Botrytis fruit rot or gray mold, caused by Botrytis cinerea, is common fruit disease in California strawberries (Koike et al. 2018).  Botrytis cinerea has a wide host range infecting several commercially important crops including blueberry (Saito et al. 2016), grapes (Saito et al., 2019), and tomato (Breeze, 2019).  Fungal infection can cause flower or fruit rot.  Fruit can be infected directly or through a latent infection in the flowers.  Moist and cool conditions favor fungal infections and increased sugar content in the ripening fruit can also contribute to the disease development.  Initial symptoms of infection appear as brown lesions and a thick mat of gray conidia is characteristic symptom in the later stages of infection.  As chemical fungicides are primarily used for gray mold control, fungicide resistance is a common problem around the world (Panebianco et al., 2015; Liu et al., 2016; Stockwell et al., 2018; Weber and Hahn, 2019).  In strawberry, cultural control options such as removing diseased plant material or using cultivars with traits that can reduce gray mold infections may not be practical when the disease is widespread in the field or cultivar choice is made based on other factors.  Non-chemical control options are necessary to help reduce the risk of chemical fungicide resistance, prolong the life of available chemical fungicides, achieve desired disease control, and to maintain environmental health.  Although there are several botanical and microbial fungicides available for gray mold control, limited information is available on their efficacy in California strawberries.  A study was conducted in the spring of 2019 to evaluate the efficacy of several chemical, botanical, and microbial fungicides in certain combinations and rotations to help identify effective options for an integrated disease management strategy.

Methodology

Strawberry cultivar San Andreas was planted late November, 2018 and the study was conducted in April and May, 2019.  Each treatment had a 20' long strawberry plot with two rows of plants replicated in a randomized complete block design.  Plots were maintained without any fungicidal applications until the study was initiated.  Table 1 contains the list of treatments, application rates and dates of application, and Table 2 contains the type of fungicide used and their mode of action.  Beauveria bassiana and Metarhizium anisopliae s.l. are California isolates of entomopathogenic fungi, isolated from an insect and a soil sample, respectively.  These fungi are pathogenic to a variety of arthropods and some strains are formulated as biopesticides for arthropod control.  However, earlier studies in California demonstrated that these fungi are also known to antagonize plant pathogens such as Fusarium oxysporum f.sp. vasinfectum Race 4 (Dara et al., 2016) and Macrophomina phaseolina (Dara et al., 2018) and reduce the disease severity.  To further evaluate their efficacy against B. cinerea, these two fungi were also included in this study alternating with two chemical fungicides.

Treatments were applied with a CO₂-pressurized backpack sprayer using 66.5 gpa spray volume.  Five days before the first spray application and 3 days after each application, all ripe fruit were harvested from each plot and incubated at the room temperature in vented plastic containers.  The level of gray mold on fruit from each plot was rated using a 0 to 4 scale (where 0=no disease, 1=1-25% fruit with fungal infection, 2=26-50% infection, 3=51-75%, and 4=76-100%) 3 and 5 days after each harvest (DAH).  Due to the rains, fruit could not be harvested after the 3^rd spray application for disease rating, but was harvested and discarded after the rains to avoid cross infection for the following week's harvest.  Data were analyzed using analysis of variance using Statistix software and significant means were separated using Least Significant Difference separation test.

Results

Gray mold occurred at low to moderate levels during the study period.  Along with B. cinerea, there were a few instances of minor fungal infections from Rhizopus spp. (Rhizopus fruit rot) and Mucor spp. (Mucor fruit rot).  Pre-treatment disease ratings were statistically not significant (P = 0.6197 and 0.5741) 3 and 5 DAH.  While the chemical standard treatment with the rotation of Captan, Merivon, Switch, and Pristine (treatment 2) appeared to result in the lowest disease rating throughout the observation period, treatments 3 and 5 after the 1^st spray application, treatments 5 and 11 along with 3, 4 and 6 after the 2^nd spray application, and treatments 3 and 5 along with 11 after the 4^th spray application also had similar disease control at 3 DAH.  When disease at 5 DAH was compared, the lowest rating was seen in treatment 2 after the 1^st and 2^nd spray applications, and treatments 2, 3, and 11 after the 4^th application.  Several other treatments also provided statistically similar control during these days.

When the average disease rating for the three post-treatment observation events was considered, treatment 2, 3, 5, and 11 had the lowest disease at both 3 and 5 DAH.  Treatments 4 and 12 at 3 DAH also had a statistically similar level of disease control to treatment 2. 

In general, most of the treatments provided moderate to high control compared to the disease in untreated control when the post-treatment averages were considered.  Only treatment 7 and 13 had lower control at 3 DAH.

Discussion

This study compared a variety of registered and developmental products along with two entomopathogenic fungi in managing B. cinerea.  Considering the fungicide resistance problem in B. cinerea in multiple crops, having multiple non-chemical control options is very important to achieve desirable control with integrated disease management strategies.  Since the active ingredients in the botanical and bacterial fungicides used in this study are not public, discuss will be limited on their modes of action and efficacy at this point.  Similarly, the active ingredient of WXF-17001 is also not known, however, an earlier study by Calvo-Garrido et al. (2014) demonstrated that a fatty acid-based natural product reduced B. cinerea conidial germination by 54% and disease severity in grapes by 96% compared to untreated control.  The product used by Calvo-Garrido et al. (2014) is thought to be fungistatic and reduce the postharvest respiratory activity and ethylene production in fruits.

While chemical fungicides have a specific mode of action, biological and other products act in multiple manners either directly antagonizing the plant pathogen or by triggering the plant defenses.  For example, amending the potting medium with biochar resulted in induced systemic resistance in tomato and reduced B. cinerea severity by 50% (Mehari et al., 2015).  Luna et al. (2016) also showed that application of β-aminobutyric acid and jasmonic acid promoted seed germination and long-term resistance to B. cinerea in tomato.  Burkholderia phytofirmans, beneficial endophytic bacterium, offered protection against B. cinerea in grapes by mobilizing carbon resources (callose deposition), triggering plant immune system (hydrogen peroxide production and priming of defense genese), and through antifungal activity (Miotto-Vilanova et al. 2016).  Similarly, entomopathogenic fungi such as B. bassiana are also known to induce systemic resistance against plant pathogens (Griffin et al. 2006).  Compared to other options evaluated in the study, entomopathogenic fungi have an advantage of controlling both arthropod pests and diseases, while also having plant growth promoting effect (Dara et al. 2017).

Rotating fungicides with different mode of actions reduces the risk of resistance development and using some combinations will also maintain control efficacy.  This study provided the efficacy of multiple control options and their combinations and rotations for B. cinerea.  This is also the first study demonstrating the efficacy of entomopathogenic fungi against B. cinerea in strawberry.

Acknowledgements: Thanks to Sipcam Agro and Westbridge for funding the study, technical assistance of Hamza Khairi for data collection, and the field staff at the Shafter Research Station for the crop maintenance.

References

Breeze, E.  2019.  97 Shades of gray: genetic interactions of the gray mold, Botrytis cinerea, with wild and domesticated tomato.  The Plant Cell 31: 280-281.  https://doi.org/10.1105/tpc.19.00030

Calvo-Garrido, C., A.A.G. Elmer, F. J. Parry, I. Viñas, J. Usall,  R. Torres,  R.H. Agnew, and  N. Teixidó.  2014.  Mode of action of a fatty acid-based natural product to control Botrytis cinerea in grapes.  J. Appl. Microbiol. 116: 967-979.  https://doi.org/10.1111/jam.12430

Dara, S. K., S. S. Dara, S.S.R. Dara, and T. Anderson.  2016.  First report of three entomopathogenic fungi offering protection against the plant pathogen, Fusarium oxysporum f.sp. vasinfectum.  UC ANR eJournal of Entomology and Biologicals https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=22199  

Dara, S. K., S.S.R. Dara, and S. S. Dara.  2017.  Impact of entomopathogenic fungi on the growth, development, and health of cabbage growing under water stress.  Amer. J. Plant Sci. 8: 1224-1233.  https://doi.org/10.4236/ajps.2017.86081

Dara, S.S.R., S. S. Dara, and S. K. Dara.  2018.  Preliminary report on the potential of Beauveria bassiana and Metarhizium anisopliae s.l. in antagonizing the charcoal rot causing fungus Macrophomina phaseolina in strawberry.  UC ANR eJournal of Entomology and Biologicals https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=28274

Griffin, M. R., B. H. Ownley, W. E. Klingeman, and R. M. Pereira.  2006.  Evidence of induced systemic resistance with Beauveria bassiana against Xanthomonas in cotton.  Phytopathol. 96.

Koike, S. T., G. T. Browne, T. R. Gordon, and M. P. Bolda.  2018.  UC IPM pest management guidelines: strawberry (diseases).  UC ANR Publication 3468. https://www2.ipm.ucanr.edu/agriculture/strawberry/Botrytis-Fruit-Rot/

Liu, S., Z. Che, and G. Chen.  2016.  Multiple-fungicide resistance to carbendazim, diethofencardb, procymidone, and pyrimethanil in field isolates of Botrytis cinerea from tomato in Henan Province, China.  Crop Protection 84: 56-61.

Luna, E., E. Beardon, S. Ravnskov, J. Scholes, and J. Ton.  2016.  Optimizing chemically induced resistance in tomato against Botrytis cinerea.  Plant Dis. 100: 704-710. https://doi.org/10.1094/PDIS-03-15-0347-RE

Mehari, Z. H., Y. Elad, D. Rav-David, E. R. Graber, and Y. M. Harel.  2015.  Induced systemic resistance in tomato (Solanum lycopersicum) against Botrytis cinerea by biochar amendment involves jasmonic acid signaling.  Plant and Soil 395: 31-44.

Miotto-Vilanova, L., C. Jacquard, B. Courteaux, L. Wortham, J. Michel, C. Clément, E. A. Barka, and L. Sanchez.  2016.  Burkholderia phytofirmans PsJN confers grapevine resistance against Botrytis cinerea via a direct antimicrobial effect combined with a better resource mobilization.  Front. Plant Sci. 7: 1236. https://doi.org/10.3389/fpls.2016.01236

Panebianco, A., I. Castello, G. Cirvilleri, G. Perrone, F. Epifani, M. Ferrarra, G. Polizzi, D. R. Walters, and A. Vitale.  2015.  Detection of Botrytis cinerea field isolates with multiple fungicide resistance from table grape in Sicily.  Crop Protection 77: 65-73.

Saito, S., T. J. Michailides, and C. L. Xiao.  2016.  Fungicide resistance profiling in Botrytis cinerea populations from blueberry in California and Washington and their impact on control of gray mold.  Plant Dis. 100: 2087-2093. https://doi.org/10.1094/PDIS-02-16-0229-RE

Saito, S., T. J. Michailides, and C. L. Xiao.  2019.  Fungicide-resistant phenotypes in Botrytis cinerea populations and their impact on control of gray mold on stored table grapes in California.  European J. Plant Pathol. 154: 203-213.

Stockwell, V. O., B. T> Shaffer, L. A. Jones, and J. W. Pscheidt.  2018.  Fungicide resistance profiles of Botrytis cinerea isolated from berry crops in Oregon.  Abstract for International Congress of Plant Pathology: Plant Health in A Global Economy; 2018 July 29-Aug 3; Boston, MA.

Weber, R.W.S. and M. Hahn.  2019.  Grey mould disease of strawberry in northern Germany: causal agents, fungicide resistance and management strategies.  Appl. Microbiol. Biotechnol. 103: 1589-1597.

Experimental plots in mid January, 2018. Photo by Surendra Dara

In a continuous effort to explore the potential of additive, soil amendment, biostimulant, and other products, a new study was conducted in a conventional strawberry field at the Manzanita Berry Farms in Santa Maria.  The following treatments were administered at different times, from planting till the end of production season, as requested by the manufacturer.

1. Untreated control: Other than the soil incorporated fertilizers during the field preparation, no other nutrient inputs were added during the study.
2. Grower standard: Transplants were dipped in Switch 62.5WG (cyprodinil+fludioxonil, at 5 oz/100 gal) before planting and a proprietary nutrient regimen that included administration of a humic acid-based product was followed.
3. Innovak Global regimen: Nutrisorb-L (a blend of polyhydroxy carboxylic acids) at 28 fl oz/ac, starting 2 wk after planting and every 3 wk thereafter through drip.  Packhard (carboxylic acids with calcium and boron) at 28 fl oz/ac, starting at the first fruit set (early January) and every 2 wk thereafter as a foliar spray. 
4. TerraVesco regimen: A microbe-rich Vermi-extract (worm extract) at 10% vol/vol as a transplant dip for 3 hours, followed by application through drip at 7.5 gal/ac after planting, and again in December, 2017 and January, 2018.
5. Fertum regimen: Transplant dip in 1% vol/vol of Germinal Plus (a product from marine algae), followed by drip applications of Booster (a biostimulant and a natural organic fertilizer made from seaweed) at 0.5 gal/ac in late November and late December, 201; Silicium PK (a biostimulant and a natural organic fertilizer based on silicon enriched with phosphorus, potassium and seaweed extracts) at 0.5 gal/ac late December, 2017 and once a month starting from mid February to early July, 2018; and Foliar (a biostimulant and a natural organic fertilizer from marine algae) at 0.5 gal/ac in mid and late January.
6. Shemin Garden regimen: EcoSil (a silica fertilizer) at 800 ml/ac once a month starting from early December, 2017 to May, 2018 through drip, and at 200 ml/ac in early May and June, 2018 as a foliar spray; ComCat (based on a plant extract) at 20 gr/ac and EcoFlora (a consortium of Azotobacter spp., Bacillus spp., Paenibacillus spp., Pseudomonas sp., Trichoderma spp., and Streptomyces spp.) at 12 oz/ac one week after EcoSil through drip until May, 2018 and ComCat at 10 gr/ac and EcoFlora at 12 oz/ac as a foliar spray in May and June, 2018.
7. GrowCentia regimen-low: Yeti containing 1% bacterial culture (of Pseudomonas putida, Citrobacter freundii, Comamonas testosterone, and Enterobacter cloacae) and 2% alfalfa extract applied at 0.6 ml/gal through drip for 90 min weekly from the first drip application.
8. GrowCentia regimen-high: Yeti at 1 ml/gal through drip for 90 min weekly from the first drip application.
9. NanoChem regimen: EX10, a biodegradable fertilizer additive containing thermal polyaspartate at 1 qrt/ac through first drip after planting with follow up applications in early January (first bloom), mid February, and mid May, 2018.  The active ingredient binds with cations such as ammonium, calcium, copper, iron, magnesium, manganese, potassium, and zinc and improves their availability for the plant.
10. BiOWiSH regimen 1: Formula 1 at 1.33 oz/gal for transplant dip followed by 3.53 oz/ac through drip starting 2 wk after planting and every 4-5 wk thereafter.
11. BiOWiSH regimen 2: Formula 1 at 1.33 oz/gal for transplant dip followed by 3.53 oz/ac as a foliar srpay starting 2 wk after planting and every 4-5 wk thereafter.
12. BiOWiSH regimen 3: Formula 1 at 1.33 oz/gal for transplant dip followed by 3.53 oz/ac through drip starting 2 wk after planting alternated with a foliar spray every 4-5 wk.
13. BiOWiSH regimen 4: Formula 1 at 1.33 oz/gal for transplant dip followed by BiOWiSH Crop 16-40-0, a microbial consortium (Bacillus amyloliquefaciens, B. lichenoformis, B. pumilus, and B. subtilis)at 3.53 oz/ac through drip starting 2 wk after planting and every 4-5 wk thereafter.

Each treatment contained a 165' long 5.7' wide bed and replicated four times in a randomized complete block design.  A 15' long plot in the center of the bed was marked and netted for collecting yield and some other parameters that were compared.  Strawberry cultivar BG 6-30214 was planted on 7 November, 2017.  Other than the untreated control, all other products were administered on top of the grower standard fertility program.  However, only the grower standard transplants were dipped in Switch 62.5WG before planting. 

Various parameters were measured during the vegetative growth and fruit production periods to evaluate the impact of the treatments on crop growth, health, and yield.  Data were analyzed using ANOVA and LSD test was used to separate significant means.

Transplant treatment (above) and drip application (below).  Photos by Tamas Zold

Tamas Zold taking canopy measurements.  Photo by Surendra Dara

Canopy growth: Canopy growth was observed on 11 December, 2017, 7 and 30 January, and 8 February, 2018 by measuring the size of the canopy along and across the length of the bed from 20 random plants per bed and calculating the area.  Canopy size significantly (P = 0.0261) different among the treatments only on the last observation date where plants treated with EX10 and the GrowCentia product at the low concentration were larger than those in the grower standard.

Electrical conductivity and temperature of soil: From two random location on each bed, electrical conductivity (EC in dS/m) and temperature (^oC) were measured about 3 inches deep from the surface on 12 and 25 January, 7 February, 19 March, 18 April, and 29 May, 2018.  Only soil temperature on 25 January significantly (P = 0.0007) varied among treatments where the difference between the highest (untreated control) and the lowest (Vermi-extract) values was 0.8^oC.

Dead plants: The number of dead plants represents empty spots in the bed due to the death of transplants.  There were no obvious signs of disease or a particular stress factor associated with those plants except that they were randomly distributed within each bed and throughout the field.  When counted on 18 April, 2018, BiOWiSH regimen 4, Fertum regimen, GrowCentia product at the high rate, and Innovak Global regimen had 

Areas where transplants did not establish.  Photo by Surendra Dara

Fruit diseases: Fruit harvested on 12 March, 3 and 13 April, and 17 May, 2018 from each marked plot was incubated at room temperature in dark in plastic containers and the fungal growth was rated 3 and 5 days after harvest (DAH) using a scale of 0 to 4 where 0=no fungal growth, 1=1-25%, 2=26-50%, 3=51-75%, and 4=76-100% fungal growth. Botrytis fruit rot or grey mold was predominant during the first two observation dates and the growth of other fungi (possibly Rhizopus spp.) was also seen during the last two dates.  In general, fruit disease occurred at low levels throughout the observation period with

Sugar content in fruit: Sugar content was measured from two harvest-ready berries per bed on 17 May, 2018 using a handheld refractometer.  Sugar content varied from 8.06 ^oBx (Innovak Global regimen) to 9.53 ^oBx (grower standard).

Fruit firmness: Fruit firmness was measured from eight randomly collected harvest-ready berries from each bed on 28 June, 2018.  Firmness varied from 0.82 kgf (Fertum and Shemin Garden regimens) to 0.98 kgf (untreated control).

Fruit yield:Strawberries were harvested from 6 February to 22 June, 2018 on 36 dates.  When compared to the grower standard, the marketable berry yield was 16.2, 15.1, 13.7, and 13% higher in Fertum regimen, EX10 treatment, Innovak Global regimen, and BiOWiSH regimen 4, respectively.  The marketable berry yield was 9.8, 9, 7.5, and 6.8% higher in those respective treatments over the yield from untreated control.  

When data were analyzed without the untreated control, there was a significant difference (P = 0.0279) among the treatments where treatment 5 had significantly higher marketable yield than the grower standard and treatments 3, 8, and 11.  Percentage difference from grower standard yield was also significantly different (P = 0.0301) among treatments where the Fertum regimen had the highest increase of 16.25.

Seasonal marketable yields (above) and percentage difference compared to grower standard (below) when untreated control data were excluded from the analyses.

There was a significant difference (P = 0.0141) among treatments only in the number of marketable berries.  There were more than 1700 in Innovak Global regimen, EX10 treatment, and BiOWiSH regimen 4 while grower standard had 1485, Vermi-extract had 1563, and the rest of the treatments had marketable berries in 1600s.  

 

The average fruit weight was a little over 33 grams in the grower standard and the Fertum regimen whereas the weight varied between 31.9 and 32.7 grams in the rest of the treatments.  

 

It took 23 harvest dates in three months (from February to April, 2018) to obtain the first third of the total seasonal yield while the remaining two-thirds were obtained from seven harvest dates in May and six dates in June.  Marketable fruit yield was higher than the grower standard in all treatments and higher than the untreated control in most treatments. 

In general, fruit yields were higher and the pest and disease pressure was lower than usual during the study period.  Aleo, a garlic oil based fungicide, at lower label rates was periodically used for disease management and bug vacuums were operated a few times against the western tarnished plant bug as a standard across all treatments.

This study evaluated some treatment regimens as recommended by the collaborating manufacturers and some of them appear to have a potential for use in strawberry production.  These results help the manufacturers fine tune their recommendations for achieving better yields through additional studies.

Acknowledgments: We thank the planting and harvest crew at Manzanita Berry Farms for their help with the crop production aspects, Chris Martinez, Tamas Zold, and Maria Murrietta for their technical assistance, Sumanth Dara for statistical analysis, and the support of the industry collaborators who funded the study.

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