Delving into new disease foes

Features - Plant Health

Not just for pests, biocontrol agents are being researched for their effect on diseases by Vineland Research and Innovation Centre, which has surveyed and spoken with growers who use them.

May 6, 2019

Vineland’s Dr. Michael Brownbridge, research director, Horticultural Production Systems (left); and Taro Saito, senior research technician in biological control

Biological controls can have a propensity to confuse and challenge growers, says Michael Brownbridge, research director, Horticultural Production Systems at Vineland Research and Innovation Centre in Ontario. Consider, for example, Bacillus subtilus, a common active ingredient. Different strains may have different properties. For example, some promote plant growth and others control disease.

“If I’m [a grower] looking for product and I’ve just gone to a talk or read somewhere that I should be using Bacillus subtilus, and I just picked the first product off the shelf that says ‘Bacillus subtilus’ on it, and then wonder why it doesn’t control a pest or disease — well, efficacy can depend on the strain,” Brownbridge says.

But grower uncertainties are not thwarting many of them from adopting biocontrols. Numerous growers, use biocontrols to avoid resistance, like they have seen against traditional synthetic chemistries, and adapt to growing public concerns about high pesticide use in greenhouses and on produce, Brownbridge says.

Vineland has found that biocontrol use for pests among Ontario floriculture growers rose from 69% in 2014 to 92% in 2018. This same demographic of growers also increased their use of biocontrols for disease control in the same timeframe, rising from 30% to 70%.

“They have adopted biocontrol as one of their main pest control strategies, and it’s amazing,” says Rose Buitenhuis, research scientist in biological control at Vineland Research and Innovation Centre.

These numbers, from a survey conducted by Vineland, reflect a larger shift among North American greenhouse growers to embracing biocontrol.

Many growers, Buitenhuis says, are influenced by (in order of importance) more pests developing resistance to pesticides, growers choosing to improve worker health and safety (which can be a concern with chemical pesticides) and growers and employees not having to adhere to a re-entry or pre-harvest interval.

Better plant health is another reason for biocontrol adoption, Buitenhuis says. “Spraying — especially if you spray a lot — there’s some phytotoxicity still involved with chemical pesticides, so they just see better plant health,” she says. Other reasons, she says, include consumer demands and the efficacy of biocontrol.

Thrips are one insect pest group with a strong resistance to chemical pesticides, Buitenhuis says. In Canada, compared to the United States, fewer pesticides are registered on thrips, and controlling these insect pests in the Great White North is particularly difficult.

Grower use of biocontrol for thrips has led to a broader adoption of biocontrol methods, Buitenhuis says.

“That was kind of how it all started,” she says. “If you have to do biocontrol for thrips, then you have to do biocontrol for other pests, and probably disease as well, because a lot of the chemistries are less, or not, compatible with biocontrol agents. You can’t be fully chemical for all the rest of the pests and fully bio for thrips. You really have to integrate it and it can be a puzzle sometimes.”

Combining biological and chemical controls

Brownbridge says he considers Ontario growers’ 70% use of biocontrols — agents such as Bacillus and Trichoderma — to be close to a full adoption rate. In other cases, the plants are in and out of the greenhouse so quickly that disease infection isn’t a concern. Meanwhile, many U.S. growers Brownbridge has met are beginning to adopt full biological programs and incorporate compatible synthetic chemistries based on their biological program, rather than the previously more common reverse scenario.

“As long as the chemical and the biological [controls] are compatible, oftentimes you get even better results by using the two components together, perhaps in a spray rotation or something like that,” Brownbridge says. “They can be very, very compatible and give you very, very good levels of disease suppression when you use the chemical and biological [controls] in combination. But there is no one-size-fits-all scenario. It’s all crop- and location-dependent.”

Dr. Michael Brownbridge, research director, Horticultural Production Systems

A clean environment

Multiple factors can help growers determine biocontrols’ effectiveness in their greenhouses. When adopting biocontrols, growers need to maintain good growing conditions and sanitation practices, Brownbridge says. “Biocontrols will not work if all of the other steps aren’t in place, as well,” he says. “They don’t give 100% control, they don’t work curatively and they don’t work when there are high pest or disease pressures, or perhaps when conditions are very, very favorable for diseases to develop.”

One favorable condition for disease, for example, is an unsanitary environment, so it works in growers’ favor to sanitize their greenhouses. Between crop cycles, Brownbridge says growers should spray a disinfectant on benches and clean floors of algae, moss and, if they have dirt floors, weeds. They must ensure that flats and pots are sterile and potting mixes remain free from diseases. A plant propagator’s reputation rests on providing clean cuttings but growers need to be aware that sometimes cuttings are infected.

Brownbridge compares the necessity of a clean greenhouse environment to the need for clean hospitals. “Ideally, a hospital is free of all sorts of diseases, because you’ve got a bunch of compromised people in there that can easily be infected if there’s a lot of dirt, filth and pathogenic organisms around,” he says. “[It’s the] same when you have a greenhouse — you want it to be as clean as possible.”

Au revoir, resistance

Biological controls have multiple modes of action: producing antibiotics to interfere with disease development, parasitizing pathogens, crowding out disease-causing organisms for space in roots, taking up resources that pathogens would otherwise use and inducing plants’ own defense pathways that protect them from infection, Brownbridge says.

“Against a chemical, it’s frequently [that] the mode of action is very specific on a specific biochemical pathway, so it’s easier for the organism to develop alternative pathways to get around it,” he says. “But with biocontrol agents having these multiple modes of action, it’s harder for the disease organisms to develop resistance to that.”

A new approach

To achieve successful results with biocontrols, growers need to understand their crop and the conditions in which it needs to be grown, and keep it healthy throughout its growth, Brownbridge says.

“For people shifting off conventional pest control into biological, it’s a little bit of a change of mindset, change of approach and a change of thinking around the whole crop cycle and the whole system in which you’re going to use this material — and ensuring all parts of that system are working together,” he says.