No matter what you’re growing in the greenhouse, stopping pathogens before they gain a foothold is the optimal way to keep them at bay. But simply cleaning isn’t enough. Cleaning involves removing organic matter and getting rid of dirt. Disinfecting requires cleaning surfaces first, but doesn’t stop there. A disinfecting step, done properly, kills pathogens lurking on surfaces, either eliminating them or reducing their numbers to prevent or decrease plant disease in the next growing cycle.
“Pests and pathogens are much easier to prevent than to cure,” Ana Pastrana, pathologist at the Vineland Research and Innovation Centre in Canada, said during a recent webinar. “Disinfecting should be done routinely. Timing does not always permit that extra effort, so take the opportunity to thoroughly clean greenhouses between crop cycles, when greenhouses are totally empty.”
Cleaning and disinfecting the greenhouse should be a priority as soon as the growing season is completed. If plant materials, debris, growing media, equipment and any surface in the greenhouse is allowed to sit for the off-season without being disinfected, pathogens can thrive and multiple. Eradicating disease when pathogen populations are low, rather than attempting to control them when they’ve had time to multiply and spread, is key in disease prevention.
Preventing Plant Disease
The first step in cleaning is to remove everything – plant material, media, containers, garbage – from every surface, including the floor. Plant material should not be composted near the greenhouse, as it’s possible and likely that pests can be dispersed back into the greenhouse environment, Pastrana said.
Next, all surfaces should be cleaned with soap and water, at least a 2% concentration by volume, using a high pressure washer. To prevent re-contamination, the greenhouse should be cleaned from back to front. Greenhouse plastic needs to be cleaned too, as pathogens can hide in the folds. Soap has to be thoroughly rinsed, or it can inhibit the actions of the disinfectant. Once rinsed, all surfaces should be allowed to air dry.
Disinfectants can’t reach the pathogens on soiled surfaces, and they will then serve as inoculant for the next season’s disease cycle. Cleaning surfaces prior to disinfection removes organic matter, so pathogens are within reach of disinfectants. Disinfectants need to be applied as per label directions.
The cleanest houses should be first in line, working toward the dirtier houses to prevent cross-contamination. Equipment should be cleaned between houses if shared. Workers need to change clothing before entering a new house. Don’t forget to clean doorknobs, light switches and computer keyboards. These all harbor potential plant pathogens.
Applying the selected disinfectant requires knowledge of which plant pathogens are of concern, Andrew Wylie, greenhouse vegetable IPM specialist at the Ontario Ministry of Agriculture, Food and Rural Affairs, said.
Not all disinfectants are effective against all forms of pathogens. Gram positive and gram negative bacteria, viruses, fungi and spores are all susceptible to these chemicals in varying degrees. Good records will indicate which diseases your plants contracted during the previous growing season and help you select from the approved disinfectants available. Wylie noted most of the diseases we see in plants come from fungi. And while fungal spores are notoriously difficult to eliminate, viruses are the most difficult pathogen to eradicate from the greenhouse.
Disinfectants include bleach, quaternary ammonium, peroxygen compounds and alkalis. Each one not only attacks different types of pathogens, but requires its own precautions, application, timing and has its own drawbacks, Wylie said. Their effectiveness is affected by solution concentration, type of surface, dry time and surface contact time.
Bleach is highly corrosive, has toxic fumes and is inactivated by any organic matter left on surfaces. It is effective against gram negative and positive bacteria and fungi and may have some effectiveness against viral pathogens. It cannot kill spores. Quaternary ammonium is not effective on gram negative bacteria and is questionably effective on viruses. It can kill fungi but not spores. It is inactivated by soap, and less active if organic matter hasn’t been removed. Calcium and magnesium in the water will inhibit its ability to eliminate pathogens.
Peroxygen compounds (like hydrogen peroxide) work best in warmer temperatures, in pH levels below 7 and are effective against viruses, bacteria and bacterial spores, and have some effectiveness against fungi. They are corrosive but can be used on plastics. They biodegrade and have some residual action. Alkalis include trisodium phosphate (TSP) and work through a negative charge, which leads them to seek hydrogen atoms. They are very corrosive, but work against bacteria, viruses and fungi.
“Alcohol is a pretty good disinfectant,” Wylie said, but it’s not commonly used in greenhouses because it cannot kill spores.
Appropriate personal protective equipment is needed, and workers should follow all safety protocols established. Disinfectants can harm the environment and can harm human cells too. “Safely handle, apply and dispose of disinfectants” as per their label, Wylie said.
Factors Affecting Efficacy
Disinfectants won’t work if they are applied incorrectly. Organic matter and biofilms can protect pathogens, acting as physical barriers. These prevent the disinfectant from reaching the pathogen.
Dirty surfaces are not able to be wetted properly. If a surface is not wet enough, evaporation occurs before pathogens are killed. Hydrophobic surfaces such as polyethylene, stainless steel and PVC are difficult to wet thoroughly. “Not only is it [disinfectant] not coating the surface as effectively as it could be, but it’s going to evaporate more quickly” if surfaces are not properly cleaned, or if they are composed of hydrophobic materials, Wylie said.
Porous materials can be problematic due to rapid drying, which occurs due to the small pockets increasing surface area. Rubber, wood and concrete are examples.
Temperature can also have an effect on efficacy. For every 18º F increase in temperature, the effectiveness of a disinfectant increases twofold, Wylie said. Exposure time matters too, and each disinfectant has to be in contact with the surface for the labeled time span. It’s also important to use an aqueous solution in greenhouse settings, as gases are not really effective against viruses.
Greenhouse Irrigation Systems
Irrigation systems can host and spread plant pathogens, and water can disperse algae and biofilms. It’s imperative to clean these, both to eliminate pathogens and to maintain water quality, Pastrana said.
Mixing tanks and irrigation systems should be emptied, and basins should then be filled with clean water to prevent any corrosion due to contact with the disinfectant. Flush pipes with clean water, then apply the disinfectant solution and allow it to sit for 24 hours before emptying the system and flushing everything again with clean water. Irrigation should then be run for a good period of time. A nitric, phosphoric or sulfuric acid solubilizer added to the system for at least three hours will remove precipitates. A final clean water flush to the system will complete the process.
Once the greenhouse is thoroughly clean, treat it as a sterile environment, Pastrana said. New plastic can be added to the ground, new growing media should be used and all equipment disinfected prior to being put back into use. Anyone entering should cover their clothes.
Not all pathogens can be fully eliminated once they gain a foothold, and decreasing their numbers via disinfection prior to a new growing season can help to better manage their spread and prevent crop losses. Keeping the greenhouse as free from pathogens as possible gives next season’s plants the healthiest start.