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United States Department of Agriculture

Agricultural Research Service

Research Project: Alternatives to Fumigating Greenhouses for Control of Soilborne Plant Pathogens

Location: Horticultural Crops Research

2013 Annual Report

1a.Objectives (from AD-416):
1. Optimize and determine the feasibility for solarization to disinfest greenhouse in-ground beds and used containers. 2. Determine if the addition of biocontrol agents to solarized soil will reduce disease caused by soilborne plant pathogens.

1b.Approach (from AD-416):
Attempt to optimize solarization conditions to achieve higher soil temperatures, and follow solarization with amendment by biological control organisms to further suppress surviving pathogen inoculum. We will also test greenhouse solarization as a means of disinfesting used containers.

3.Progress Report:

This research was conducted in support of objective 1 "Identify and characterize key pathogens to pinpoint critical pathogens vulnerabilities and develop targeted disease management strategies" of the parent project. Inoculum of three plant pathogens, Rhizoctonia solani, Pythium irregulare, and Phytophthora pini was placed in sachets as a bioassay for determining the effectiveness of solarization in greenhouse media and used containers. Rhizoctonia solani was on grown potato slices and mixed with autoclaved sand. Pythium irregulare was grown as cornmeal sand inoculum, and P. pini was produced by preparing infested leaf disks after dipping rhododendron leaves in a zoospore suspension.

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.

Last Modified: 4/25/2014
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