Abstract
The purpose of this study was to investigate the use of potassium phosphite (Pi) alone and in combination with a reduced dose of synthetic fungicide (myclobutanil) to control pear scab (Venturia pirina) under field conditions. Irrespective of Pi and myclobutanil concentration, no leaf or fruit phytotoxic effects were observed throughout the 2006 and 2007 experiments. In both field experiments, Pi alone significantly reduced the incidence and severity of V. pirina on leaves and fruit compared to water-treated control with the degree of scab reduction similar to that of a reduced dose of myclobutanil. The efficacy of Pi at 20 ml per liter water in inhibiting V. pirina scab incidence and severity was superior than that of Pi at 10 ml per liter water and a reduced dose of myclobutanil in virtually all monthly assessments. Combining a reduced dose of myclobutanil with either Pi at 10 ml or 20 ml per liter significantly improved the efficacy of scab control compared to stand alone applications of each product at most monthly assessments. Data analyzed with Limpel’s formula indicated a positive synergistic effect between Pi and a reduced dose of myclobutanil. Greatest reductions in V. pirina incidence and severity on leaves and fruit were, however, achieved by stand alone applications of myclobutanil at manufacturers recommended strength. Irrespective of year, crown volume, number of fruit per tree, and total fruit yield were higher in Pi and myclobutanil treated trees irrespective of concentration applied compared to water-treated controls. A combined mix of Pi with a reduced dose of myclobutanil proved effective in increasing crown volume, number of fruit per tree and fruit yield compared to stand alone applications of each treatment. Greatest increases in crown volume, number of fruit per tree and fruit yield were achieved by applications of myclobutanil at the manufacturers recommended strength. In virtually all cases, Pi combined with a reduced dose of myclobutanil induced positive synergistic effects on crown volume and fruit yield greater than their additive effects alone. Mean fruit weight per tree were in all cases higher in Pi and myclobutanil treated trees irrespective of concentration applied compared to water-treated controls, however, these differences were not statistically significant in all cases.
- Fungicides, Integrated Disease Management
- Orchard Management
- Pathogen Control
- Plant Health Care
- Synergism
- Urban Landscapes
Pear scab caused by Venturia pirina causes significant economic losses annually in many countries where pears are grown for human consumption (Villalta et al. 2004). In addition, ornamental pear species are planted into urban landscapes (e.g., streets, public recreation areas, car parks) for aesthetics such as flowers, bark, berry, and leaf color. Repeated pear scab infection can result in tree mortality and/or undesirable reductions in aesthetic appearance. Controlling pear scab requires the frequent application of synthetic fungicides each season depending on weather conditions (Washington et al. 1999). Public demands to reduce fungicide use, stimulated by greater awareness of environmental and health issues, as well as development of fungicide tolerant strains of scab has placed greater emphasis on the development of reduced fungicide control strategies (Gozzo 2003; Fobert and Després 2005; Ilhan et al. 2006). Likewise, increased legislative restrictions regarding the registration, use and application of pesticides has led to a situation within Europe whereby more fungicides are withdrawn on an annual basis than released onto the commercial market, which in turn increases selection pressure for fungicide resistance in surviving populations (Anonymous 2009). For these reasons, research has focused on the development of a range of novel plant protection products that have limited or no known adverse effect on the environment and human health (Garbelotto et al. 2007). Pertinent examples include application of inorganic calcium and boron fertilizers, biologically inert film forming polymers, systemic inducing agents, and biostimulants (Sutherland and Walters 2002; Ippolito et al. 2005; Rolshausen and Gubler 2005; Akbudak et al. 2006; Percival et al. 2006; Percival et al. 2009). The consensus of research opinion in the majority of cases however, concludes that most of these novel plant protection products are generally less effective and consistent than standard synthetic fungicides for pathogen control (Agostini et al. 2003; Krokene et al 2008; Percival and Haynes 2008; Percival et al. 2009). This has led to the suggestion that a more appropriate role for these types of products would be in combination with a reduced dose of synthetic fungicide to achieve control comparable or significantly higher than stand alone applications of fungicides at full dose (Bécot et al. 2000; Van Loon et al. 2002; Ilhan et al. 2006). This in turn would reduce potential environmental impacts and extend the working life of existing fungicide products.
One family of potential plant protection products are inorganic potassium and phosphite salts. When applied to plants either as a foliar spray or soil drench, phosphites exhibit a complex mode of action, acting both on the pathogen (direct) and by stimulating plant host defense responses (indirect), such as: the accumulation of phytoalexins, hypersensitive cell death, cell wall lignification and fortification and formation of lytic enzymes that inhibit pathogen growth (Guest and Grant 1991; Garbelotto et al. 2007). Research in Australia and the U.S. has found potassium phosphite salts to be extremely effective in the control of pathogens belonging to the Oomycetes group, such as Phytophthora spp., Pythium spp., and the Downy Mildew diseases (Jackson et al. 2000; Miller et al. 2006). In addition, potassium phosphite has been shown to suppress fungal pathogens that fall outside this group such as Venturia inaequalis (apple scab) (MacHardy and Jeger 1983), and bacterial diseases such as Erwinia amylovora (apple fire blight). The objective of this study was to investigate the use of potassium phosphite (Pi) alone and in combination with a reduced dose of synthetic fungicide (myclobutanil) to control pear scab under field conditions. Within the UK, myclobutanil, a systemic, protectant, and curative triazole fungicide, is commercially registered for the control of pear scab.
MATERIALS AND METHODS
Field Trials
The pear trial site consisted of a 0.90 ha block of Pyrus communis ‘Williams’ Bon Chrétien’ interspersed with individual trees of Pyrus communis Beth and Concorde. Pyrus communis ‘Williams’ Bon Chrétien’ was chosen for experimental purposes due to its sensitivity to pear scab infection. Planting distances were based on 2 m × 2 m spacing. The trees were planted in 2003 and trained under the central-leader system to an average height of 2.5 m ± 0.25 m, and with mean trunk diameters of 12 cm ± 1.4 cm at 45 cm above the soil level. The trial site was located at the University of Reading Shinfield Experimental Site, University of Reading, Berkshire, UK (51°43N, −1°08W).
The soil was a sandy loam containing 3%–5% organic matter, pH of 6.1. Weeds were controlled chemically using glyphosate (Roundup; Green-Tech, Sweethills Park, Nun Monkton, York, UK), throughout experiments. No water, fertilizer, or plant growth regulators were applied during the two-year trial. Historically, the pears suffered from pear scab infection on an annual basis. Consequently, prior to the trial commencing in 2006 and 2007, trees were inspected in September 2005 and 2006 and only those trees with > 50% of leaves affected with pear scab infection were used in the trials. A minimal insecticide program based on the residual pyrethroid insecticide deltamethrin (product name Bandu, Headland Agrochemicals, Ltd., Saffron Walden, Essex, UK) was applied every three months during each growing season commencing in May 2006 to September 2007. All sprays were applied using a Tom Wanner Spray Rig sprayer at 40 ml deltamethrin (Bandu) per 100 liters of water. Trees were sprayed until runoff, generally 0.35 liter insecticide per tree.
Spray Treatments
Potassium phosphite (Pi; 300 g phosphorous acid per liter water), trade name Phoenix (Orion Future Technology, Ltd., Henwood House, Henwood, Ashford, Kent, UK) and myclobutanil, trade name Systhane 20EW (Landseer, Ltd., Chelmsford, Essex, UK) sprays were applied at four growth stages identified as key treatment times for scab control under field conditions (Bevan and Knight 2001), namely: bud break (March 11, 2006; March 17, 2007), green cluster (April 1, 2006; April 7, 2007), 90% petal fall (May 13, 2006; May 19, 2007), and early fruitlet (June 1, 2006; June 8, 2007). Prior to spray treatments, trees were inspected and no visible symptoms of pear scab were apparent. During spray treatments polythene screens 2.5 m high were erected around each tree to prevent dispersal of sprays and possible cross contact with other trees. The base of the tree was covered with a 0.5 m × 0.5 m polythene mulch to prevent potential soil percolation.
The experimental treatments and the application protocol for the treatments were as follows:
1) water-treated control
2) Pi at 10 ml per liter water
3) Pi at 20 ml per liter water
4) Myclobutanil at 0.3 ml per liter water (manufacturer’s recommended strength)
5) Myclobutanil at 0.075 ml per liter water (25% of manufacturer’s recommended strength).
6) Pi at 10ml + myclobutanil at 0.075ml per liter water.
7) Pi at 20ml + myclobutanil at 0.075ml per liter water.
The treatments (one water control; two Pi; two myclobutanil; two Pi + myclobutanil combinations), were applied in 10 randomized complete blocks with a single tree as the experimental unit, giving a total of 70 observations per response variable. Foliar sprays of each product were applied until runoff using a hand sprayer (Cooper Pegler, Watling Street, Clfton upon Dunsmore, UK).
Scab Incidence and Severity
The degree of protection conferred by each treatment was assessed by recording scab incidence and severity at monthly intervals from July to September.
Scab incidence: At each assessment, 100 leaves and 30 fruits were chosen arbitrarily from different sides of a tree. A leaf or a fruit was considered to be infected if at least one visible scab lesion was present.
Scab severity of leaves and fruit was assessed visually. Leaf scab severity of each tree was rated using a visual indexing technique and ratings on the scale: 0 = No scab observed; 1 = less than 5% of leaves affected and no aesthetic impact; 2 = 5%–20% of leaves affected with some yellowing but little or no defoliation; 3 = 21%–50% of leaves affected, significant defoliation and/or leaf yellowing; 4 = 51%–80% of leaves affected, severe foliar discoloration; 5 = 81%–100% of leaves affected with 90%–100% defoliation.
Scab severity on fruit was calculated on the following scale: 0 = no visible lesions; 1 = <10% fruit surface infected; 2 = 10%–25% fruit surface infected; 3 = 25%–50% fruit surface infected; 4 = >50% fruit surface infected. Leaf scab severity ratings used in this study was based on UK and Ireland market standards for fungicide evaluation of scab control (Butt et al. 1990; Swait and Butt 1990). Fruit scab severity was based a scale used by Ilhan et al. (2006). Scab severity ratings were undertaken by three independent BASIS (British Agrochemical Standards Inspection Scheme) qualified crop protection specialists.
Fruit Yield and Crown Volume
Mean fruit weight per tree was determined by counting the number of fruit still attached to the tree at harvest and dividing by total fruit weight per tree. Yield per tree was determined by weighing all fruit on each tree at harvest and dividing by the number of trees per treatment.
Crown volume (Cv) was estimated from the crown width (D) and crown depth (L) using the paraboloid form of the crown (Kupka 2007):
Statistical Analysis
Mean scab severity values for all treatments were transformed using the Arcsin (sine−1) transformation. Analysis of variance was performed separately for each month and each year and means were separated by LSD (P = 0.05) following appropriate checks for homogeneity using the Genstat for Windows program. Back transformed pathogen severity values are presented here to ease interpretation of these data. Limpel’s formula, as described by Richer (1987), was used to describe synergistic interactions between the reduced dose of myclobutanil and both phosphite combinations. Limpel’s formula is Ee = A + B − (AB/100) in which Ee is the expected effect from additive responses of two treatments and A and B are the percentages of reductions in disease incidence or severity relative to each treatment alone. Thus, if the combination of the two treatments produces any value of reduction greater than Ee, then this is evidence for synergistic activity (Lorito et al. 1993).
RESULTS
Irrespective of Pi and myclobutanil concentration no leaf or fruit phytotoxic effects were observed throughout the 2006 and 2007 experiments (data not shown). In the first field experiment, conducted in 2006, Pi at 10 ml and 20 ml per liter of water significantly reduced the incidence and severity of V. pirina on both leaves and fruit compared to water-treated control (Table 1). The efficacy of Pi at 10 ml and 20 ml per liter water was similar to that of a reduced dose of myclobutanil (0.075 ml) per liter of water in all monthly assessments from July to September. In general, the efficacy of Pi at 20 ml per liter of water in inhibiting scab incidence and severity was superior to that of Pi at 10 ml per liter water and a reduced dose of myclobutanil in virtually all monthly assessments. Combining Pi at 10 ml and 20 ml per liter of water with a reduced dose of myclobutanil in most instances significantly improved the efficacy of stand alone applications of treatments. In addition, data analyzed with Limpel’s formula demonstrated synergistic effects between Pi at 10 ml and 20 ml per liter of water and a reduced dose of myclobutanil in inhibiting V. pirina leaf incidence and severity in July and September assessments (Table 1). Similar synergistic activity was recorded between Pi at 20 ml per liter of water and a reduced dose of myclobutanil in the assessments made in September with respect to V. pirina incidence and severity on fruit (Table 2). Greatest reductions in V. pirina incidence and severity on leaves and fruit were achieved by stand alone applications of myclobutanil at the manufacturers recommended strength of 0.3 ml per liter water (Table 1).
The efficacy of Pi alone or in combination with reduced dose of myclobutanil in reducing the disease incidence and severity on leaves and fruit caused by Venturia pirina in 2006.
The efficacy of Pi alone or in combination with reduced dose of myclobutanil in reducing the disease incidence and severity on leaves and fruit caused by Venturia pirina in 2007.
In the second field experiment, conducted in 2007, weather conditions were more favorable and scab incidence and severity were higher than in 2006. Pi at 10 ml and 20 ml per liter of water and reduced dose of myclobutanil treatments were effective in reducing V. pirina incidence and severity on leaves in all monthly assessments from July to September; in addition, the incidence and severity on fruit was recorded at the cessation of the growing season (Table 2). Similar to the results of the experiment conducted in 2006, the efficacy of Pi at 20 ml per liter of water in inhibiting V. pirina incidence and severity was superior than that of Pi at 10 ml per liter of water and a reduced dose of myclobutanil at most monthly assessments. Combining Pi at 10 ml and 20 ml per liter of water with a reduced dose of myclobutanil significantly improved the efficacy of stand alone applications of treatments. Accord ing to Limpel’’s formula, a number of synergistic effects between Pi at 10 ml and 20 ml per liter of water and a reduced dose of myclobutanil were found to reduce scab incidence in the July (Pi at 10 ml liter only) and September assessments on both leaf and fruit (Pi at 10 ml and 20 ml per liter; Table 2). Similar to the 2006 trial maximal reductions in V. pirina, incidence and severity on leaves and fruit was achieved by stand alone applications of myclobutanil at manufacturers recommended strength (Table 2).
Irrespective of year, the crown volume, number of fruit per tree, and fruit yield were higher in Pi and myclobutanil treated trees in all cases, irrespective of concentration applied in comparison with water-treated controls. Application of Pi at 10 ml and 20 ml per liter water proved more effective in increasing crown volume, number of fruit per tree, and fruit yield than that of a reduced dose of myclobutanil. However, a combined mix of Pi at 10 ml and 20 ml per liter of water with a reduced dose of myclobutanil proved effective in further increasing crown volume, number of fruit per tree, and fruit yield compared to stand alone applications of each treatment. Greatest increases in crown volume, number of fruit per tree, and fruit yield were achieved by applications of myclobutanil at the manufacturers recommended dose. In all cases except one, fruit yield in September 2007 following a Pi 20 ml per liter of water and reduced dose of myclobutanil, Pi and a reduced dose of myclobutanil combination induced synergistic effects on crown volume and fruit yield greater than their additive effects alone based on Limpel’s equation. Irrespective of year, mean fruit weight per tree were in all cases higher in Pi and myclobutanil treated trees irrespective of concentration applied compared to water-treated controls; however, in most cases these differences were not statistically significant.
The efficacy of Pi alone or in combination with reduced dose of myclobutanil on growth of pear (Pyrus communis ‘Williams’ Bon Chretien’) at the cessation of the growing season.
DISCUSSION
Results of this study demonstrated that Pi at 10 ml and 20 ml per liter of water applied at four growth stages (bud break; green cluster; 90% petal fall; early fruitlet) under field conditions significantly reduced the incidence and severity of pear scab. Previous studies have reported that application of Pi to be extremely effective in the management of pathogens, particularly those that belong to the Oomycetes group such as Phytophthora citricola, P. capsici, P. infestans, Peronosclerospora sorghi, Peronospora parasitica in several economically important food crops to include potato, maize, pepper, and crucifers (Ouimette and Coffey 1989; Jackson et al. 2000; Wilkinson et al. 2001; Miller et al. 2006). In addition, phosphite application reduced pathogen severity of several powdery mildews to include Sphaerotheca fuliginea (cucumber powdery mildew), grape powdery mildew, nectarine, and mango powdery mildew (Bécot et al. 2000), as well as suppress the bacterial pathogen Pseudomonas syringae pv. syringae (pear blast) (Moragrega et al. 1998). Within Australia, phosphites are now extensively used for the control of Phytophthora cinnamomi in avocados, Downy Mildew (Plasmopara viticola) in grapevines, and Phytophthora cinnamomi and P. parasitica (heart rot) in pineapples. However, to the best knowledge of the authors of the present study, this is the first report of the efficacy of Pi against V. pirina.
In the two field experiments conducted on Pyrus communis ‘Williams’ Bon Chrétien’, a highly sensitive pear scab variety, applications of Pi at 20 ml per liter of water significantly and consistently reduced pear scab incidence and severity on leaves and fruit to a greater degree than Pi applied at 10 ml per liter of water. Such a response was recorded even when greater conditions of rainfall were more conducive for scab development in 2007. Manufacturers recommend a 10 ml per liter dose of Pi for pathogen suppression and/or stimulation of plant vitality. Results of this study indicate an increase in Pi concentration to 20 ml per liter of water can achieve a greater degree of scab reduction without phytotoxicity symptoms developing on leaves or fruit. In addition, Pi at this higher concentration resulted in a greater fruit yield and crown volume compared to Pi applied at 10 ml per liter of water and stand alone applications of a reduced dose of myclobutanil.
The combined effects of Pi at 10 ml and 20 ml per liter of water with a reduced dose of myclobutanil were more than their additive properties according to Limpel’’s formula. Pi and a reduced rate of myclobutanil acted synergistically in both the July and September assessments to reduce V. pirina incidence and severity. Such synergistic effects were particularly pronounced with respect to crown volume and fruit yield recorded in September 2006 and 2007. Consequently, combining Pi’s with higher concentra tions than the reduced dose of myclobutanil used in this study may result in even greater synergistic activity. Such a hypothesis is given credence by the fact that myclobutanil applied at full strength at bud break, green cluster, petal fall, and early fruitlet formation proved the most effective treatment for pear scab control. Under these circumstances leaf and fruit scab severity was reduced by 75%–85%. Such a result is comparable with other workers following similar spray regimes [i.e., four times between flower bud formation to initial early fruit set (Butt et al. 1990; Swait and Butt 1990; Bevan and Knight 2001). Within the UK, total scab control is generally achieved via a minimum of 10–12 fungicide sprays throughout the growing season (Butt et al. 1990; Swait and Butt 1990)].
Irrespective of year, the crown volume, number of fruit per tree, and fruit yield were in all cases higher in Pi and myclobutanil treated trees irrespective of concentration applied compared to water-treated controls. However, little effects on mean fruit weight per tree were recorded. Detrimental effects of scab infection include premature fruit drop and reduction in fruit size (Agrios 2004; Villalta et al. 2004). Results of this investigation indicate reductions in fruit yield were caused by scab-induced fruit drop rather than specific reductions in individual fruit size. Such a response may be related to source-sink relationships. Scab induced premature fruit drop would mean the smaller remaining number of fruit having a greater sink strength that in turn would account for their comparable mean weight compared to Pi and myclobutanil-treated trees (Blanke 2007).
From a commercial aspect, producers, suppliers and vendors of pears adopt a zero tolerance policy toward pear scab on fruit (Butt et al. 1990). Consequently, to reduce scab levels to commercially accepted standards, several follow-up fungicide sprays would need to be applied. However, where fruit produce is sold under a naturally produced label within the UK, scab severity levels tend to be less stringent (Bevan and Knight 2001). Likewise, ornamental pears are grown and planted for aesthetic reasons within town and city landscapes where lower scab levels are acceptable (Percival and Haynes 2008). In these instances, Pi application can possibly be used alone or in rotation with synthetic fungicides as part of an IPM strategy.
In conclusion, results of this study indicate that application of Pi alone is useful in reducing the incidence and severity of pear scab. Combination of Pi with a reduced dose of myclobutanil, however, significantly improved the efficacy compared to stand alone applications of each product to further reduce the incidence and severity of pear scab on leaves and fruit. Such a response indicates that Pi and myclobutanil combinations offer potential to reduce the risk of fungicide resistant strains of scab developing by decreasing fungicide selection pressure. This is based on the fact that higher degrees of scab control were achieved with lower myclobutanil inputs when combined with Pi. These findings would be useful to arborists as they indicate synthetic fungicide usage can be reduced yet achieve control comparable or significantly higher than stand alone applications of fungicides at full dose. This in turn promotes a greater awareness of environmental and health issues to the public and government legislative bodies. Likewise phosphites cost in general 40%–80% less than conventional fungicides. Further research is ongoing evaluating Pi combinations with higher doses of myclobutanil and other forms of synthetic fungicides.
Acknowledgments
The authors are grateful for funding, in part, from the TREE FUND (Hyland Johns Grant).
- © 2010, International Society of Arboriculture. All rights reserved.
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