March 2014

PAA: Managing Potato Disease With Phosphites

During the past eight years, we have conducted a number of trials and investigations that have examined the efficacy of phosphorous acid-based products or phosphites (Phi) for control of pink rot and late blight and more recently, control of other potato diseases including silver scurf, early blight and Verticillium wilt (Figure 1). These products are mono- and dibasic sodium, potassium or ammonium salts of phosphorous acid, and when applied foliarly or as a root drench, can move systemically within the plant. An added bonus is that these products have a low environmental risk, compared to some of the more toxic protectant fungicides. They can act by directly inhibiting the growth and reproduction of the pathogen, but we also have acquired recent evidence that applying Phi to the foliage can stimulate the production of natural defence chemicals within the potato plant that makes it less susceptible to disease.

Figure 1. Various potato diseases that have been effectively managed with phosphites. A=foliar late blight; B=late blight tuber rot; C=Verticillium wilt; D=pink rot; E=silver scurf; F=early blight.

Phosphates vs. Phosphites

Although the terms sound similar, phosphates and phosphites are very different! Phosphates (PO4) are utilized in fertilizers for plant nutrition. Phosphites (PO3) have mainly fungicidal properties and can be phytotoxic at high rates, especially in undernourished plants. Beware of fertilizer products that claim to have disease control properties – they may or may not have the Phi concentrations needed for adequate disease control! Gefu Wang-Pruski and her team at Dalhousie University have developed an ion chromatography method to detect and quantify Phi in products and in plant tissues. They have been able to use this technology to determine Phi concentrations in various products and to correlate Phi concentrations in leaf and tuber tissues with levels of disease control.

Mode of Action

Phosphites can be directly fungitoxic, but we have also documented the stimulation of a range of natural plant defence compounds following application of Phi. In this way, Phi application contributes to overall plant health and yield and the ability to resist many foliar diseases including Verticillium wilt (Figure 1C) and early blight (Figure 1F). Control of early blight with foliar Phi application has been similar to that afforded by protectant fungicides in our trials. As well, Verticillium spp. have been found to be inhibited by Phi in plate cultures and field trials have realized a 20% reduction in wilt with foliar Phi application.

Late Blight and Pink Rot

Late blight, caused by Phytophthora infestans, is a devastating disease of potatoes and tomatoes that occurs worldwide and causes significant crop losses annually. In recent years, new strains of the late blight pathogen, including US-22, US-23 and US-24 have caused significant epidemics in parts of Canada and the U.S. Late blight symptoms can be found on the foliage (Figure 1A) and serious epidemics can cause complete defoliation resulting in significant yield losses. Spores from infected leaves and stems can also wash into the soil during rain events to infect the tubers, leading to tuber rot in the field and in storage (Figure 1B). Late blight control is challenging since the pathogen overwinters as mycelium in seed tubers, in tubers in cull piles, and in un-harvested diseased tubers which survive the winter and become sources of inoculum. Control of late blight now relies heavily on the use of protectant fungicides which are applied frequently when weather conditions are conducive to disease development.

Pink rot, caused by Phytophthora erythroseptica, is most prevalent when wet conditions occur late in the growing season. The pathogen can overwinter in soils and is often found together with P. infestans in rotting tubers. Tubers with pink rot degrade rapidly and can be diagnosed if internal tissues turn pink after exposure to air (Figure 1D).

Strains of the pink rot and late blight pathogens with resistance to metalaxyl-m (Ridomil Gold), a systemic fungicide, have become commonplace in potato production areas of Canada and the U.S. Clearly, other systemic products useful for the management of late blight and pink rot would be of great benefit to growers. Many studies have documented the benefits of using phosphites for managing both late blight and pink rot. To describe some of our experience, field trials conducted in New Brunswick and Prince Edward Island in 2005 and 2006 compared Phi products (45 to 55 percent active ingredients), applied foliarly, with Ridomil Gold 480EC (metalaxyl-m), applied in-furrow for control of late blight and pink rot. Plants were treated foliarly with Phi from 3-5 times at rates from 2-10 L of product per hectare (0.8 to 4 L per acre). Plants were inoculated with local strains of the pathogens in late August. After harvest, the number and weight of tubers showing pink rot or late blight symptoms was assessed. In addition, tubers from field trials were inoculated post-harvest to see if there was a residual disease suppression conferred to tubers from plants treated in the field.

We found that foliar application of Phi provided excellent control of both late blight and pink rot in tubers, which was equal to or better than that found with metalaxyl-m (Ridomil Gold 480EC). Higher rates of foliar Phi application provided better tuber rot control than lower rates, however a rate of approximately 5L of product per hectare (2 L per acre) was determined to be an appropriate choice. Application of Phi after pathogen inoculation (curative) was generally less effective than application prior to inoculation (preventative) and at least three applications during the growing season were needed to confer disease control (Figure 2). As well, tubers from plants receiving Phi in the field were less susceptible to disease than control tubers when we inoculated the tubers with the pathogens after harvest. In this way, tubers from plants receiving Phi in the field enter storage with a significant level of disease resistance.

Figure 2. Percent of harvested tubers with pink rot from a field trial in Prince Edward Island. C=control, R=Ridomil in-furrow, PA=phosphorous acid (Phi) applied foliarly at either 2.5, 5 or 10 L per hectare 5X or 3X during the growing season.

Trials at Cavendish Farms, Prince Edward Island

Replicated field trials were established at the Cavendish Farms research farm in Prince Edward Island from 2007-2009. The initiation of late blight in the field plots was dependent on wind-blown spores from near-by natural field infections.

The field experiments were established to assess the effect of Phi (Confine) alone and in combination with Bravo (chlorothalonil) to suppress the development of late blight in foliage of both Shepody and Russet Burbank cultivars. The four treatments were: 1) untreated check (no fungicides applied), 2) Bravo alone @ 2L product per hectare applied 10-11 times at weekly intervals, 3) Phi alone @ 5.8L product per hectare, applied every second week for a total of five applications and four) Bravo + Phi, similar rates as above, plots sprayed every week, alternating between Bravo alone and Bravo + Phi. After the first evidence of disease symptoms in the plots, foliar disease was assessed on a weekly basis thereafter. As well, the effect of the fungicide treatments on tuber rot, at harvest and after storage was also assessed. Finally, healthy tubers were taken from each of the plots (10 tubers from each plot) and inoculated with either P. infestans (US-8 strain) or P. erythroseptica (PEI strain). These tubers were then stored at 15°C and 95 percent RH for 2-3 weeks to encourage disease development. Following post-harvest inoculation and incubation, tubers were rated for the incidence and severity of pink rot and late blight tuber rot.

Data was similar in all years of the study and for both cultivars. The data on development of foliar late blight for Russet Burbank in 2008 is summarized in Figure 3. In the check plots, late blight symptoms developed more quickly in the foliage of Shepody plants than in Russet Burbank plants. The most extensive disease development occurred in the check plots. Within two weeks after detection in the check plots, symptoms started to appear in plots treated with Phi. Later in the season, foliage damage caused by late blight occurred in the plots treated with Bravo and Bravo + Phi.

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 tubers than in Russet Burbank tubers (Table 1). There was a direct association between the extent of late blight in foliage and losses in total biological yield of tubers. Although extensive foliar blight eventually developed in foliage of plants/plots sprayed with Phi, significantly less tuber rot was observed in these plots compared to the check plots (Table 1). Very little tuber rot was observed in plots where plants had been treated with Bravo alone or Bravo + Phi.

Following post-harvest inoculation, healthy tubers from plots receiving Phi during the field season were significantly more resistant to the development of pink rot and late blight tuber rot in storage (Table 2)

Figure 3. Seasonal development of late blight in Russet Burbank potatoes treated with various foliar fungicides. PA=phosphorous acid (Phi)

Table 1. Effect of fungicide treatments on tuber rot at harvest and potato yields.

Table 2. Effect of foliar fungicide treatments on percentage of tubers with rot following post-harvest inoculation of healthy tubers with late blight (LB) and pink rot (PR) pathogens.

Replicated field trials have repeatedly shown that Phi products can provide significant disease control benefits when incorporated into a late blight and pink rot management program. Phi, when applied alone, delayed foliar disease epidemic development by about two weeks in our studies. This could be an important benefit for growers during those times when application of protectant products in a timely fashion is difficult due to adverse weather conditions. The combination of Phi with Bravo was a particularly effective approach likely due to the synergies captured by combining a truly systemic (Phi) with a protectant (Bravo) product. In addition to achieving foliar disease suppression, the suppression of tuber rot achieved by incorporating a Phi product into a late blight/pink rot management program must be stressed. This is the major benefit of using Phi products because not only is tuber rot prevented in the field, tubers from plants receiving Phi have intrinsic resistance properties that prevent subsequent infection and disease development in storage.

In 2011, Cavendish Farms staff sampled tubers from fields using various fungicide programs (Table 3) that incorporated a Phi product (Confine). We inoculated these tubers with spores of the late blight pathogen and the resulting disease development is shown in Figure 4.

Table 3. The incorporation of a Phi product into various late blight management programs in Prince Edward Island in 2011.

Figure 4. Mean incidence of late blight (percent of infected tubers) harvested from different fields (Fields 1-6) following inoculation with spores of the late blight pathogen.

The programs used in Fields 3, 4, and 5 provided good tuber rot control. Field 2 had insufficient application of Phi for good disease suppression. Interestingly, tubers from Field 6 were somewhat more susceptible to disease. A recent analysis of the loading of phosphite into the tubers (by Gefu Wang-Pruski and her team at Dalhousie University) exactly matched the disease suppression data, such that the highest levels of phosphite were found in tubers most resistant to disease. The reason for the lack of translocation of phosphite into tubers from Field 6 is unknown, but could be due to a number of factors including the age of plants, plant stress, weather conditions following application and interactions with other chemical components of the fungicide program.

The technology employed by the Dalhousie University team to detect Phi in plant tissues has also led to several related discoveries. The strong correlation between application rate, Phi loading into tissues, and disease control has been confirmed. As well, maximum movement of Phi into leaf tissues after application can take up to two days – therefore, applications should be made during a period of dry weather. Phi concentrations steadily drop in tissues up to two weeks after application and hence repeated applications are needed to maintain disease control.

Post-Harvest Use of Phosphites

During harvest operations, pathogen spores can be splashed from diseased to healthy tubers before they enter storage resulting in potential rot during storage. Early work in New Brunswick and Prince Edward Island with Phi products showed that, when applied as a post-harvest spray to potatoes entering storage, complete suppression of late blight tuber rot and pink rot caused by these splashed spores could be achieved. It should be noted however, that tubers already infected in the field could not be cured with this approach. Also, it is important to apply the spray as soon after harvest as possible (preferably within six hours of harvest) to prevent the spores from germinating and infecting the tubers.

An additional benefit of post-harvest application has been suppression of silver scurf (Figure 1E) development on tubers in storage. No negative impacts on tuber skin colour or fry colour have been noted following post-harvest Phi application. As always, it is important to accurately measure the amount of moisture applied to tubers entering storage, since excessive moisture can lead to increased bacterial soft rot and decay.

A common question that is often asked is which approach to take – foliar or post-harvest application? Since most infections occur in the field, foliar application of Phi is best to deal with these infections. As mentioned, Phi is also translocated to the tubers to confer tuber protection from infection in the field and after harvest. However, when concerns about late blight and pink rot spores are present at harvest, post-harvest application can certainly provide benefits. Work on detection of Phi in plant tissues by the Dalhousie University team has clearly shown that Phi is predominantly found in the tuber cortex following foliar application, compared to being found mainly in the skin following post-harvest application. The Phi application profile should be tailored to individual operations and the disease pressures in any given year.

Negative Plant Impacts After Phosphite Application

A note of caution – application of Phi to seed potatoes, either as a post-harvest treatment, to cut seed or to the foliage of seed fields has been shown to have detrimental effects on seed sprouting (Figure 5A), delay emergence (Figure 5B) and reduce the vigor of plants grown from this treated seed in some situations and thus, these applications should be avoided until further research has been completed. As well, high rates of application of Phi products to aging plants late in the growing season have caused leaf tip burn in some cases (Figure 5C,D). Research to determine the levels of Phi in plant tissues, combined with various environmental and climatic factors, that lead to negative plant impacts is on-going.

Figure 5. Potential negative plant impacts of phosphite application. A=inhibition of sprouting with post-harvest application; B=delay in emergence and reduced vigour in seed from fields receiving foliar phosphite aplications; C,D=late season tip burn following high rates of foliar phosphite application to aging, drought-stressed plants

In conclusion, our studies indicate that Phi products can provide significant disease control benefits when incorporated into an integrated management program. A summary of best management practices for using phosphites would include:

Foliar

• start early! (especially before foliar disease is found)
• excellent control of oomycetes (late blight and pink rot)
• stimulates plant health and management of other pathogens
• need sufficient dose to get disease control (season total of 7-10 L/ac is reasonable)
• 3-4 spaced applications
• reduce rates as plants age or in times of stress (drought)
• watch the weather to avoid wash-off immediately after application
• be cautious of use on fields destined for seed since some adverse plant effects have been noted (more research is needed on quantity of phosphite in plant tissue that may cause concern and interactions with other nutrients and environmental and crop factors)

Post-Harvest

• apply at label rate as soon as possible after harvest
• manage water volumes
• control of silver scurf a bonus
• still cautious about application to seed

Rick D. Peters, Agriculture and Agri-Food Canada, Charlottetown, PE, C1A 4N6 Canada; Gefu Wang-Pruski, Dalhousie University, PO Box 550, Truro, NS, B2N 5E3 Canada; Khalil I. Al-Mughrabi, New Brunswick Department of Agriculture and Aquaculture, Wicklow, NB, E7L 3S4 Canada; Zenaida Ganga, Cavendish Farms, Summerside, PE, C1N 5J5 Canada ; Robert H. Coffin, Privar Farm Inc., North Wiltshire, PE, C0A 1Y0 Canada ;Tudor Borza, Dalhousie University, PO Box 550, Truro, NS, B2N 5E3 Canada; Xingxi Gao, Dalhousie University, PO Box 550, Truro, NS, B2N 5E3 Canada;and Sangyhun Lim, Dalhousie University, PO Box 550, Truro, NS, B2N 5E3 Canada;