In the Field, Controlling Pink Rot in Potatos Spud Smart Fall 2010

Phosphate controls the spread of pink rot—if used correctly—and new research show pink rot pathogen in above ground plants parts.

Pink rot, a disease caused by a soil-borne fungus called Phytophthora erythroseptica, can cause significant losses in the field and in storage for growers in Canada and worldwide. Developing resistance in certain strains of the pathogen to metalaxyl-m—an important fungicide in the control and
prevention of pink rot— prompted researchers to find and evaluate another fungicide to help control the disease.

Early on, a phosphite (mono- and di-potassium salts of phosphorus acid)-based fungicide, also known as Confine in Canada, caught researchers’ attention. “It seemed to be the new it product that all the growers were happy to use and to try,” says Khalil Al-Mughrabi, a potato pathologist with the New Brunswick Department of Agriculture, Aquaculture and Fisheries. Registered for use in Canada for two years now, phosphite showed promising results in a series of studies  conducted by Al- Mughrabi, but those results also indicated the fungicide must be used properly or it won’t work.

For example, the results of a study of the in-furrow application at planting of phosphite and metalaxyl-m showed that metalaxyl-m worked fine against metalaxylm- sensitive strains of pink rot, but phosphite applied infurrow was ineffective.

Post-Harvest Application of Phosphite

Yet another study indicates if used in a different way— by applying phosphite on the tubers as they are loaded into storage on the bin piler after harvest—phosphite works very well by killing the fungus on the surface of  the tubers. “The product can potentially protect tubers 100 per cent if they were not infected, which means if the fungus was on the surface of the tuber, the phosphite prevents pink rot from attacking other tubers,” says Al- Mughrabi. If a tuber is already infected, nothing will help because you can’t make a diseased tuber healthy, he notes.

An added bonus to applying phosphite post-harvest is it controls other diseases by default. “Potatoes that were sprayed with [phosphite] post-harvest did have less silver scurf than the ones that were not sprayed.” Also, post-harvest application of phosphite eliminates the spread of late blight.

Next, Al-Mughrabi, in collaboration with scientists at Agriculture and Agri-Food Canada, looked at the foliar application of phosphite in the prevention of disease in daughter tubers. Results indicated that if the fungicide is applied on the foliage in the correct way—at the right time after planting and proper frequency—it prevents infection. The product must be applied four weeks after planting and then once every four weeks (three applications in total) at five litres per hectare. Progeny tubers were protected against pink rot and late blight.

To determine if phosphite would be as effective with irrigation, the product was also applied as a soil drench using the same rates. Researchers found soil drench  worked as well as foliar application. However, if applied to infected tubers as a soil drench or to foliage, the product was ineffective against pink rot and late blight.

Effect of Phosphite on Seed Potatoes and Skin Colour

Al-Mughrabi also wanted to determine the effect of phosphite on seed potatoes. After planting seed potatoes sprayed with phosphite, he discovered the potatoes grew slower than ones not treated with the product. Confirmation of these results from colleagues at AA FC prompted Al-Mughrabi to conduct another experiment using 11 varieties of seed potatoes, storing them at the correct temperatures and humidity to assess sprouting. He found that the product negatively affects sprouting and sometimes made the tuber less serviceable as a seed tuber.

In another experiment conducted by Al-Mughrabi to prove or disprove claims that phosphite changes potato skin colour, he tested 11 varieties of potatoes ranging in colour from red, yellow and purple to russet and chipping varieties by applying the fungicide to those potatoes at the recommended, and twice the recommended, rates. Using charts to determine colour and colour changes, Al-Mughrabi tested skins to determine the colour before application and at the end of the season after application of phosphite. “We concluded it did not affect the colour of the skin for the table market,” he says.

New Discovery

An important new discovery recently made by Al-Mughrabi and other scientists is the pink rot pathogen can translocate in the stem and to the leaves above ground. “In the past, everybody thought pink rot could only be found in the underground parts of the plant— which means the stem, stolon and the tuber itself,” he says, “We did a study where we found it can translocate in the rest of the stem and the leaves.” This information has significant implications for growers: if a plant is infected and growers leave the plant debris on the ground after harvest, growers are adding fungus or pathogen to the soil, says Al-Mughrabi. So, not only are infected tubers a source of inoculum for the next year, but also plant debris can serve as a source as well. Al-Mughrabi cautions growers to be careful with what’s left on top of the soil as well as with what’s left underground.

Pink rot develops in saturated soils and is often found in wet, low-lying areas or those with poor drainage, and is associated with excessive precipitation and irrigation. The surfaces of infected potatoes when exposed to air will turn bright pink in 20–30 minutes. Unlike the pathogen that causes late blight, the cell walls of pink rot fungal spores are thick, thus enabling them to survive adverse weather conditions such as drought or freezing. The fungus that causes pink rot can remain viable in the soil for two to three years. When you plant potatoes the chemicals produced by the roots of the plant attract that fungus and it starts growing, eventually entering the stem and infecting the daughter tubers, says Al- Mughrabi. Thus, a three-year crop rotation should help control the disease.

Al-Mughrabi also advises to choose fields with good drainage, plant disease-free seed and harvest when pulp temperatures are between 7°C and 10°C. Also, if growers suspect their fields have been infested with pink rot, he recommends harvesting the infected area last, so as not to contaminate harvesters and other equipment, and those tubers can be picked and destroyed or stored closer to the door to be easily monitored or taken out quickly.

Used correctly, studies show phosphite is another excellent tool to control pink rot in the field, and with good harvest management practices the incidence of disease should dramatically decrease.

Strategies to Control Pink Rot

  • Plant clean, disease-free seed.
  • Carry out a regular three-year crop rotation.
  • Plant potatoes in well-drained soil.
  • Prior to harvesting check low-lying areas in the feld and rogue diseased plants and tubers
  • Avoid harvesting during wet conditions and kill vines at least two weeks before harvest.
  • Delay harvesting until pulp temperature is between 7°C and 10°C.
  • Tubers with signifcant pink rot should be harvested and stored separately for immediate grading and marketing.
  •  Remove rotted tubers before placing potatoes in storage. In storage, turn on ventilation systems immediately to move air through the pile to dry rotting tubers. Cool tubers as quickly as possible. Keep humidity low to prevent secondary bacterial infection from developing. The fungus is usually inactive at temperatures below 4.4°C.
  •  Fungicides applied in-furrow at planting (metalaxyl) or applied to the foliage during early tuberization (metalaxyl or phosphite) can reduce pink rot losses.

Information provided courtesy of Khalil Al-Mughrabi, potato pathologist for the New Brunswick Department of Agriculture, Aquaculture and Fisheries.

Kari Belanger