Controlling the environment is a key component of preventing diseases on edible crops.
As an increasing number of growers start growing edible crops in controlled environment structures they may be facing some diseases that they haven’t encountered before. For ornamental plant growers who are adding edible crops, they will not have as many or as effective chemical controls as they have access to with their ornamental crops.
While providing the proper soluble salts and pH levels are important in hydroponic systems, don’t overlook the significance of maintaining the optimum temperature, oxygen concentration and microbe level in the nutrient solution.
Maintaining the proper soluble salts (electrical conductivity) level and pH are critical in hydroponic systems like nutrient film technique and floating rafts. While monitoring these properties are important, growers should not overlook the importance that temperature, oxygen level and microbial activity play in the growth of plants in these production systems.
“It’s not as much about maintaining root health as it is about managing the conditions in the rhizosphere, which is the region around the plant roots,” said Rosa Raudales, assistant professor of horticulture and greenhouse extension specialist at the University of Connecticut. “The area around the roots undergoes a lot of biological and chemical activity. Microorganisms in the rhizosphere feed on the exudate of the roots. Managing the rhizosphere and the conditions in the nutrient solution are critical to maintaining plant health.”
Maintain optimum root temperatures
While providing the proper air temperature in a greenhouse or controlled environment agriculture system is important, maintaining the optimum root temperature can have a bigger impact on the health and production time of a crop.
“If higher temperatures are maintained in the root zone then the plants are going to lose a lot of energy,” Raudales said. “Temperatures above the optimum in the root zone affect the cell membrane integrity of the roots. A disruption of the cell membranes affects the function of the roots resulting in less nutrient uptake, which affects crop cycles and yields.
“If plants are grown at root temperatures lower than the optimum, the plants grow slower because their metabolism is slower. In the worst case scenario, if freezing temperatures occur then ice crystals could form in the cells resulting in cell leakage and cell disruption.”
Cornell University researchers have conducted studies (http://www.cornellcea.com/attachments/Cornell%20CEA%20Lettuce%20Handbook%20.pdf) to identify the specific temperatures that are ideal for hydroponically-grown vegetables.
“Cornell researchers found the temperature of the nutrient solution had a greater effect than the air temperature,” Raudales said. “Lettuce plants exposed to air temperatures ranging between17ºC (62.6ºF) and 31ºC (87.8ºF) had consistent yields as long as the nutrient solution had a consistent temperature of 24ºC (75.2ºF). This research was done in the 1990s, but it still has application today.
“Cornell researchers did a similar study with spinach and they found the optimum root temperature was 22ºC (72ºF). They tested air temperatures ranging from 16ºC to 33ºC (60.8ºF-91.4ºF) and they found as long as the root temperature was 22ºC, the air temperature could be in that range and plants still produced optimum yields. For tomatoes the optimum root temperature is 25ºC (77ºF).”
Raudales said growers who are producing hydroponic leafy greens like lettuce and spinach have the option of installing a water heater to maintain the optimum root temperatures.
“It is easier and less expensive to heat the nutrient solution than to keep the whole greenhouse warm,” she said. “Heating the greenhouse does not make economic sense, when the research indicates that the temperature of the nutrient solution is a more important factor. If a grower is producing lettuce and spinach, which can tolerate lower air temperatures, it makes sense to run the greenhouses cooler and to install a water heater to adjust the nutrient solution temperature.”
Maintain adequate oxygen levels
Raudales said the dissolved oxygen level in a hydroponic solution needs to be maintained so respiration can occur in the roots.
“When oxygen levels are low in the root zone, the roots do not take up the nutrients required for growth,” she said. “Low oxygen levels cause increased ethylene production in the roots. If there are higher ethylene levels in the roots then the roots start to mature and die. The more oxygen present, the better the nutrient uptake and the better the root system.”
Raudales said there is also an inverse relationship between the oxygen level and solution temperature.
“If the root zone temperature is high, then the oxygen level is going to go down,” she said. “This is another reason why the root zone temperature is so important. The optimum oxygen level should be greater or equal to 6 parts per million of dissolved oxygen in the root zone. Plants should be able to handle 6-10 ppm without any problems.”
Raudales said growers who are using nutrient film technique systems typically don’t need to do any type of aeration. The movement caused by the flow of the water is usually enough to keep the oxygen level high enough in the solution.
Raudales said growers who are using floating rafts usually incorporate some type of oxygen-generating system.
“There are different ways of oxygenating the water,” she said. “One is aerating the water where air is being pumped into the water. Air is not pure oxygen, but it contains enough oxygen for what is needed in the hydroponic solution.”
Raudales said another reason for maintaining a high oxygen level in the hydroponic solution is the effect it can have on pathogenic fungal zoospores.
“If there is more oxygen, then zoospores don’t survive as well,” she said. “Zoospores don’t want completely anaerobic conditions, but they do better in conditions where there is less oxygen.
Pathogens of concern include Phytophthora, Pythium, Thielaviopsis basicola and Xanthomonas.
“Growers should try to keep the oxygen level high. If there are warmer temperatures, then there are lower oxygen levels. When there are lower oxygen levels the plants are not as healthy and more zoospores tend to survive. This is one of the reasons why there tends to be more disease issues during the summer than during the winter.”
Raudales said growers who are using floating rafts should be measuring the oxygen level regularly. Meters for measuring dissolved oxygen look like pH meters and are simple to operate.
Maintain beneficial microbes
Raudales said beneficial microbes are present naturally in water. Commercial products with beneficial microbes can also be incorporated into the hydroponic solution.
“Growers who are using the floating rafts tend to treat the nutrient solution like gold,” she said.
“They don’t want to replace it because they have a solution which is very high in beneficial microbes. Growers can inoculate the nutrient solution with a commercial biocontrol product or they can allow the good microbes to build up with time.
“As long as growers maintain the other parameters at optimum levels, including root temperature, pH, nutrients and oxygen levels, there typically isn’t a problem with diseases. This is very comparable to what happens with plants grown in substrates. The microbes build up naturally in the water just like in a substrate. These microbes feed on the exudates of the roots.
They need carbon sources that they wouldn’t get just from the nutrient solution. The system has to be clean, but it doesn’t have to be clean to the point of having to start with a fresh solution every time a new crop is planted.”
For more: Rosa Raudales, University of Connecticut, Department of Plant Science and Landscape Architecture; (860) 486-6043; firstname.lastname@example.org; http://www.greenhouse.uconn.edu.
David Kuack is a freelance technical writer in Fort Worth, Texas; email@example.com.
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