers were sent to Peters’ lab by provincial, federal and industry agencies across the country. A total of 345 isolates were tested to determine if they were sensitive, moderately resistant or highly resistant to metalaxyl-m.

Isolates with moderate or high resistance were found in samples from P.E.I., Nova Scotia, New Brunswick, Ontario, Manitoba and Alberta.

“The results were pretty consistent from year to year, with most of the resistance from Ontario east. In those production areas, about 50 per cent of the populations showed some form of resistance to metalaxyl-m. In the western provinces, less than 10 per cent of the populations were resistant,” summarizes Peters.

“So, there is much more resistance in Eastern Canada versus Western Canada. But the fact that we are starting to see resistance in the west for the first time is also a concern. Pink rot resistance is certainly something to keep on everyone’s radar.”

Effective Alternatives in the Field

From 2014 to 2016, Peters’ team conducted replicated field trials at AAFC’s Harrington Research Farm near Charlottetown to assess alternatives to metalaxyl-m, including some experimental products.

Peters says, “We looked at Orondis (oxathiapiprolin) as an in-furrow application. This is a Syngenta product used foliarly for late blight control, but it is not registered yet as an in-furrow treatment for anything. We also tested Presidio (fluopicolide) as an in-furrow treatment; it is now registered in Canada for pink rot control. We looked at a biocontrol product called Serenade SOIL (Bacillus subtilis) as an in-furrow treatment [it is registered for this use] as a possible option for organic growers or others. For phosphite fungicides, we had either Phostrol or Confine Extra as an in-furrow application or as multiple foliar sprays during the growing season [Phostrol is registered for both types of applications; Confine Extra is registered for foliar applications but not for potatoes intended for seed].”

The check treatments included: non-inoculated healthy tubers; tubers inoculated at planting with a resistant pink rot strain; and tubers inoculated with a sensitive strain. Ridomil Gold was applied as an in-furrow treatment for comparison with the other products.

The research team rated the effectiveness of the fungicides based on the percentage of diseased daughter tubers at harvest (see chart).

“We found that the phosphites were the most effective products, but only when they were applied foliarly through the growing season. These products are very systemic and have the capacity to move down into the tubers to provide protection in the field. Also those tubers still have some of that protection as they move into storage,” explains Peters.

He adds, “There’s a management program to go along with those products that is very important because rate and timing are really significant. If you have too high rates or improper timing you can get some phytotoxicity, such as leaf tip burn, with those products. But if you use those products properly, they are dynamite as a pink rot management tool – and for late blight tuber rot, too.”

In contrast, the phosphites were not effective when applied in-furrow. “Applying a product to the soil is very different than applying it foliarly,” notes Peters. “The soil environment is very complex – there is a dilution effect, there are other organisms that can impact the product’s efficacy and there is plant uptake that is needed.”

The in-furrow applications of Presidio and Orondis were almost as good as the foliar-applied phosphites, providing significant suppression of pink rot.

Serenade provided some pink rot suppression, but it was much less effective than the foliar phosphites, Presidio or Orondis.

As expected, Ridomil worked very well against the sensitive pink rot strain but did not work at all against the resistant strain. “So if a grower knows he is dealing with sensitive strains – and we provided that information to people who submitted samples to the survey – Ridomil is still an effective option,” says Peters.

Good Option for Storage

“At harvest time, you’ll often get soil still adhering to the tubers or the tubers are banged up a little so there is some wounding. If the pathogen is in that soil, then it can infect the tubers going into storage,” says Peters. “So applying a product going into storage can be effective to manage pink rot spores that are around the tuber surface at that time.”

Replicated storage trials have been conducted by Peters in recent years using phosphite-based products. Before storage, the research team inoculated tubers with spores of either a sensitive or a resistant strain of the pink rot pathogen, plus they had a non-inoculated check treatment.

The fungicide treatments – Phostrol, Confine Extra, Rampart – were applied one hour after inoculation with the pathogen. The tubers were stored in conditions favouring pink rot development. Fungicide effectiveness was evaluated based on the degree of disease in the tubers.

All phosphite-based products provided effective post-harvest control of pink rot. “We have also seen some suppression of silver scurf development in storage with phosphite-based products,” notes Peters.

However, for management of pink rot, he still encourages foliar field application of phosphites since this approach prevents field infections and provides tubers with the capacity to resist infections at harvest and into storage as well.

Present and Future

This research has identified several effective options for managing metalaxyl-resistant pink rot, including some products that are recently registered or might be registered in the future.

Another benefit from this research – whether or not you have metalaxyl-resistant pink rot – is the opportunity for fungicide rotations to reduce the risk of developing fungicide resistance.

“Now that we have a few alternatives for pink rot management that look fairly effective, like the phosphites or Presidio and perhaps Orondis down the road, fungicide rotation is an excellent idea,” he says. Strategies like fungicide rotation and tank mixing, and knowing what pathogen populations you’re dealing with are always important, but are especially valuable for fungicide chemistries at high risk for pathogen resistance.

If you have concerns about the possibility of metalaxyl-resistant pink rot in your fields, you can still send samples to Peters’ lab. He explains, “Even though the pink rot project is completed, we have an ongoing project in the oomycetes in general, which includes diseases like late blight, pink rot and Pythium leak. So we would still accept samples but not in the volume that we did with the pink rot project.”

Peters and Crane, who graduated in August 2017, are currently speaking at meetings and conferences to share their findings with growers, specialists and researchers.

Moving forward, Peters will be working on fine-tuning the foliar application practices for the phosphite products so growers will be able to use them successfully. As well, in a year or two, he would like to conduct a snapshot pink rot survey in selected locations in Western Canada to see if resistance is spreading.

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