Efficacy of new and experimental biological fungicides against botrytis fruit rot in strawberry: a 2021 study

Botrytis cinerea infection appears as wilted flowers and a layer of spores on ripe fruit.  Photo by Surendra Dara

Botrytis fruit rot or gray mold caused by Botrytis cinerea is an important disease of strawberry and other crops damaging flowers and fruits.  Pathogen survives in the plant debris and soil and can be present in the plant tissues before flowers form.  Infection is common on developing or ripe fruits as brown lesions.  Lesions typically appear under the calyxes but can be seen on other areas of the fruit.  As the disease progresses, a layer of gray spores forms on the infected surface.  Severe infection in flowers results in the failure of fruit development.  Cool and moist conditions favor botrytis fruit rot development.  Sprinkler irrigation, rains, or certain agricultural practices can contribute to the dispersal of fungal spores.

Although removal of infected plant material and debris can reduce the source of inoculum in the field, regular fungicide applications are typically necessary for managing botrytis fruit rot.  Since fruiting occurs continuously for several months and fungicides are regularly applied, botrytis resistance to fungicides is not uncommon.  Applying fungicides only when necessary, avoiding continuous use of fungicides from the same mode of action group (check FRAC mode of action groups), exploring the potential of biological fungicides to reduce the risk of resistance development are some of the strategies for effective botrytis fruit rot management.  In addition to several synthetic fungicides, several biological fungicides continue to be introduced into the market offering various options for the growers.  Earlier field studies evaluated the potential of various biological fungicides and strategies for using them with synthetic fungicides against botrytis and other fruit rots in strawberry (Dara, 2019; Dara, 2020).  This study was conducted to evaluate some new and soon to be released fungicides in fall-planted strawberry to support the growers, ag input industry, and to promote sustainable disease management through biological and synthetic pesticides.

Methodology

This study was conducted at the Manzanita Berry Farms, Santa Maria in strawberry variety 3024 planted in October, 2020.  While Captan and Switch were used as synthetic standards, a variety of biological fungicides of microbial, botanical, and animal sources were included at various rates and different combinations and rotations.  Products and active ingredients evaluated in this study included Captan Gold 4L (captan) from Adama, Switch 62.5 WG (cyprodinil 37.5% + fludioxinil 25%) from Syngenta, NSTKI-14 (potassium carbonate 58.04% + thyme oil 1.75%) from NovoSource, A22613 [A] (botanical extract) from Syngenta, Regalia (giant knotweed extract 5%) from Marrone Bio Innovations, EXP14 (protein 15-20%) from Biotalys, Gargoil (cinnamon oil 15% + garlic oil 20%) and Dart (caprylic acid 41.7% + capric acid 28.3%) from Westbridge, Howler (Pseudomonas chlororaphis strain AFS009), Theia (Bacillus subtilis strain AFS032321), and Esendo (P. chlororaphis strain AFS009 44.5% + azoxystrobin 5.75%) from AgBiome, ProBlad Verde (Banda de Lupinus albus doce – BLAD, a polypeptide from sweet lupine) from Sym-Agro with Kiplant VS-04 (chitosan 2.3%) or Nu-Film-P spreader/sticker, AS-EXP Thyme (thyme oil) from AgroSpheres, and AgriCell FunThyme (thyme oil) provided by AgroSpheres.

Table 1. List of treatments color coded according to the kind of fungicide (light blue=synthetic fungicide; dark blue=synthetic+biological fungicide active ingredient; peach=synthetic and biological fungicides alternated; green=biological fungicides)

Excluding the untreated control, rest of the 24 treatments can be divided into synthetic fungicides, a fungicide with synthetic + biological active ingredients (a formulation with two application rates), synthetic fungicides alternated with biological fungicides, and various kinds of biological fungicides  (Table 1).  Treatments were applied at a 7-10 day interval between 22 April and 17 May, 2021.  Berries for pre-treatment disease evaluation were harvested on 19 April, 2021.  Each treatment had a 5.67'X15' plot replicated four times in a randomized complete block design.  Strawberries were harvested 3 days before the first treatment and 3-4 days after each treatment for disease evaluation.  On each sampling date, marketable-quality berries were harvested from random plants within each plot during a 30-sec period and incubated in paper bags at outdoor temperatures under shade.  Number of berries with botrytis infection were counted on 3 and 5 days after harvest (DAH) and percent infection was calculated.  This is a different protocol than previous years' studies where disease rating was made on a 0 to 4 scale.  Treatments were applied with a backpack sprayer equipped with Teejet Conejet TXVK-6 nozzle using 90 gpa spray volume at 45 PSI.  Water was sprayed in the untreated control plots.  Dyne-Amic surfactant at 0.125% was used for treatments that contained Howler, Theia, Esendo, AgriCell FunThyme, AS-EXP Thyme, and EXP 14.  Research authorization was obtained for some products and crop destruction was implemented for products that did not have California registration.

Percent infection data were arcsine-transformed before subjecting to the analysis of variance using Statistix software.  Significant means were separated using the least significant difference test.

Results

Pre-treatment infection was very low and occurred only in some treatments with no statistical difference (P > 0.05).  Infection levels increased for the rest of the study period.  There was no statistically significant difference (P > 0.05) among treatments for disease levels 3 or 5 days after the first spray application.  Differences were significant (P = 0.0131) in disease 5 DAH after the second spray application where 13 treatments from all categories had significantly lower infection than the untreated control.  After the third spray application, infection levels were significantly lower in eight treatments in 3 DAH observations (P = 0.0395) and 10 treatments in 5 DAH observations (P = 0.0005) compared to untreated control.  There were no statistical differences (P > 0.05) among treatments for observations after fourth spray application or for the average of four applications.  However, there were numerical differences where infection levels were lower in several treatments than the untreated control plots.

In general, the efficacy of both synthetic and biological fungicides varied throughout the study period among the treatments.  When the average for post-treatment observations was considered, infection was numerically lower in all treatments regardless of the fungicide category.  Multiple biological fungicide treatments either alone or in rotation with synthetic fungicides appeared to be as effective as synthetic fungicides.

Conclusions

Botanical and microbial fungicides can be effective against either for using alone or in rotation with synthetic fungicides for suppressing botrytis fruit rot in strawberry.  Additional studies can help optimize the application rates and use strategies for those fungicides that were not as effective as others.  Sanitation practices and use of synthetic and biological fungicides help manage botrytis fruit rot.

Acknowledgements: Thanks to AgBiome, AgroSpheres, Biotalys, NovaSource, Sym-Agro, Syngenta, and Westbridge for funding and Chris Martinez for his technical assistance.

References

Dara, S. K.  2019.  Five shades of gray mold control in strawberry: evaluating chemical, organic oil, botanical, bacterial, and fungal active ingredients.  UCANR eJournal of Entomology and Biologicals.  https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=30729

Dara, S. K. 2020.  Evaluating biological fungicides against botrytis and other fruit rots in strawberry.  UCANR eJournal of Entomology and Biologicals. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=43633

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