Phytophthora species and other plant pathogens can be killed by exposure to high temperatures for a sufficient length of time. The most common methods for applying heat in nursery operations are via:
· steam or aerated steam (steam/air mixtures)
· hot water
· dry heat (e.g., insulated or noninsulated containers heated by electricity or natural gas)
· solarization (solar heating via greenhouse effect under clear plastic or glass)
Materials to be heat-treated should be moist before treatment because target organisms are killed more readily and at lower temperatures if they are hydrated. For 30 minute heat treatments, temperatures had to be increased by up to 36°F (20°C) to kill dry propagules of some plant pathogenic fungi compared to temperatures required for propagules premoistened for 16 hours (van Loenen et al 2003). If materials to be treated (e.g., potting mix, residues on used pots) have not been moist for at least 12 hours, treatment temperature or time should be increased well above minimum standards to ensure efficacy.
Effective treatment times decrease as temperatures increase. For instance, metal tools can be sterilized by exposure to flame for a short period. Standard treatments for killing plant pathogens in water include 203°F (95°C) for 30 seconds and 185°F (85°C) for 3 minutes (Runia and Amsing 2001). However, longer treatment times at lower temperatures are more useful for treating large volumes and bulky materials (e.g., used pots and potting media) because of the time required to uniformly heat the materials to the desired temperatures without overheating. Based on multiple studies, heating of moist materials to 140°F (60°C) or higher for at least 30 minutes will kill propagules of Phytophthora and other water molds, as well as most plant pathogenic fungi.
In all heat treatment procedures, the timing of the heat exposure period starts when the coolest portion of the heated material reaches the target temperature. Total heating time can be reduced by ensuring that that treated materials are as warm as possible before treatment; preheating via solarization or simply warming materials in the sun will help reduce energy needs. Total heating time will also be minimized if the heated material (e.g., water, potting media) is agitated and that heat is uniformly distributed. In all heat treatments, some margin for error should be allowed to account for non-uniform heating. Use treatment times substantially longer than the minimum if it is difficult to ensure uniform heating.
For materials heated via solarization, temperatures fluctuate based on sun exposure. The treatment duration is related to the total amount of time above target temperatures of about 110-125°F (43-52°C). Typical treatment duration for soil solarization is 4 to 6 weeks at the hottest time of the year, but may be shorter if the coolest portions of the treated material routinely reach 125°F (52°C) or more.
Phytophthora species and other water molds are relatively sensitive to heat. Temperatures of 122°F (50°C) for 30 minutes will kill propagules of many Phytophthora species, though more heat tolerant Phytophthora species can survive up to about 72 minutes at this temperature (Funahashi and Parke 2016). The differential between the temperatures that are lethal to Phytophthora and to plant propagules (seeds, bulbs, cuttings, etc.) provides an opportunity for freeing plant propagules from these pathogens though carefully controlled heat treatments. Vegetative plant materials tend to tolerate heat treatment better if they are in a dormant condition, under slight water stress, and have low nitrogen levels. Plant materials should be selected or preconditioned to be in their most tolerant state before treatment.