July 2023 Plant Health, Production

Is Your greenhouse really clean? By David Kuack

Maintaining a clean greenhouse can mean the difference between growing a high-quality, profitable crop and scrambling to reduce losses from pathogens and pests. Here are five steps that can help you lower your chances of disease and pest outbreaks.

1. BE CONCERNED ABOUT GREENHOUSE SANITATION.

Growers have to build from a good sanitation program to operate a greenhouse that minimizes pathogen and pest losses.

“You can usually walk into a greenhouse and know quickly whether the greenhouse is oriented towards good sanitation or not. It looks clean,” said Margery Daughtrey, plant pathologist and senior Extension associate at Cornell University. “By taking preventive actions, growers aren’t going to run into as many disease and pest problems in the course of growing their crops.”

Organic debris is where pathogens survive between opportunities to attack plants. Many pathogens can live on dead plant material as well as attacking living plants.

“If there are dead leaves on the floor, on or under the benches or in plant containers, anywhere within the greenhouse, there are pathogens in that tissue that can be activated by moisture,” Daughtrey said. “Growers need to take a clean approach to growing to avoid having a lot of disease inoculum carry over, especially for root and stem diseases.

“Powdery mildew isn’t going to persist unless a crop or weeds are kept in the greenhouse which have the fungal spores on them. The pathogen can hitchhike at a low level on plants brought into the greenhouse or enter as spores through the vents and eventually a new infestation can occur.”

Although greenhouse sanitation applies more often to pathogens than pests, there are some exceptions. Slow-moving insects like mealybugs are unlikely to crawl into the greenhouse through vents or doors. These pests could potentially be hiding under benches and crawl up onto plants if the greenhouse hasn’t been cleaned between crops.

Fungus gnats prefer high organic matter and plant debris. A residual population of fungus gnats that can fly up to bench level and go after young plants is going to be much higher if greenhouse surfaces have not been cleaned prior to bringing in new crops.

2. REMOVE WEEDS IN AND AROUND THE GREENHOUSE.

Don’t ignore weeds. Weeds can be a haven for pathogens and pests. One of the biggest concerns is the spread of tospoviruses, which can infect many greenhouse crops. Tospoviruses include impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TSWV).

If thrips are introduced into a greenhouse, they can feed on virus-infected weeds on the greenhouse floor and then fly up and spread tospoviruses into a crop.

Powdery mildew can also survive on weeds on the greenhouse floor and then move up to crops on the benches.

“The powdery mildew species Golovinomyces cichoracearum has a multi-crop host range,” Daughtrey explained. “The fungal spores can be carried up to plants of the same family and start an infestation even during the winter. Disease control can be more difficult for someone who is growing crops year-round.”

Another factor in the control of weeds is greenhouse construction.

“Greenhouses with dirt floors are going to provide more opportunity for weed seeds to germinate,” Daughtrey said. “If the floors are concrete, they can be thoroughly cleaned, which should minimize weed problems unless they are in crop containers or just outside the greenhouse so that weed seeds can be brought in through vents and doors. Growers need to be aware of areas where weeds can accumulate or where there is soil that can support weeds.”

3. FOCUS ON KEEPING GREENHOUSE SURFACES CLEAN.

Greenhouse surfaces can never really be clean unless they are precleaned.

“Anyone who tries to use a disinfectant on greenhouse surfaces without first removing debris isn’t going to be satisfied with the results,” Daughtrey emphasized. “The basic idea of pressure washing first and then using some type of greenhouse disinfectant is a better way to go. Precleaning eliminates trying to clean through a layer of debris.”

Disinfectants are biocides and broad-spectrum compounds. They don’t have to be rotated like pesticides and fungicides.

“A grower’s ability to get proper control with a disinfectant is going to be related to how much time it makes contact with the surface and whether it makes good contact with the surface,” Daughtrey shared. “This is where precleaning comes into play so that the disinfectant makes good contact with the surface.

“Generally, these compounds have an oxidizing effect. They burn the organisms on contact. The chemistry of these compounds breaks down so they don’t have a residual effect to continue killing what falls on the greenhouse floor or bench. They have their effect and then their effect is gone. They have to be reapplied.”

After cleaning the greenhouse, it has to be kept clean by not bringing soil in from outside.

“Don’t track field soil into the greenhouse because there are many pathogens in field soil, including rhizoctonia, pythium and phytophthora,” Daughtrey said. “When these pathogens are tracked into the greenhouse, they can be brought up to crop level and clean surfaces can be contaminated. Staying in continual keep-it-clean mode is important.”

4. DON’T REUSE CONTAINERS WITHOUT DISINFECTING THEM FIRST.

Growers may reuse containers for a couple of reasons, including minimizing plastic waste and trying to be frugal.

“Growers who choose to recycle their containers should make a special point of removing any debris and then soaking them in a disinfectant before they are reused,” Daughtrey emphasized. “This is particularly important for crops exposed to thielaviopsis or black root rot. This fungus produces a resistant spore that is very good at surviving extreme conditions.

“Thielaviopsis is a particularly serious disease on pansies, vinca and calibrachoa. Reusing containers with these crops without properly cleaning them could lead to future plant losses. If there was black root rot in pansies and containers were reused with vinca or vice versa, there could be a return of the disease.”

5. ALWAYS REMOVE PLANTS THAT SHOW DISEASE SYMPTOMS OR ARE HEAVILY INFESTED WITH PESTS.

Disease- or pest-infested plants are inoculum sources and need to be removed from the greenhouse.

“Scouting has to be part of a sanitation program so that a problem is recognized when it begins to happen,” Daughtrey detailed. “This can help to resolve a problem before it expands to other plants or areas of the greenhouse. Scouting regularly provides an option of being able to discard the fewest number of plants. This buys growers some time so a control action can be initiated and reduces the opportunity for an epidemic to occur.”

If a problem is not recognized, the plants should be carefully bagged and sent off for diagnosis.

“If there is a problem and growers don’t find out what caused the problem, then they won’t know what they are trying to protect against with future crops,” Daughtrey said. “Never getting a diagnosis of problems is a sanitation error. Diagnosis has a cost in terms of money and time. There are growers who just throw plants away and don’t necessarily worry about what caused plant loss. These growers are not making informed decisions when they go to clean up their greenhouses. The clean up between crops is designed to protect against whatever problems occurred previously. If growers don’t know what caused a problem, how can they implement the right steps to avoid it in the future?”

For more: Margery Daughtrey, Cornell University, Long Island Horticultural Research and Extension Center; mld9@cornell.edu; https://cals.cornell.edu/margery-daughtrey

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