Strawberry, a high-value specialty crop in California, suffers from several soilborne, fruit, and foliar diseases.  Verticillium wilt caused by Verticillium dahliae, Fusarium wilt caused by Fusarium oxysporum f. sp. fragariae, and Macrophomina crown rot or charcoal rot caused by Macrophomina phaseolina are major soilborne diseases that cause significant losses without proper control.  Chemical fumigation, crop rotation with broccoli, nutrient and irrigation management to minimize plant stress, and non-chemical soil disinfestation are usual control strategies for these diseases.  Botrytis fruit rot or gray mold caused by Botrytis cineaea is a common fruit disease requiring frequent fungicidal applications.  Propagules of gray mold fungus survive in the soil and infect flowers and fruits.  A study was conducted to evaluate the impact of drip application of various fungicides on improving strawberry health and enhancing fruit yields.

Methodology

This study was conducted in an experimental strawberry field at the Shafter Research Station during 2019-2020.  Cultivar San Andreas was planted on 28 October 2019.  No pre-plant fertilizer application was made in this non-fumigated field which had Fusarium wilt, Macrophomina crown rot, and Botrytis fruit rot in the previous year's strawberry planting.  Each treatment was applied to a 300' long bed with single drip tape in the center and two rows of strawberry plants.  Sprinkler irrigation was provided immediately after planting along with drip irrigation, which was provided one or more times weekly as needed for the rest of the experimental period.  Each bed was divided into six 30' long plots, representing replications, with an 18' buffer in between.  Between 6 November 2019 and 9 May 2020, 1.88 qt of 20-10-0 (a combination of 32-0-0 urea ammonium nitrate and 10-34-0 ammonium phosphate) and 1.32 qt of potassium thiosulfate was applied 20 times at weekly intervals through fertigation.  Treatments were applied either as a transplant dip or through the drip system using a Dosatron fertilizer injector (model D14MZ2).  The following treatments were evaluated in this study:

i)  Untreated control: Neither transplants nor the planted crop was treated with any fungicides.

ii) Abound transplant dip: Transplants were dipped in 7 fl oz of Abound (azoxystrobin) fungicide in 100 gal of water for 4 min immediately prior to planting.  Transplant dip in a fungicide is practiced by several growers to protect the crop from fungal diseases.

iii) Rhyme: Applied Rhyme (flutriafol) at 7 fl oz/ac immediately after and 30, 60, and 90 days after planting through the drip system.

iv) Velum Prime with Switch: Applied Velum Prime (fluopyram) at 6.5 fl oz/ac 14 and 28 days after planting followed by Switch 62.5 WG (cyprodinil + fludioxinil) at 14 oz/ac 42 days after planting through the drip system.

v) Rhyme with Switch: Four applications of Rhyme at 7 fl oz/ac were made 14, 28, 56, and 70 days after planting with a single application of Switch 62.5 WG 42 days after planting through the drip system.

Parameters observed during the study included leaf chlorophyll and leaf nitrogen (with chlorophyll meter) in February and May; fruit sugar (with refractometer) in May; fruit firmness (with penetrometer) in April and May; severity of gray mold (caused by Botrytis cinereae) twice in March and once in May, and other fruit diseases (mucor fruit rot caused by Mucor spp. and Rhizopus fruit rot caused by Rhizopus spp.) once in May 3 and 5 days after harvest (on a scale of 0 to 4 where 0=no infection; 1=1-25%, 2=26-50%, 3=51-75% and 4=76-100% fungal growth); and fruit yield per plant from 11 weekly harvests between 11 March and 14 May 2020.  Leaf chlorophyll and nitrogen data for the Abound dip treatment were not collected in February. Data were analyzed using analysis of variance in Statistix software and significant means were separated using the Least Significant Difference test.

Results and Discussion

Leaf chlorophyll content was significantly higher in plants that received drip application of fungicides compared to untreated plants in February while leaf nitrogen content was significantly higher in the same treatments during the May observation.  There were no differences in fruit sugar or average fruit firmness among the treatments.

The average gray mold severity from three harvest dates was low and did not statistically differ among the treatments.  However, the severity of other diseases was significantly different among various treatments with the lowest rating in Abound transplant dip on both 3 and 5 days after harvest and only 3 days after harvest in plants that received four applications of Rhyme.  Unlike the previous year, visible symptoms of the soilborne diseases were not seen during the study period to evaluate the impact of the treatments.  However, there were significant differences among treatments for the marketable fruit yield.  The highest marketable yield was observed in the treatment that received Rhyme and Switch followed by Velum Prime and Switch and Rhyme alone.  The lowest fruit yield was observed in Abound dip treatment.  Unmarketable fruit (deformed or diseased) yield was similar among the treatments.  Compared to the untreated control, Abound dip resulted in 16% less marketable yield and such a negative impact from transplant dip in fungicides has been seen in other studies (Dara and Peck, 2017 and 2018; Dara, 2020).  Marketable fruit yield was 4-28% higher where fungicides were applied to the soil. 

Although visible symptoms of soilborne diseases were absent during the study, periodic drip application of the fungicides probably suppressed the fungal inocula and associated stress and might have contributed to increased yields.  The direct impact of fungicide treatments on soilborne pathogens was, however, not clear in this study due to the lack of disease symptoms.  Considering the cost of chemical fumigation or soil disinfestation and the environmental impact of chemical fumigation, treating the soil with fungicides can be an economical option if they are effective.  While this study presents some preliminary data, additional studies in non-fumigated fields in the presence of pathogens are necessary to consider soil fungicide treatment as a control option.

Field assistants Tamas Zold and Marjan Heidarian Dehkordi

Acknowledgments: Thanks to FMC for funding this study and Marjan Heidarian Dehkordi and Tamas Zold for their technical assistance.

References

Dara, S. K. 2020.  Improving strawberry yields with biostimulants and nutrient supplements: a 2019-2020 study.  UCANR eJournal of Entomology and Biologicals. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=43631

Dara, S. K. and D. Peck.  2017.  Evaluating beneficial microbe-based products for their impact on strawberry plant growth, health, and fruit yield.  UCANR eJournal of Entomology and Biologicals.  https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=25122

Dara, S. K. and D. Peck.  2018.  Evaluation of additive, soil amendment, and biostimulant products in Santa Maria strawberry.  CAPCA Adviser, 21 (5): 44-50.

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