Location: Horticultural Crops Research
2013 Annual Report
Solarization of greenhouses for two weeks was highly successful in eliminating all three pathogens in artificially infested media, regardless of their location within the greenhouse. Maximum temperatures exceeded 65°C in all three greenhouses. Temperature data were analyzed according to the number of hours above the thresholds of 50°C and 60°C. Rhizoctonia, Phytophthora, and Pythium have been shown to be eliminated by 30 min. exposure to moist heat at 50°C, whereas some other plant pathogens require 30-min. treatment with 60°C (Baker and Cook, 1974). A confounding factor in this experiment was that fungivorous mites were present in the commercial potting medium. Mites were killed by greenhouse solarization treatments, but those not exposed to high temperature were observed grazing on the Rhizoctonia hyphae. Therefore recovery of Rhizoctonia in the outdoor or lab locations were lower than expected. The mites did not graze on the oomycetes Pythium and Phytophthora.
Solarization of pots was inconsistent in eliminating soilborne pathogens except when pallets were placed inside the greenhouse. Although recovery of Phytophthora pini was 0% from solarized pots, it was but only 23% from the lab, suggesting that another factor such as low moisture or immature chlamydospores reduced survival of this pathogen across locations. Temperature maxima ranged from 37°C to 73.5°C. Temperatures were highly variable among pot position within the pallet load and across locations. Interestingly, the pots at the top corners of the pallet load obtained the highest temperatures, but also lost the greatest amount of heat at night. The greatest number of hours above the 50°C threshold occurred in position “V” (top SE corner of pallet load) in three of the four locations, but in one location, the bottom center position achieved the greatest number of hours above 50°C. All of the pot positions of the pallet solarized within the greenhouse achieved at least 20 hours above 50°C. One factor that could have contributed to inconsistent kill was lack of moisture throughout the pallet load. Heat could also have been lost from the base of the pallet not covered by the plastic film.