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A chemical-free steaming technique developed by scientists at the National Ornamentals Research Site at Dominican University of California (NORS-DUC) to control the spread of plant disease is now helping one of the world’s largest berry growers reduce the use of disposable plastic containers.

NORS-DUC scientists focus on diseases of ornamental and forest plants, especially those caused by members of the genus Phytophthora, such as P. ramorum, the causal agent of sudden oak death. Phythopthora, which can infest hundreds of different host plants, is one of the pathogens blighting the strawberry industry, says Vernon Huffman, NORS nursery manager.

The NORS-DUC team recently spent three days steam treating more than 175,000 used strawberry containers at Driscoll’s strawberry growing ground in Northern California. The goal was to allow the grower to safely reuse the plastic pots – saving money and reducing plastic waste – by ensuring the pots did not contain traces of Phytophthora, a highly destructive plant pathogen spread by the movement of contaminated soil, equipment, or plant debris.

“Phytophthora is mainly a soil borne pathogen, therefore growing plants in pots on the ground means there is a risk of infection,” Huffman says. “The way to control the spread of the disease is to take a preventative measure – an extra step – but you have to understand disease and be willing to spend money to invest in preventing disease.”

While decontaminating research plots at Dominican University in which experiments with P ramorum and other quarantine pathogens were conducted, NORS-DUC researchers, led by Dr. Wolfgang Schweigkofler, discovered that heat treatment at a temperature of 50⁰ C for only 30 minutes will kill P. ramorum. Based on this published research, the USDA’s Animal and Plant Health Inspection Service now accepts steaming as a treatment for nursery soil infested with P. ramorum.

The traditional ‘top down’ approach, with heat penetrating the treated material from steaming socks placed on top of it, works well for many situations, but the technique can be slow and inefficient when treating large amounts of soil or potting mix. 

In order to speed up the process, the NORS-DUC scientists developed a ‘manifold system’. The manifold is basically a steel scaffolding consisting of a central pipe connected at a right angle to a number side pipes. The steam is released from exit holes drilled at the bottom of the pipes and moves ‘bottom up’ trough the soil which is placed on top of it.

“We compared the two steaming methods at our NORS-DUC facility and treated 7.5 cubic yard soil mix: using the ‘top down’ approach the target temperature of 50°C was not reached after seven hours on the bottom of the soil pile, whereas using the ‘bottom up’ approach with the manifold the whole pile reached the target temperature after 1.5 hours,” says Huffman.

Using the manifold not only resulted in a much faster and efficient heat treatment, but also reduced the costs and environmental impact by considerably reducing the fuel needed to run the steamer engine. Manifolds can be designed in different sizes and shapes based on the needs of the stakeholder using it.

 “Steaming is now an established part of the best management practices in several nurseries in and outside of California,” Dr. Schweigkofler says. “Steaming can be used to mitigate existing soil beds in situ, but also potting mixes, pots, and other nursery equipment and will reduce the risk of establishment and transmission of plant diseases.”&�Բ�����;

NORS-DUC recently started offering a ‘Steaming-on-the-Go’ service for nurseries and other facilities in the San Francisco Bay Area and surrounding counties concerned with the spread of Phytophthoras. NORS-DUC brings its own equipment, including a SIOUX SF-20 steam engine and temperature sensors, to the treatment site, and will plan, set up, conduct, and monitor the steaming event. 

Steamings have been held already at several native plant nurseries as well as other properties. Last year, with a grant from the USDA Forest Service, Huffman successfully treated wood infested by the Mediterranean Oak Borer beetle, an invasive ambrosia beetle attacking mature Valley oaks in Northern California, particularly in Napa County and the Sacramento valley.

The Driscoll’s facility in MacArthur, located about an hour east of Redding, is the largest site Huffman and the NORS-DUC steam engine have treated.

Driscoll’s grows plant rootstocks in McArthur before moving them to a production field in Central California for out planting. Although no Phytophthoras have been found in the used pots, the company traditionally had a policy of ‘single us’ for their planting pots to prevent the movement of potentially contaminated soil and plant debris.

After a first successful trial in 2021, the company asked NORS-DUC to treat 175,000 used pots in a ‘steaming marathon’ earlier in 2022. 

“These were small, starter pots – plastic containers used to start strawberry plants before they are sent to other facilities for planting,” Huffman says. “These are all pots that previously would have been discarded after being used just one time.”&�Բ�����;

The treated pots can now be reused for growing new plants, which not only saves the company a considerable amount of money – an estimated $120,000 – but also helps to reduce plastic waste and is environmentally friendly. 

Huffman also treated 93,000 reusable plastic containers (RPCs) that Driscoll’s uses to store strawberry roots at a warehouse facility near Merced in the Central Valley. In previous years, the grower used cardboard storage containers to stack and freeze the pots. However, as cardboard holds moisture the containers were at risk to develop mold.

 “They called us in to do a preventative treatment of the RPC crates so they could reuse them knowing that they do not have Pythophthora.”&�Բ�����;

“Plant disease is an issue that impacts every grower,” Huffman says. “It’s just hard for those on a budget to treat the unknown. It’s a preventative measure – an extra step – but you have to understand that preventing a disease is cheaper than fixing it in the long run.”&�Բ�����;

Photo above of Vernon Huffman, NORS nursery manager, with stream treatment device