Soil Solarization on Populations ofPythiumspp., FluorescentPseudomonas, and Damping-off of Broccoli and Cucumber

Soil solarization is the use of solar radiation to disinfest moist soil covered with clear polyethylene mulch during warm months. Beneficial effects of soil solarization are lower pathogen populations, reduced disease, and higher yield. Information on the duration of solarization benefits is necessary to achieve effective and efficient implementation of this management strategy. A 3-year study investigated effects of single- and multiple-year soil solarization on Pythium spp., Pseudomonas spp., and damping-off of broccoli (Brassica oleracea L.) and cucumber (Cucumis sativus L.). Soil was solarized 10 weeks in 2003 only; 2004 only; 2003 and 2004; 2004 and 2005; 2003, 2004, and 2005; or not solarized. Soil was sampled from 0 to 10 cm deep and population densities of Pythium spp. and Pseudomonas spp. assayed at monthly intervals from the start of solarization until 2 months after solarization. Cucumber and broccoli were planted one month after solarization ended or plots were left fallow. During solarization, soil temperatures increased 2.5 to 4.5°C in solarized soils compared to those in nonsolarized soils. In general, microbial counts in all years were lower throughout the solarization period in solarized soils than in nonsolarized soils. During the 2-month period following solarization, Pythium spp. increased in soils solarized 1 or 2 years but not in soils solarized 3 years. However, postsolarization Pythium spp. densities in soils solarized 3 years were high enough that there were no differences in Pythium spp. densities among solarized soils. No differences in microbial populations were found among solarization treatments applied in the current year irrespective of whether the soil was previously solarized, planted, or fallowed. One year after solarization, there were no differences in population densities between soils solarized or not solarized the previous year. Incidences of damping-off were not different among treatments. Under conditions in the southeastern United States, soil solarization is a disinfestation method that should be applied each season prior to planting, because beneficial effects of reducing microbial populations do not persist in the following years.