In the vast majority of seasons, powdery mildew (PM) (caused by Podosphaera aphanis) is the most economically important foliar disease of strawberries in California. Many growers have observed that PM is particularly destructive in glasshouses and under plastic tunnels. This could be due to the difference in temperature and relative humidity or to filtering UV radiation that is detrimental to the pathogen or to a combination of these factors.
A recent paper authored by Onofre et al. (2022) reports on the effect of different plastic types on PM in strawberries. They found that the foliar severity of PM within plastic tunnels was inversely related to their UV transmissibility. In other words, the more UV that gets through the plastic, the less PM develops.
The details. Onofre et al. evaluated 3 plastic types: 1) KoolLite Plus (polyethylene that blocks 99% of solar UV-B); 2) F-Clean Gruv (ethylene tetraflouroethylene [ETFE] + UV blocker that blocks 80% of solar UV-B); and 3) F-Clean Clear (ETFE without UV-B blocker that blocks 17% of solar UV-B). These were compared to open field growing conditions in two consecutive seasons (Fig. 1). Strawberries grown under UV-B blocking plastic resulted in PM severity that was three to five times higher than those grown in the open field (Fig. 2).
Implications for California strawberries. Of the competing explanations for increased PM in strawberries grown under plastic (i.e., temperature, humidity or UV filtering), this study concluded that exclusion of solar UV radiation was the principal factor contributing to higher disease severity.
Most of California’s 40,700 acres of strawberries (California Strawberry Commission 2022 acreage report) are produced in open fields. However, there are a few hundred acres where plants are grown in field soil under high plastic tunnels or in a table-top production system (Fig. 3).
• What plastics are we using for these tunnels and what is their capacity for blocking UV solar radiation?
• Are there other considerations like worker safety and comfort that accompanies the degree of UV filtering?
• Could we change the plastic used on our high tunnels as part of an integrated disease management program to fight PM?
I don’t know the answers to those questions, but we should at least be aware of the effect plastics can have on powdery mildew severity and be prepared for the potential of increased disease when growing under plastic.
Figure 3. Two California growing systems where high plastic tunnels are used: in soil (left) and table tops (right) (photos by G. Holmes).
Reference
Onofre, R. B., Gadoury, D. M., Stensvand, A., Bierman, A., Rea, M. and Peres, N. A. 2022. UV-transmitting plastics reduce powdery mildew in strawberry tunnel production. Plant Disease 106:2455-2461. https://doi.org/10.1094/PDIS-10-21-2195-RE
This is a really interesting observation.
Other researchers have reported that some plant protectants have longer residuals under poly tunnels due to UV filtering –many pesticides, especially biopesticides break down quickly in unfiltered sunlight.