This post first appeared on the CalPoly Strawberry Center Blog on January 3, 2023, and was written by Gerald Holmes.  The work reported is supported by a USDA NIFA grant and is a multi-disciplinary group of scientists that includes Dr. David Gadoury from Cornell University.  To read the entire paper, please see the link in the references section.    

A recent publication by Mello et al. reports that strawberry powdery mildew (SPM) was reduced by nighttime application of UV-C to field plots at 200 J m-2 . Disease reduction was equivalent to that provided by weekly fungicide applications in CA and superior to that provided by twice weekly fungicide applications in FL.  No negative effects to the plants’ appearance (phytotoxicity) or yield were measured. 

It’s nice to see this work carried out in both CA and FL and telling the same story with a few caveats. In CA, where our seasons are longer, the fungicide program used for comparison consisted of weekly fungicide applications from July 31 through November 20 (17 applications).  In FL, there were 8 applications from November 19 to March 3.  UV-C was applied once per week (CA and FL) or twice per week (FL only).  In CA, total cumulative SPM severity was reduced 74% in the fungicide treated and 73% in the weekly UV-C treatment when compared to non-treated plants.  In FL, SPM severity was equal in the non-treated and fungicide treated plots and reduced 45% in the weekly UV-C treatment and 90% in the twice weekly UV-C treatment.  The lack of control in the fungicide treatment in FL might be explained by resistance of the pathogen to the fungicides used.  Similar trends in treatment efficacy resulted for the incidence of SPM on fruit.  Marketable yield tended to increase as SPM decreased although not as dramatically as disease severity. 

A fair amount of work has preceded this in many areas of the world. It seems that the CA strawberry industry may be poised to adopt this technology on a grander scale. Whether this occurs will depend on the cost of treatment, ease of use and the benefits. If UV-C is shown to reliably reduce other pests such as two-spotted spider mites, this will add to the value of UV-C and likely increase adoption. 

Reference 

Mello, P. P., Onofre, R. B., Rea, M., Bierman, A., Gadoury, D. M., Ivors, K., Ganci, M., Broome, J. C. and Peres, N. A. 2022. Design, construction, and evaluation of equipment for nighttime application of UV-C for management of strawberry powdery mildew in Florida and California. https://doi.org/10.1094/PHP-01-22-0002-RS