Suppression of Monilinia Blight: Strategies for Today and Potential Fungicide Options for Tomorrow

Abstract

Fungicide efficacy field trials were conducted in Nova Scotia and Prince Edward Island during the 2009 growing season to address concerns over: (i) the possible tolerance of the fungal pathogen [Monilinia vaccinii-corymbosi (Reade) Honey] causing Monilinia blight or mummy berry to presently registered fungicides; (ii) increasing scrutiny of specific fungicides by end users; and (iii) the challenge of trying to apply a curative fungicide in a timely manner after an infection period has likely occurred. Results indicated that the active ingredients propiconazole (Topas® and Orbit®), prothioconazole (Proline®), and penthiopyrad (LEM17) were very effective in suppressing Monilinia blight, tolerance to propiconazole did not appear to be present, and the use of multiple active ingredients with different modes of action and persistence also provided excellent suppression. In addition, the treatments had a profound influence on berry yield with the propiconazole, prothioconazole, and penthiopyrad treatments having berry yields that were 73.6, 111, and 112% greater than the untreated control, respectively, at the Nova Scotia site. However, these berry yield results need to be viewed with caution given the known suppressive attributes of some of these fungicides to other diseases present during Monilinia infection periods.

Keywords:

• Vaccnium angustifolium Ait
• Monilinia floral blight

INTRODUCTION

The lowbush (i.e., “wild”) blueberry (Vaccinium angustifolium Ait.) is an endogenous plant that has evolved into an important horticultural commodity in northeastern North America. Wild blueberry fields are commercially managed on a 2-year cycle with the perennial shoot being pruned in alternate years to maximize floral bud initiation, fruit set, yield, and ease of mechanical harvest. Pruned plants grow for a noncropping or “sprout” year in which vegetative growth occurs along with floral bud initiation. Selective herbicides are applied to control competing weeds in the spring of the first year and granular fertilizers are applied. During the following “cropping year,” bloom and fruit set occurs followed by harvest in late summer. The majority of wild blueberries are sold to processing plants where most berries are individually quickly frozen.

Monilinia blight, or mummy berry disease, is caused by the fungus Monilinia vaccinii-corymbosi (Reade) Honey. The disease is of economic importance to highbush, lowbush, southern highbush, and rabbiteye blueberries, and has also been observed to occur on Vaccinium myrtilloides, V. myrsinites, V. elliottii, and V. fuscatum (Hildebrand et al., 1995). The disease is common in many blueberry production areas, and can be especially destructive when extended wet periods are encountered for several weeks after bud break (Delbridge and Hildebrand, 1995). In addition, fields with heavy soil, poor air flow, poor drainage, and previous occurrence of Monilinia are typically more prone to the disease.

Monilinia blight infects leaves, blossoms, and fruit, and the first infections occur in the spring after bud break (Milholland, 1977) (Fig. 1). Several weeks after bud infections, symptoms appear as water soaked or dark brown areas along the midrib and veins of leaves, which soon wilt (Delbridge and Hildebrand, 1995) (Fig. 1). Infected floral inflorescences become dark purple-brown in color, and shrivel (Ramsdell et al., 1975) (Fig. 1). A whitish-grey growth of spores (conidia) is produced on the midrib of infected leaves and at the base of infected blossoms (Delbridge and Hildebrand, 1995). Several weeks before harvest, infected fruit shrivel, harden, and turn salmon in color (Delbridge and Hildebrand, 1995) (Fig. 1). The blueberry skin eventually becomes silver in color and is sloughed off, exposing a hard, black fungal mass called a mummy berry (Ramsdell et al., 1975).

FIGURE 1 Damage to leaves, inflorescences, and flowers by Monilinia (left and central pictures). Resulting damage and presence of “mummyberries” with lowbush blueberries. (Left and right photographs are courtesy of the Nova Scotia Department of Agriculture.) (color figure available online)

The fungus overwinters in fields as pseudosclerotia (mummified fruit) from a previous crop, and these structures are the sole survival structure for over-summering and over-wintering of M. vaccinii-corymbosi (Scherm, 2006). Early in the growing season (i.e., during bud break), the pseudosclerotia germinate to produce small cup-like structures (apothecia) that produce primary spores (ascospores). Under favorable weather conditions (wet), infection occurs if the vegetative or leaf buds and floral buds are at susceptible stages of development. Once ascospore infection has occurred, disease symptoms become evident in 10 to 20 days (Delbridge and Hildebrand, 1995). Secondary asexual spores (conidia) are produced on this infected tissue and are carried by wind and pollinating insects to blossoms where infection occurs. Infected blossoms and fruit remain symptomless until the fruits are almost mature. They then drop to the ground, completing the life cycle.

Efficacy concerns have been raised regarding the use of propiconazole to control Monilinia blight with erratic control being obtained in field trials in 2007, 2008, and 2010. This is coupled with concerns regarding the use of propiconazole with export markets including the European Union reviewing the use of this active ingredient, and requests by end users to only use those agrochemicals approved for use in their respective country. In addition, successive, prolonged conditions of wet and cool weather during Monilinia infection periods resulted in the inability for equipment operators to apply fungicides in a timely manner (i.e., application of fungicides within 72 hr of a Monilinia infection period). Given these developments, two fungicide trials were conducted during the 2009 growing season to examine the influence of different modes of action and multiple active ingredient combinations on the ability to suppress Monilinia.

MATERIALS AND METHODS

Plots were established in commercial wild blueberry fields in the cropping phase of production (approximately 860 stems·m⁻²; please note that substantial variability exists between clones) located near Farmington, Nova Scotia (coordinates = 45° 33′, 63° 53′) and Mount Stewart, Prince Edward Island (coordinates = 46° 21′ 62° 52′).

The fungicide efficacy trials were established in April 2009 at the Farmington and Mount Stewart sites. The trial consisted of a randomized complete block design with five replications, a plot size of 4 × 6 m, and 2 m buffers between plots. Treatments consisted of (i) an untreated control, (ii) Allegro® 500F (a.i. fluazinam) applied at 2.24 L∙ha⁻¹, (iii) Fontelis® (a.i. penthiopyrad in the form of LEM17 200 SC) applied at 300 ml product∙ha⁻¹, (iv) Pristine® (a.i.'s boscalid and pyraclostrobin) applied at 1.6 kg product∙ha⁻¹, (v) Proline® (a.i. prothioconazole) applied at 400 ml product∙ha⁻¹, Serenade® Max™ (QST 713 strain of dried Bacillus subtilis) applied at 6.0 kg product∙ha⁻¹, Switch™ DF (a.i.'s cyprodinil and fludioxonil) applied at 975 g product∙ha⁻¹, and Topas® (a.i. propiconazole) applied at 500 ml product∙ha⁻¹.

The multiple active ingredient trial at the Farmington site consisted of a randomized complete block design with five replications, seven treatments, a plot size of 4 × 6 m, and 2 m buffers between plots. Treatments consisted of (i) an untreated control, (ii) Lance® WDG (a.i. boscalid) applied at 560 g product∙ha⁻¹ and Proline® applied at 400 ml product∙ha⁻¹, (iii) Lance® WDG (a.i. boscalid) applied at 560 g product∙ha⁻¹ and Topas® 250E applied at 500 ml product∙ha⁻¹, (iv) Pristine® applied at 1.6 kg product∙ha⁻¹, (v) Bravo® 500 Weatherstik™ applied at 5.4 L product∙ha⁻¹ and Proline® applied at 400 ml product∙ha⁻¹, (vi) Bravo® applied at 5.4 L product∙ha⁻¹ and Topas® applied at 500 ml product∙ha⁻¹ and (vii) Topas® applied at 500 ml product∙ha⁻¹.

Compounds were applied using a Bellspray Inc. (Opelausas, LA, USA), hand held, 2 meter carbon dioxide propelled boom sprayer with 4 nozzles and using 2-liter sample bottles. The nozzle type consisted of TeeJet Visiflow 8003VS (Wheaton, IL, USA), and a sprayer pressure of 32 PSI (220 kPa) was used. The nozzle discharge rate was 12.5 ml·s⁻¹ and application groundspeed was approximately 1.19 m·s⁻¹. Fungicide applications occurred on April 30, May 4, May 11, and May 19, 2009 at the Farmington site, and this coincided with the dispersion of ascospores and the floral buds being at stages of F2 (separation of scales), F3 (separation of scales and appearance of green tissue), F4 (floral bud scales had separated and individual flowers could be observed), and F4 stages of floral development, respectively. Upon consulting with the Monilinia forecasting system (which uses temperature, wetness duration, and spore dispersal), infection periods were estimated to have occurred immediately prior to the last two treatment applications. In addition, the last treatment application was applied with the intention of minimizing secondary infection of flowers and resulting formation of mummy berries.

At the Mount Stewart site, fungicide applications occurred on May 5, May 12, and May 21, 2009 and this also coincided with the dispersion of ascospores and the floral buds being at stages of F2, F3, and F4, respectively. Upon consulting with the Monilinia forecasting system, infection periods were thought to have occurred prior to the last two treatment applications.

Fifteen randomly selected stems were collected from all of the field trials using a line transect (samples were collected every 30 cm along the 4.5 m transect) and taken back to the laboratory to be further examined for phytotoxicity and disease incidence and severity within 2 weeks of treatment application. In addition, other parameters that were measured included stem length, vegetative nodes per stem, floral buds per stem, number of flowers per stem, and the presence of Monilinia including infected vegetative and/or floral nodes.

The impact of the treatments on yield potential (including the various yield components) was examined by randomly selecting blueberry stems after fruit set had occurred. This was done by randomly selecting 15 stems from each plot using a line transect, placing the stem samples in plastic bags, placing the bags in a cooler with ice, and bringing the stem samples back to the Nova Scotia Agricultural College for further diagnosis. The parameters examined included stem length, vegetative nodes per stem, floral nodes per stem, set fruit per stem, pinheads (small, presumably underfertilized and unusable berries) per stem, and side branches per stem. This information was used to measure potential yield in accordance with each treatment.

On August 16 and August 22, 2009, berries were harvested from the Farmington and Mount Stewart sites respectively. A forty-tine, commercial wild blueberry hand rake was used to harvest berries from four, randomly selected 1-m² quadrats in each plot, and harvested berry yield was recorded using a digital balance (Mettler PE 6000, Burlington, Ontario). A composite 500-ml berry sample from each plot was collected from each plot, placed in a cooler (∼4°C), and brought back to the lab for further analysis (e.g., individual berry weight and if required, for residue analysis).

Analysis of variance was complete using the General Linear Model procedure of SAS (SAS Institute, Cary, NC, USA). Normality and constant variance was tested using the Univariate procedure of SAS.

RESULTS AND DISCUSSION

Severe Monilinia pressures were observed at the Farmington site making this trial an excellent assessment of the suppressive attributes of the fungicides examined. Results from the fungicide efficacy trial at the Farmington site indicated that penthiopyrad, prothioconazole, propiconazole, and the combination boscalid and pyraclostrobin (i.e., Pristine™) treatments were effective in suppressing Monilinia, with these treatments having 82.7, 93.7, 88.4, and 78.6% fewer floral nodes with symptoms of Monilinia than the control (Table 1). Similarly, upon examining the vegetative nodes for symptoms of Monilinia, the penthiopyrad, prothioconazole, propiconazole, and the combination boscalid and pyraclostrobin treatments were effective in reducing Monilinia symptoms, with these treatments having 71.6, 90.8, 63.0, and 56.5% fewer vegetative nodes with symptoms than the control (Table 1). Interestingly, the fluazinam and B. subtilis treatments were not effective in suppressing Monilinia. This may have occurred as a result of the protectant attributes of these treatments and Monilinia infections having already been present within the blueberry shoots prior to treatment application. In addition, low temperatures incurred during the application of the B. subtilis (less than 12°C), may have also reduced the effectiveness of this biological control treatment. Very few mummified berries (i.e., less than 1%) were found in the treatments examined in this study, illustrating the predominately destructive nature of this disease on the developing flowers (data not reported).

TABLE 1 Influence of Seven Fungicides with Varying Modes of Action on the Suppression of Monilinia Blight of Wild Blueberries at a Commercial Field Located Near Farmington, Nova Scotia during the 2009 Growing Season

Treatments, active ingredients and associated fungicide resistance action committee classification	Percentage of floral nodes with visual symptoms of Monilinia	Percentage of vegetative nodes with visual symptoms of Monilinia	Set fruit (berries per stem)	Harvestable berry yield (g berries·m^−2)
Control (untreated)	22.3a	17.3a	10.7a	431.4c
Allegro™ (a.i.: fluazinam) Group 29 fungicide	24.0a	4.07b	13.3a	827.6ab
Penthiopyrad Group 7 fungicide	3.85abc	4.89b	10.5a	915.1a
Pristine^™ (a.i.'s: boscalid and pyraclostrobin) Group 7 and 11 fungicides	4.77abc	7.53b	14.1a	803.5ab
Proline (a.i.: prothioconazole) Group 3 fungicide	1.40c	1.59b	12.9a	912.1a
Serenade^™ (a.i.: QST 713 strain of dried Bacillus subtilis) Group 44 fungicide	15.7abc	18.0a	9.15a	449.8c
Switch^™ (a.i.'s: cyprodinil and fludioxonil) Group 9 and 12 fungicides	23.2a	18.4a	12.1a	705.2b
Topas® (a.i.: propiconazole) Group 3 fungicide	2.58bc	6.40b	14.2a	749.1ab
ANOVA results and associated p value	Treatment (p < 0.0001)	Treatment (p < 0.0001)	NS	Treatment (p < 0.0001)

Similar results were obtained with the fungicide efficacy trial at the Mount Stewart site, with the penthiopyrad, prothioconazole, and propiconazole treatments having 71.1, 72.8, and 72.9% fewer floral nodes with symptoms of Monilinia (Table 2). Different results from the Farmington site were obtained with the fluazinam, B. subtilis, and combination cyprodinil and fludioxonil treatments, with these treatments having 66.5, 40.6, and 75.2% fewer floral nodes with symptoms of Monilinia. The suppressive effects of these protective treatments may have occurred as a result of the treatments being applied prior to infection, and/or these treatments persisting sufficiently long to suppress Monilinia. A non-significant effect of the fungicide treatments was observed at the Mount Stewart site and this may have been due to the low levels of Monilinia observed with the vegetative nodes and resulting leaves at this site (Table 2).

TABLE 2 Influence of Seven Fungicides with Varying Modes of Action on the Suppression of Monilinia Blight of Wild Blueberries at a Commercial Field Located Near Mount Stewart, Prince Edward Island during the 2009 Growing Season

Treatments, active ingredients and associated fungicide resistance action committee classification	Percentage of floral nodes with visual symptoms of Monilinia	Percentage of vegetative nodes with visual symptoms of Monilinia	Set fruit (berries per stem)	Harvestable berry yield (g berries·m^−2)
Control (untreated)	17.5a	0.875a	4.55b	363.0ab
Allegro™ (a.i.: fluazinam) Group 29 fungicide	5.87ab	0.417a	4.49b	557.4a
Penthiopyrad Group 7 fungicide	5.05ab	0.659a	9.95a	549.2a
Pristine^™ (a.i.'s: boscalid and pyraclostrobin) Group 7 and 11 fungicides	17.6a	6.67a	3.73b	588.4a
Proline® (a.i.: prothioconazole) Group 3 fungicide	4.76b	0.604a	4.55b	465.5av
Serenade^™ (a.i.: QST 713 strain of dried Bacillus subtilis) Group 44 fungicide	10.4ab	2.91	4.46b	496.2ab
Switch^™ (a.i.'s: cyprodinil and fludioxonil) Group 9 and 12 fungicides	4.34b	1.51a	6.49ab	315.5b
Topas® (a.i.: propiconazole) Group 3 fungicide	4.75b	1.00a	6.32ab	467.1ab
ANOVA Results and associated p value	Treatment (p < 0.0001)	Treatment (p = 0.0231)	Treatment (p = 0.0186)	Treatment (p < 0.0001)

Upon examining the impact of the fungicide treatments on yield components and harvestable berry yield of the fungicide efficacy experiment, significant differences were present. At the Farmington site, there was no significant effect of the fungicide treatments on the number of set berries per stem (Table 1). However, there were substantial effects of the penthiopyrad, picoxystrobin, Pristine™, and propiconazole treatments having harvestable yields that were 112, 86.5, 79.1, and 86.5% greater than the control (Table 1). At the Mount Stewart site slightly different results were obtained with the penthiopyrad, picoxystrobin, Pristine™, and propiconazole treatments having set berry numbers per stem that were 119% greater, 91.4% greater, 18.0% less than, and 38.9% greater than the control, respectively (Table 2). However, similar to the Farmington site, the penthiopyrad, picoxystrobin, Pristine™, and propiconazole treatments at Mount Stewart had harvestable berry yields that were 51.2, 53.4, 61.9, and 28.7% greater than the control, respectively (Table 2).

Significant effects of treatments with multiple active ingredients were observed at the Farmington site. The chlorothalonil and prothioconazole and the boscalid and pyraclostrobin treatments had 56.7 and 17.3% more set fruit per stem than the control treatment (Table 3). The effects of the boscalid and pyraclostrobin treatments were not likely due to suppression of Monilinia (Tables 1– 3), but most likely due to (i) the ability of the treatment to suppress Septoria leaf spot (Percival and Dawson, 2009), which was also infecting the blueberries, and (ii) the growth promoting characteristics of the strobiluron active ingredient contained in the treatment.

TABLE 3 Influence of Multiple Active Ingredient Formulations and Associated Modes of Action on the Suppression of Monilinia Blight of Wild Blueberries at a Commercial Field Located Near Farmington, Nova Scotia during the 2009 Growing Season

Treatments, active ingredients and associated fungicide resistance action committee classification	Percentage of floral nodes with visual symptoms of Monilinia	Percentage of vegetative nodes with visual symptoms of Monilinia	Set fruit (berries per stem)	Harvestable berry yield (g berries·m^−2)
Control (untreated)	11.0a	10.6a	10.4ab	470.7e
Lance^™ (a.i.: boscalid) + Proline® Group 7 and 3 fungicides	4.19b	0.474c	11.7ab	611.8d
Lance^™ + Topas® Group 7 and 3 fungicides	5.96b	3.26bc	9.76b	806.7bc
Pristine^™ (a.i.'s: boscalid and pyraclostrobin) Group 7 and 11 fungicides	10.9a	6.56b	12.2a	896.5b
Bravo® + Proline® Group M5 fungicide	0.440c	1.69bc	16.3a	1072a
Bravo® + Topas® Group M5 and 3 fungicides	8.25a	2.46bc	14.4ab	734.4d
Topas® (a.i.: propiconazole) Group 3 fungicide	2.28b	3.60bc	11.5ab	773.4bc
ANOVA results and associated p value	Treatment (p < 0.0001)	Treatment (p < 0.0001)	Treatment (p < 0.0431)	Treatment (p < 0.0001)

Similarly, significant effects of the fungicide treatments on harvestable berry yields were present with the multiple active ingredient trial with the (i) boscalid and prothioconazole, (ii) boscalid and propiconazole, (iii) boscalid and pyraclostrobin, (iv) chlorothalonil and prothioconazol, and (v) chlorothalonil and propiconazole treatments having berry yields that were 30.0, 71.4, 56.0, 90.5, 128, and 56.0% greater than the control. When compared with the propiconazole check (i.e., industry standard), the chlorothalonil and prothioconazole treatment had significantly different and substantially greater harvestable berry yields (i.e., 38.6%). However, as previously mentioned, the fungicide treatments may have also had an influence on other debilitating diseases present that may have reduced berry yield (i.e., Septoria leaf spot).

Nonetheless, with climatic conditions making it difficult to apply fungicides in a timely manner, multiple active ingredient applications offer the advantages of different modes of action being present which decrease the likelihood of fungicide resistance occurring, and depending on the protectent fungicide used, a longer effective duration (protection window) than propiconazole. However, this consideration needs to be factored in with (i) the lack of protective and curative effects of the fungicides in new floral and vegetative growth that may occur between treatment applications (Scherm and Stanaland, 2001), and (ii) the inability to use chlorothalonil in the cropping year of production due to residue concerns in processed berries destined for the European Union. Despite these challenges, in the early stages of Monilinia infection (F2 and F3 stages), new growth is minimal and protectant fungicides, such as chlorothalonil, have a limited ability to spread and provide protection to new growth.

CONCLUSIONS

Results from this study illustrate that tolerance of M. vaccinii-corymbosi to propiconazole does not appear to be widespread, and alternatives to propiconazole exist that can be effectively used to manage Monilinia in wild blueberry production. Prothioconazole has the advantage of being very efficaceous, having an established maximum residue level for blueberries in the European Union and is in the final stages of registration for commercial use in Canada. In addition, no detectable residue levels were observed in this study from composite samples collected from each plot (data not reported). Penthiopyrad is also undergoing global registration for use in wild blueberry production and appears to be more efficacious than other active ingredients examined in the FRAC grouping (i.e., boscalid). However, uncertainties exist with respect to the effective duration of the fungicide treatments used and the reliance on using only apothecial viability, floral and/or leaf wetness duration and temperature in determining if a fungicide application should be applied. Other factors, including ascospore viability on host tissue prior to an infection period and movement of ascospores and conidia by insects, need to be examined more carefully in the future. In addition, despite the apparent lack of Monilinia resistance to the active ingredients examined, in-vitro tolerance studies need to be conducted with the various active ingredients to ensure these products are being used to their fullest potential now and in the foreseeable future.

ACKNOWLEDGMENTS

This research was financially supported by the Atlantic Innovation Fund and the Technology Development Program of the Nova Scotia Department of Agriculture. We gratefully thank Bragg Lumber Company, Oxford Frozen Foods Limited, the Wild Blueberry Producers Association of Nova Scotia, Mr. Elwood Lawton (owner of the Mount Stewart field), Ms. Holly Hines, and various research assistants and research interns for the contributions made to this project. Mention of a product or trade name does not constitute a guarantee or warranty of the product by the Nova Scotia Agricultural College nor an endorsement over similar products mentioned.