Report to the Research Subcommittee of the Maine Potato Board

 

Management of Sclerotinia sclerotiorum, the causative organism of white mold in potatoes in Aroostook County, Maine.

 

PPrincipal Investigator:                                        Andrew B. Plantt

           UMCE- Potato IPM Program

           P.O. Box 727

           Presque Isle, ME 04769

 

Co- Investigator:                                               Dr. David H. Lambert

 

Department of Plant, Soil, and Environmental Sciences

5722 Deering Hall, Rm. 9

University of Maine

Orono, ME 04469-5733

 

Executive Summary:

 

 

Sclerotinia sclerotiorum (Lib.) de Bary is an increasingly more important issue in this growing area as we continue to increase and improve irrigation practices and incorporate rapeseed, sunflower, and soybean rotations into our potato-cropping areas.  Sclerotinia sclerotiorum (Lib.) de Bary has a wide host range, affecting many families of plants, and current estimates include over 400 plant species.  In potatoes, the white mold fungus causes senescence and premature death of aboveground plant parts but is typically not associated with tuber infection.  Potato cultivars have been generally observed to vary in their susceptibility to the fungus; some varieties seem to have relatively low incidence and severity, yet other varieties have shown yield losses estimated to be 50 percent (Johnson and Dwyer, personal communication).  Crop rotation is an important factor for white mold inoculum levels.  Crop susceptibility to white mold plays an important role in the selection of rotation crops and may be a determining factor for disease incidence and severity for subsequent potato crops.  White mold of potatoes can be controlled by fungicide application; however, in the past, issues of application timing made chemical control economically unfriendly.  Research of other susceptible crops has provided increased knowledge as to the biology, epidemiology, and subsequent improvement in timing of fungicide control measures.  Presently, there are several available commercial fungicides that show efficacy against S. sclerotiorum.

In 2004, research, applying information from multiple crops and geographic regions, was initiated to make an assessment of white mold in potatoes in the Aroostook County geographic region.  Infection biology, varietal susceptibility, and fungicide efficacy and timing trials were conducted to evaluate white mold in potatoes.  The conclusions drawn from this must be considered preliminary, as they are based on one season’s data.  From the trials conducted it appears that:

 

1.) There were no significant yield differences within or between Supeior, Shepody, and Russet Burbank treatments.  There were, however, significant disease incidence and severity differences at both geographic locations (Houlton and Presque Isle).  White mold infection was seen to significantly reduce yields in other trial plots that included only cv. Superior.

 

2.) A proportion of stem infection results from spore infection of flowers and subsequent blossom drop, but by the observation of white mold occurrence in certain treated plots, it appears that a proportion of infection may also result from direct infection of stems via fungal mycelium, direct infection of stem wounds by fungal spores, and direct plant-to-plant contact.

 

3.) Tested materials of Endura®, Topsin M®, Omega®, Rovral®, Sonata®, and an untreated control resulted in statistically significant differences in ratings yet no statistically significant differences in total or marketable yield.

 

4.) Fungicide timing trials, resulted in the conclusion that flowering stage (proportion of crop in bloom) is of importance to the success of white mold control.  There was a statistically significant linear relationship in ratings and yields between treatment times.

 

 

Acknowledgements

We would like to thank Ms. Erica Fitzpatrick, Mr. Albert Fitzpatrick, Maysville Farms, and Mr. James Dwyer for their graciousness, cooperation, and hard work.  The efforts of these people were essential for the completion of these research trials.

 

 

 

Introduction

 

Sclerotinia sclerotiorum (Lib.) de Bary is an increasingly more important issue in this growing area as we continue to increase and improve irrigation practices and incorporate rapeseed, sunflower, and soybean rotations into our potato-cropping areas. In 2004, research, applying information from multiple crops and geographic regions, was initiated to make an assessment of white mold in potatoes in the Aroostook County geographic region.  Infection biology, varietal susceptibility, and fungicide efficacy and timing trials were conducted to evaluate white mold in potatoes.

 

Methods

 

Two sites were selected based upon the probability of high inoculum.  Sites in Houlton, ME and Presque Isle, ME were selected based upon the previous growing season rotation of sunflowers, a crop highly susceptible to white mold and having an increased likelihood of providing sufficient inoculum levels to produce effects in the following potato crop.  The plot in Presque Isle, ME, due to size restrictions, was only used for a varietal susceptibility trial.  All plots were planted with fresh-cut seed using a plant spacing of 36 inches between rows and 14 inches within rows. Disease presence and severity in all plots was determined by scouting 10 plants within the middle rows of each plot.  Plants were rated using the ratings scale of, 0= no stem/leaf symptoms, 1= x <10% stem/leaf symptom, 2= 10%<x<30% stem/leaf symptoms, stem lesion at base, 3= 30%<x<60% stem/leaf symptoms, slight girdling, external sclerotia, 4= x>60% stem/leaf symptoms—girdling and death of 25% to 50% primary stems, 5= death of >50% of primary stems, stem/leaf symptoms on remaining plant tissue, internal sclerotia. 

 

Varietal Susceptibility Trials

 

            Three varieties of potato (Russet Burbank, Shepody, and Superior) were evaluated for their susceptibility to white mold in Houlton and Presque Isle, Maine.  Two of the three varieties were subjected to three different treatments employing Endura® brand fungicide as an industry standard fungicide for white mold control.  Russet Burbank received treatments of no Endura® application (untreated control), and 1 Endura® application (due to weather conditions at appropriate spray times), while Shepody and Superior received treatments of zero (untreated control), one, and two applications of Endura®.  Single application treatments were timed to approximate “full bloom” of the respective variety, while the two application treatment included an Endura® application at “full bloom” and another application two weeks post “full bloom.”  For both trials, plots were laid out in a randomized complete block design with five replications.  Plot size was 40 feet by four rows.  Fertilization, cultivation, and foliar fungicide applications to prevent late blight, done by the cooperating grower, were of regional standard.  Applications of Endura® were performed using a pressurized backpack sprayer fitted with a two-row boom with 2-ft drop nozzles.  White mold incidence, severity, and yield effects were evaluated.  Plots at Presque Isle, ME were rated three times throughout the season while plots in Houlton, ME were rated only twice due to weather conditions.  and 28 row feet from the middle two rows of each plot were harvested to determine yields.  Data was analyzed using analysis of variance (ANOVA), and Duncan’s Multiple Range Test (DMRT).

 

 

 

 

Infection Biology Trial

 

            Superior variety potatoes under three different treatments were evaluated for white mold incidence and severity, and yield at one site in Houlton, ME.  Selected treatments were; fungicide treatment (Endura®) at “full bloom” and 2 weeks post “full bloom”; blossom picking, in which all of the flowers were picked from the plants within a plot; and an untreated control (no Endura®, no blossoms picked).  Plots were laid out in a randomized complete block design with four replications.  Plot size was 25 feet by 4 rows.  Fertilization, cultivation, and foliar fungicide applications to prevent late blight, done by the cooperating grower, were of regional standard.  Applications of Endura® were performed using a pressurized backpack sprayer fitted with a two-row boom with 2-ft drop nozzles.  White mold incidence, severity, and yield effects were evaluated.  Plots were rated three times throughout the season and 20 row feet were harvested from the middle two rows of each plot to determine yields.  Data was analyzed using analysis of variance (ANOVA), and Duncan’s Multiple Range Test (DMRT).

 

Fungicide Efficacy Trial

 

            Superior variety potatoes were planted at Houlton, ME.  Plots consisted of four 25-ft long rows.  Tested materials included Endura®, Topsin M®, Omega®, Rovral®, Sonata®, and an untreated control.  Treatments were arranged in randomized complete block design with four replications.  Applications of tested materials were performed using a pressurized backpack sprayer fitted with a two-row boom with 2-ft drop nozzles. First applications of tested materials were timed to “full bloom” of Superiors or approximately 50% flowering; applications were repeated only once, and were timed to 2 weeks post “full bloom.”  Disease presence and severity was rated three times throughout the season and potatoes were harvested from two 10 ft. sections in middle rows of each plot to determine yields.  Analyses of variance and Tukey’s HSD were performed on yield and ratings data.  Fertilization, cultivation, and foliar fungicide applications, done by the cooperating grower, were of regional standard.

 

Fungicide Timing Trial

 

            Superior variety potatoes were planted at Houlton, ME in plots consisting of four 25-ft long rows.  Selected times of fungicide (Endura®) application were timed to flowering stage.  Five different treatments in which only one fungicide application occurred included “Initial Flowering”, “Full Bloom”, “1 week post Full Bloom”, “2 weeks post Full Bloom”, and an untreated control.  20 plants within each plot were randomly selected at different sampling times throughout the flowering stage and the number of flowers in bloom and number of buds (potential flowers not in bloom) were recorded on each plant.  Calendar based spray times were then able to be “fixed” to a unique proportion of bloom, providing us a picture of spray timing according to flowering stage.  Treatments were arranged in randomized complete block design with four replications.  Applications of Endura® were performed using a pressurized backpack sprayer fitted with a two-row boom with 2-ft drop nozzles.  White mold incidence, severity, and yield effects were evaluated.  Plots were rated three times throughout the season and 20 row feet were harvested from the middle two rows of each plot to determine yields.  Data was analyzed using analysis of variance (ANOVA), and Duncan’s Multiple Range Test (DMRT).

 

 

 

Results and Discussion

 

Monthly rainfall totals for June, July, and August were all above normal.  Disease pressure and potato yield potential was very high for the 2004 growing season and therefore discernment of treatment effects on yields was generally minimal.  With some trials we were able to produce some inference as to the biology and management of white mold in potatoes; while in other trials uncontrollable factors likely played a part in the discrepancy of insignificant results.

 

Varietal Susceptibility Trials

 

            In the varietal susceptibility trials we were able to observe statistically significant differences in white mold disease ratings within and between varieties due to treatment effects.  However, we were not able to elucidate significant differences in total or marketable yield due to treatment effects at Presque Isle, ME (Table 1) or Houlton, ME (Table 2).  The trial at Presque Isle, ME provided us more of a discernable difference, albeit non-significant.  Observations in other trials conducted in the 2004 growing season (Infection Biology and Spray Timing trials) however produced results that show yields for the potato variety Superior are highly dependent upon the level of white mold infection and are significantly different among treated and untreated plots.  There are various auxiliary hypotheses to describe the conflicting observations between varietal susceptibility and other trials:

a.)Inoculum levels may have been unevenly dispersed throughout the varietal susceptibility trials leading to an unforeseen uniformity between treatments that experimental design (blocking) could not have accounted for. 

b.)Observations of European corn borer (ECB), blackleg, and gray mold were made throughout the season, most notably at the Houlton, ME site.  Counts were taken from the varietal susceptibility plot in Houlton, ME, and a probability of incidence model was produced (Figure 1).  From the model we are able to see that there is a significant variety interaction, as well as an interaction with the selected fungicide (Endura®).  We were unable to provide any significant evidence of yield effects upon cross referencing the ECB data with the white mold data.  It may, however, be said that one may expect with increased ECB infestation, an increase in blackleg incidence.  With the wet conditions that occurred this past growing season, the differing levels of blackleg incidence may have played an important factor in determining yield differences in the varietal susceptibility plots.  Information regarding blackleg incidence was not collected.

            To account for these hypotheses in retrospect is nearly impossible; however, experimental design for trials to be conducted next year will include a “nested” randomized complete block design in which treatments will be done in grouped variety plots.  Attempts at acquiring more in depth ECB and blackleg data will also be made.

 

Table 1.) Yield and Ratings Data:  Varietal Susceptibility Trials, Presque Isle

                                                        Disease Ratings (DAP)­­­­a

                                 Rate                                                                   TotalYield                Marketable Yield

Treatment              Per Acre                 96      110       117                     (cwt/ac.)                    (cwt/ac.)___

Superior

0                                  -                      3.68ab   4.74a    4.90a                   286.31abc                     261.71ab

1                               5.5oz                  2.62bc  4.20b    4.76ab                 303.27a                         278.18a

2                             2*5.5oz                 2.34cd  3.82c    4.56b                   304.63a                         282.19a

Shepody

0                                  -                      2.94b    3.26c    3.86c                   248.76c                         209.03c

1                               5.5oz                  2.18de  3.10de  3.50d                   266.29abc                     223.83c

2                             2*5.5oz                 2.14de  2.88ef   3.38d                   254.54bc                       215.04c

Russet Burbank

0                                  -                      1.90ef   3.26d    3.62dc                 281.08abc                     211.90c

1                               5.5oz                  1.61f     2.77f     3.32d                   295.67ab                       235.56bc

aDAP= Days After Planting

bData were analyzed using ANOVA (P=0.05). Values followed by the same letter are not significantly different at P=0.05 (DMRT).

 

Table 2.) Yield and Ratings Data: Varietal Susceptibility Trials, Houlton.

                                                        Disease Ratings (DAP)­­­­a

                                 Rate                                                                   TotalYield                Marketable Yield

Treatment              Per Acre                114     120                                (cwt/ac.)                    (cwt/ac.)___

Superior

0                                  -                      3.82ab   4.72a                               376.17a                         352.59a

1                               5.5oz                  2.98b    4.42b                               350.16a                         322.82ab

2                             2*5.5oz                 2.84bc  4.48b                               360.04a                         336.18ab

Shepody

0                                  -                      2.60cd  3.26c                               333.96a                         296.61b

1                               5.5oz                  2.44de  3.08cd                             336.44a                         304.53ab

2                             2*5.5oz                 2.20e    3.00de                             331.43a                         302.64ab

Russet Burbank

0                                  -                      1.52f     3.04cde                           370.26a                         317.40ab

1                               5.5oz                  1.24f     2.81e                               369.58a                         320.84ab__

aDAP= Days After Planting

bData were analyzed using ANOVA (P=0.05). Values followed by the same letter are not significantly different at P=0.05 (DMRT).

 

 

Figure 1.) Varietal Susceptibility Trial: Houlton, ME, ECB infestation versus Endura® application.

 

 

 

Infection Biology Trial

 

            In the infection biology trial, there were significant differences between treated plots and the untreated control (Table 3).  The Endura® and “Picked blossoms” treatments rated statistically similar to each other and significantly less than the untreated control plots for white mold incidence and severity.  Total and marketable yield for Endura® and “Picked blossoms” displayed similar results, both yielding significantly greater than the untreated control.  Preliminary conclusions include; picking blossoms results in the decrease of white mold symptoms and increased yield to a value statistically similar to fungicide treatment; varieties less inclined to produce flowers may have a decreased occurrence of white mold infection; a proportion of stem infection results from spore infection of the petals and subsequent blossom drop.  Flowers from this trial and others were plated throughout the season onto the white mold-selective growth medium of “Rose-Bengal” agar.  Plating blossoms on this growth medium provided us observable white mold spore infection of blossoms uniformly throughout all trial plots.  Infection rates of blossoms ranged from 90% to 100%, providing evidence that blossoms are extremely susceptible to infection by white mold.  With the observation of white mold occurrence in the “Picked blossoms” plots, it appears that a proportion of infection may also result from direct infection of stems via fungal mycelium, direct infection of stem wounds by spores, and direct plant-to-plant contact transferring inoculum from one plant to another. 

 

Table 3.) Yield and Ratings Data: Infection Biology Trial.

                                                        Disease Ratings (DAP)a _­­­­___       

                                                                                             Total Yield   Marketable YieldTotal Yield  Marketable Yield  Marketable Yield

Treatment        Per Acre       104         114             120 P               (cwt/ac.)          (cwt/ac.)

Untreated Check    -            2.80aba      3.45a          4.52a           377.18b          350.69b

Endura                5.5oz        2.30b       2.95b          4.23b          424.20a          409.23a

Picked Blossoms    -           2.28b        3.13b          4.28b          416.89ab        397.23a

Data were analyzed using ANOVA (P=0.05). a DAP= Days After Planting

ba.) Data were analyzed using ANOVA (P=0.05). Values followed by the same letter are not significantly different at P=0.05 (DMRT).

 

Fungicide Efficacy Trial

 

            In the fungicide efficacy trial, there were statistically significant ratings between materials tested, however no statistically significant differences in total or marketable yield (Table 4).  The lack of statistical difference in yield may have been a result of spray timing; where had the plots been treated prior to 50% flowering yield differences may have been greater.  Endura and Omega treatments had the lowest disease ratings.  Sonata AS treatment yielded the greatest, with approximately a 10% yield increase compared to the untreated check, but was not significantly different from other treatments.  Of note, disease ratings for Sonata AS were not significantly different from the untreated check, providing circumstantial evidence that there may be a secondary benefit or control using Sonata AS treatment.  Infestations of European corn borer, blackleg, and gray mold were noted in this plot.  No phytotoxic effects were observed with any treatment.

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 4.) Yield and Ratings Data: Fungicide Efficacy Trial.

                                                                    Disease Ratings (DAP)­­­­a

Treatment                 Rate                                                                   TotalYield               Marketable Yield

Formulation           Per Acre                104      114      120                   (cwt/ac.)                    (cwt/ac.)___

Untreated Check         -                     4.05ab   3.55a   4.48a                 400.44a                       373.22a

Endura                     5.5oz                  2.78c   3.13b   4.08a                 404.33a                       382.13a

Omega 500F          5.5fl oz                2.70c   3.18b   4.15a                 409.99a                       390.01a

Rovral 4F              32.0fl oz               3.45b   3.25b   4.33a                 413.69a                       389.48a

Topsin M 70WP     16.0oz                 3.13bc 3.15b   4.20a                 390.53a                       369.62a

Sonata AS            128.0fl oz              3.63ab 3.33ab 4.35a                 439.54a                       410.83a

a DAP= Days After Planting

bData were analyzed using ANOVA (P=0.05). Values followed by the same letter are not significantly different at P=0.05 (Tukey’s HSD).

 

Fungicide Timing Trial

 

            In the fungicide timing trial, there were statistically significant differences in ratings and yields between treatment times and the untreated control (Table 5).  Proportion of bloom at each treatment time was estimated to infer the effect of flowering stage on ratings and yields.  Treatments at “Initial Flowering” were estimated to be at 37% flowering at time of treatment, “Full Bloom” was estimated to be at 61% flowering at time of treatment, “1 week post Full Bloom” was estimated to be at 80% flowering at time of treatment, “2 weeks post Full Bloom” was estimated to be at 92% flowering at time of treatment, and an untreated control was considered 100% flowering (no time of treatment).  Treatment times of 37% flowering and 61% flowering rated and yielded statistically similar, however it appears as though treatment times prior to 50% flowering would be more practical where this data expresses an approximate10% yield difference between treatment prior to 50% flowering and treatment post 50% flowering.  Figure 2 shows a linear relationship between proportion of flowering at treatment times versus total and marketable yield.  It is thought that the relationship may not be truly linear, in that early treatment times may not provide adequate residual carryover to sufficiently protect a great proportion of flowers at peak bloom.  It is therefore of question what the true relationship may be, and will be proposed for next year’s research trials.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 5.) Yield and Ratings Data: Spray Timing Trial.

                                                       Disease Ratings (DAP)a­­­­

                            Proportion                                                              TotalYield               Marketable Yield

Treatment              Flowered               104      114      120                   (cwt/ac.)                    (cwt/ac.)___

Untreated Check       1.00                   3.20ab   3.65a   4.60a                 361.60c                       338.41c

2wk> Full Bloom      0.92                   2.90ab 3.50a   4.50ab               387.36bc                     361.04bc

1wk> Full Bloom      0.80                   2.65bc 3.25b   4.38b                 401.01bc                     379.75bc

Full Bloom                0.61                   2.38c   3.03c   4.20c                 417.72ab                     395.14ab

Initial Flowering         0.37                   2.78bc 3.18bc 4.38b                 459.36a                       435.51a

a DAP= Days After Planting

bData were analyzed using ANOVA (P=0.05). Values followed by the same letter are not significantly different at P=0.05 (DMRT).

 

Figure 2.) Flowering Stage Versus Yield.

 

 

 

 

 

 

 

 

Summary

 

 

Infection biology, varietal susceptibility, and fungicide efficacy and timing trials were conducted to evaluate white mold in potatoes.  The conclusions drawn from this must be considered preliminary, as they are based on one season’s data.  From these trials it appears as though:

 

1.)  There were no significant yield differences within or between Superior, Shepody, and Russet Burbank treatments.  There were, however, significant disease incidence and severity differences at both geographic locations (Houlton and Presque Isle).  Conflicting observations as to yield effects remain to be verified; white mold infection was seen to significantly reduce yields in other trial plots that included only Superiors, yet we had no significant effect of treatment in varietal susceptibility plots.  Hypotheses as to this discrepancy are listed in the “Results and Discussion” section of this report.

 

2.) Flowers from this trial and others were plated throughout the season onto the white mold-selective growth medium of “Rose-Bengal” agar.  Plating blossoms on this growth medium provided observable white mold spore infection of blossoms uniformly throughout all trial plots.  Infection rates of blossoms ranged from 90% to 100%, providing evidence that blossoms are extremely susceptible to infection by white mold.  It appears as though picking blossoms results in the decrease of white mold symptoms and increases yield to a value statistically similar to fungicide treatment; varieties less inclined to produce flowers may have a decreased occurrence of white mold infection; a proportion of stem infection results from spore infection of the petals and subsequent blossom drop, but by the observation of white mold occurrence in the “Picked blossoms” plots, it appears that a proportion of infection may also result from direct infection of stems via fungal mycelium, direct infection of stem wounds by fungal spores, and direct plant-to-plant contact.

 

3.) Tested materials of Endura®, Topsin M®, Omega®, Rovral®, Sonata®, and an untreated control resulted in statistically significant differences in ratings yet no statistically significant differences in total or marketable yield. Endura and Omega treatments had the lowest disease ratings.  Sonata AS treatment yielded the greatest, with approximately a 10% yield increase compared to the untreated check, but was not significantly different from other treatments.  No phytotoxic effects were observed with any treatment.  Of note, infestations of European corn borer, blackleg, and gray mold were also noted in this plot. 

 

4.) Fungicide timing trials, resulted in the conclusion that flowering stage (proportion of crop in bloom) is of importance to the success of white mold control.  There were statistically significant differences in ratings and yields between treatment times and the untreated control.  A linear relationship between proportion of flowering at treatment times versus total and marketable yield was evident, where as the proportion of crop in bloom increased total and marketable yield decreased.  Of note, further investigation is warranted as to whether this relationship is truly linear. 

 

Appendix

 

Our original proposal included an evaluation of rotation e