Storage Science

The latest on controling late blight and pink rot.

Phosphorous acid has captured the attention of most potato storage researchers in Canada and the United States over the past few years as a novel remedy for some of the most serious storage-related diseases, including post-harvest late blight and pink rot.

Late blight symptoms can be found on plant foliage and serious epidemics can cause complete defoliation, resulting in significant yield losses. Spores from infected leaves and stems can wash into the soil during precipitation infecting tubers, resulting in tuber rot in the field and later in storage.

Pink rot is most prevalent when wet conditions occur late in the growing season. The pathogen can overwinter in Canadian soils and is often found together with late blight in rotting tubers. The internal tissues of tubers infected with pink rot turn pink after exposure to the air and degrade rapidly.

Research originating from the University of Idaho has shown in many dedicated trials over several years that the phosphite in phosphorous acid-based fungicides directly inhibits the growth of water mould fungi.

The pathogens causing late blight and pink rot belong to this group of fungi. Researchers at the university’s Kimberly Research and Extension Center concluded it is likely that both direct fungicidal activity as well as promotion of natural plant defenses play a role in the efficacy of phosphorous-based fungicides for control of late blight and pink rot. This conclusion is based on the fact that the active ingredient phosphite has also been shown in previous research studies to induce natural defense reactions in plants.

It is important that growers note the difference between phosphorous acid and phosphoric acid. When phosphorous acid dissociates it releases phosphonate (also called phosphite). When phosphoric acid dissociates it releases phosphate—a fertilizer.

Here in Canada, scientists confirmed these conclusions by U.S. researchers with respect to the natural defence reactions in plants when phosphorous acid-based fungicides are utilized. Canadian researchers say they have acquired evidence that foliar application of phosphorous acid can stimulate the production of natural defence chemicals within the potato plant making it less susceptible to disease.

Rick Peters, a research scientist for Agriculture and Agri-Food Canada in Charlottetown, and his team have been conducting trials since 2005 to examine the efficacy of phosphorous acid-based products for control of pink rot and late blight tuber rot, and, more recently, for the control of foliar late blight. In a recently published research paper entitled Managing late blight and pink rot of potatoes with foliar and post-harvest applications of phosphorous acid, and based on numerous studies conducted in Prince Edward Island and New Brunswick, researchers revealed they are optimistic about the efficacy of these products.

The researchers found when phosphorous acidbased products are applied foliarly or as a root drench, the active ingredient moves systemically within the plant. These products act by directly inhibiting the growth and reproduction of the pathogen, as well as inducing natural defense reactions in plants. Also, they have a low environmental impact compared to some of the more toxic protectant fungicides.

Fungicide resistance is a serious problem and a harsh reality for the control of many potato-related diseases—and this is also true for control of late blight and pink rot. The new US-8 strain of late blight dominating populations of the pathogen in Central and Eastern Canada is of the A2 mating type, which is very aggressive and is resistant to metalaxyl-m, a systemic fungicide that was effective against the A1 strain (US-1). Control of late blight now relies heavily on the use of protectant fungicides such as chlorothalonil or mancozeb.

Most strains of pink rot are still sensitive to metalaxyl-m; therefore, this systemic product still works well at controlling this disease, particularly if it is applied in-furrow during planting. However, in recent years, strains of pink rot with resistance to metalaxyl-m have been found in New Brunswick.

Clearly, researchers concluded, other options for the management of late blight and pink rot are needed by growers. And this is where phosphorous acid  comes into play.

During the 2005 and 2006 growing seasons, Peters and his team conducted several field trials in New Brunswick and Prince Edward Island to compare the efficacy of phosphorous acid-based products (applied foliarly) and the in-furrow application of metalaxyl-m to control late blight and pink rot. They found that foliar application of phosphorus acid provided excellent control of both late blight and pink rot in tubers, which was equal to or better than that found with metalaxyl-m. Higher rates of foliar phosphorus acid application provided better tuber rot control than lower  rates, but an application rate of 5 litres per hectare was determined the best choice in terms of control and economy.

Replicated field trials were conducted in Prince Edward Island from 2007 to 2009. Field experiments assessed the effects of Confine, a commercial phosphorous acid-based product, on its own as well as in combination with Bravo on the suppression of the development of late blight in the foliage of both Shepody and Russet Burbank cultivars.

The researchers also took healthy tubers from each of the plots and inoculated them with late blight (US-8 strain) or pink rot (P.E.I. strain). The tubers were then stored at 15°C and 95 per cent relative humidity for two to three weeks to encourage disease development. After postharvest inoculation and incubation, tubers were rated for the incidence and severity of pink rot and late blight.

The results showed that in check plots, late blight symptoms developed more quickly in the foliage of Shepody than Russet Burbank plants. The most extensive disease development occurred in the check plots. When tubers were graded at harvest, rot was most extensive in the tubers from the check plots. As well, rot was more extensive in Shepody than Russet Burbank tubers.

Although extensive foliar blight eventually developed in foliage of plants sprayed with phosphorus acid alone, significantly less tuber rot was observed in these plots compared to the check plots. Very little tuber rot was observed in plots where plants had been treated with Bravo alone or Bravo in combination with phosphorus acid. The combination of phosphorus acid with Bravo was particularly effective likely due to the synergies captured by combining a truly systemic product (phosphorus acid) with a protectant fungicide (Bravo).

The researchers further observed that following post-harvest inoculation, healthy tubers from plots receiving  phosphorus acid during the field season were significantly more resistant to the development of pink rot and late blight tuber rot when in storage.

Application of phosphorus acid after pathogen inoculation was generally less effective than application prior to inoculation. At least three applications during the growing season were needed to confer disease control. The researchers observed no phytotoxicity when phosphorous acid was applied to potato foliage. However, post-harvest application to seed tubers is not recommended (and indeed is not a registered application) due to the inhibition of sprouting that can occur. Delayed emergence has also been observed from seed tubers harvested from plants receiving foliar applications of phosphorous acid during the previous field season, and, therefore, growers should be cautious about using these products for potato seed production.

Tubers already infected in the field could not be cured, notes Peters. He warns growers if spraying phosphorus acid for in-storage prevention of late blight or pink rot, it is important to apply it as soon as possible after harvest— preferably within 6 hours—to prevent spores from germinating and infecting other tubers. Lukie Pieterse