Uncovering a delicate root zone balance

Sight unseen, small microbes play big parts in the root zone below.
October 28, 2019
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When it comes to root zone management, water and nutrients most likely come to mind, but there’s one component which doesn’t receive nearly as much attention.

Sometimes known as ‘root zone microflora,’ the term points to both disease-causing pathogens and good microbes that can benefit the crop.

With beneficial microbials, there are a few basic modes of action, explains Dr. Youbin Zheng,  professor at the University of Guelph and known researcher in the field of controlled environment agriculture. They can compete with pathogens for resources, produce antibiotics, parasitize or attack other microbes and colonize the rhizosphere, he says. Some beneficial microbes can even help trigger the plant’s innate defences or promote crop health.

But ridding the root zone of bad microbes can be difficult without harming the good ones. Zheng points out that residual activity from some water disinfecting technologies can kill off good microbes in the root zone. “But having said that,” he continues, “when we grow plants normally, either for vegetables in rockwool or ornamentals in coco [coir] or peat moss, those substrates or their root zones contain a lot of organic matter,” which can tie up any disinfectant residuals such as ozone or chlorine coming from the disinfected water.

As for the pathogens already established in the root zone, not many technologies can get rid of them, Zheng says. Water-sanitizing methods may remove harmful pathogens from the water and nutrient solution, but their spores exist everywhere – in the air, in the substrate – making complete disinfection very difficult.

One approach is to create an ecosystem. At the beginning of each crop, the root zone starts off with low levels of microorganisms – that’s when a grower should consider applying beneficial microbes. This gives the good guys a head start, colonizing the space so they can better compete with harmful pathogens later on. “Start clean, introduce your beneficial microbials first, [and] make your system more resistant and more resilient to pathogens,” he says.

Another way is to create a good root zone environment for the crop, using a comprehensive approach known as ‘integrated root zone management’ (IRM). “If you manage your root zone well, your plants are healthier and stronger,” Zheng explains. Similar to the concept of integrated pest management, the goal of IRM is to optimize the root zone environment through different components of production, including growing substrates, containers, irrigation and so forth, to achieve the right oxygen content, nutrient levels, pH – in other words, the right environment for healthy roots. “Say you water too much,” he says, that creates a low-oxygen root zone environment conducive to Pythium, which could eventually lead to root rot.

For researchers, the challenge is to find and isolate beneficial microbes to target different pathogens. However, these microbes must be active in the crop’s root zone environment. In greenhouse soilless production, for instance, the target pH is between 5.5 and 6.25, says Zheng. That means an isolated microbe found to work against Pythium in the lab can only be used in the greenhouse if the microbe can survive and perform under low pH conditions. The best approach is to optimize the root zone environment for that particular crop, then choose a microbial product that would work best against the pathogen in question, he says.

Zheng’s team has just finished part of a collaborative study with researchers at Laval University. Having previously tested six commercially available biocontrol products against Pythium without success, the team began to try applying fish sludge to potted gerbera. A combination of fish excrement, uneaten fish food and water from local fish cultivating facilities, fish sludge is rich with microbials. Performing their trials in the greenhouse, researchers found that weekly doses of fish sludge were able to reduce or control both Pythium and Phytophthora. The next step would be to isolate the microbe or combination of microbes responsible.

Critical values of various water treatment residues are available for different crops and pathogens. Visit http://www.ces.uoguelph.ca/water/pathogen.shtml
More in this category: « Growing lettuce in the deep

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