- Nurseries are an important part of the agriculture industry of the United States. The nursery industry provides ornamental, forest, restoration and specialty products to many markets in N. America. The disease sudden oak death, caused by Phytophthora ramorum, has devastating environmental and economic impacts on forested land, the urban-forest interface, and nursery stock. This pathogen has caused the loss of millions of trees on the west coast and still threatens coastal forests of California and Oregon. P. ramorum, along with other Phytophthora spp., continues to impact nursery production and marketability. In 2014, P. tentaculata, a species new to N. America, appeared in California nurseries and restoration sites, causing the death of plant stock, and threatening native ecosystems in the area.
Once infested plants are introduced into restoration sites it is difficult to eradicate the pathogens, as they are often located on public lands or in other areas where chemical pesticides are not allowed. While soil steaming has been demonstrated to be effective in some nursery situations, it is generally impractical to implement in remote areas. Moreover, many restoration sites are at high risk for disturbance of rare or sensitive species.
Soil solarization is a low-cost, non-toxic method that can be highly effective in killing soilborne Phytophthora spp. in infested nurseries, croplands and restoration sites. This method consists of using a transparent plastic film to trap solar radiant heat to kill pests. Understanding and implementing the best methods for soil solarization are critical for successful eradication of pathogens.
We examined how the size of the solarization treatment area, irrigation of soil field capacity, and duration of solarization affected the survival of Phytophthora spp. Research sites were established at the Oregon State University Botany and Plant Pathology (BPP) Farm Corvallis, Oregon, and the National Ornamentals Research Site at Dominican University (NORSDUC) quarantine facility in San Rafael, California in the summers of 2017 and 2018. At the CA site, we tested survival of P. ramorum, and P. pini; at the OR site, we tested only P. pini. We used three plot sizes (0.5 x 0.5m, 1.0 x 1.0 m, and 2.2 x 2.2 m) plus a non-solarized control.
Inocula consisting of infected rhododendron leaf disks (P. ramorum and P. pini) or infested oregano stems or rye seeds (P. tentaculata) were buried in each plot at three depths: 5 cm, 15 cm, and 30 cm. Plots were either irrigated to field capacity just prior to solarization or left in a non-irrigated state. Inoculum was collected from each plot at 2, 4, 6 and 12 weeks to assess survival and plated on selective media to assess viability of inoculum. A non-solarized treatment was included as a control.
In both sites and years, viable Phytophthora inoculum was recovered from non-solarized plots over the 12-week sampling period, with the exception of 5 cm depth, where inoculum in all plot sizes were killed. Among solarized plots (0.5, 1.0, and 2.2 m), recovery of Phytophthora inoculum was lowest in the largest plots and greatest in the smallest plots. Recovery increased with soil depth (5, 15, or 30 cm) and declined over the 12 week sampling period. In most trials, recovery was slightly greater from irrigated plots as compared to non-irrigated plots.
In both sites and years, 0.5 m plots Phytophthora spp. survived at 15 cm and 30 cm, except for BPP in 2017, where P. pini was not recovered at 15 cm after 12 weeks. In the 1.0 m plot size, both species were killed to a depth of 15 cm at both sites in 2017, and at NORSDUC in 2018 in non-irrigated plots. At BPP during both years, P. pini was killed in the 2.2 m at 30 cm depth by two weeks in the non-irrigated treatment. At NORSDUC during both years in non-irrigated 2.2 m plots, P. pini and P. ramorum at 30 cm depth were killed by two weeks. In irrigated 2.2 m plots, both species were killed by 4 weeks in all trials, except for NORSDUC 2018, where P. pini and P. ramorum were killed by 2 weeks.
Average daily maximum soil temperatures increased with plot size and decreased with soil depth. Temperatures in 0.5 m and 1.0 m plots were likely insufficient to kill Phytophthora spp. at 30 cm; this was only achieved in the 2.2 m plots. Non-irrigated plots yielded slightly less Phytophthora recovery than irrigated plots, with the most noticeable difference being at 30 cm. Gravimetric water content was used to analyze moisture data. Irrigated plots reached field capacity (0.20 - 0.35 g water/g soil), where non-irrigated plots remained between 0.03 - 0.10 g water /g soil. Non-irrigated soils were able to diffuse heat more efficiently and thus maintained higher temperatures as compared to irrigated plots, reducing recovery. Historical recommendations have suggested that soil should be irrigated prior to solarization. Based on the results of this study, combined with previous work, we believe that soil should be moist, but not irrigated to field capacity prior to solarization, and under our conditions, the plot size used should be at least
2.2 m x 2.2 m (4.84 m2) to kill Phytophthora spp. at 30 cm depth.
Results from this research have many practical applications. Understanding the effects of minimum plot size and increasing awareness about soil moisture effects on soil temperature should lead to more effective implementation of solarization. In restoration sites, public municipalities, small-scale nurseries, and organic agriculture there is a push to use pesticide alternatives for disease management. Solarizing the soil is a non-toxic management strategy that can be readily used in this context. It has already been shown to be effective for disinfesting contaminated beds in container nurseries and field production nurseries and could be used as a pre-plant method to manage both weeds and soilborne plant pathogens restoration sites, or to mitigate infested sites.