Producers of greenhouse food crops can’t be sloppy growers

Growers can reduce the chance of disease infestation on greenhouse vegetable crops by incorporating a strict sanitation program and minimizing plant exposure to moisture.

Sanitation and moisture management are key factors in controlling diseases on greenhouse food crops, said Michigan State University plant pathologist Mary Hausbeck.

“Preventing greenhouse diseases starts with sanitation,” she said. “I can walk into a greenhouse head house and predict how many disease problems I’m going to find just by what I see in the head house area. Is it clean? Is it neat and orderly? Sanitation is a mindset. It either carries through from the head house to the growing areas or it doesn’t.”

Hausbeck said growers should try to do a thorough job of removing any plant debris that may be left in the greenhouse, including “pet” plants that may be left in hallways or in the corners of the greenhouse.

“When a grower is making a changeover to another crop there shouldn’t be any other plants left in the greenhouse,” she said. “That is the safest approach.”

Hausbeck said growers should also make sure there is a weed-free perimeter that goes all the way around the outside of the greenhouse.

“There should not be any weeds growing around the greenhouse,” she said. “Growers need to have a critical eye to make sure there is nothing around the greenhouses that can harbor pests or diseases. Ideally all of that needs to be removed.”

Moisture management

Hausbeck said growers should make every effort to limit plant exposure to moisture.

“This includes moisture in the air—relative humidity, moisture on the foliage—leaf wetness, and moisture in the growing medium,” she said. “Moisture is a big driver for disease prevention and control. Growers should limit the amount of moisture by decreasing the relative humidity, watering during the time of day when the foliage can dry rapidly or not getting the foliage wet at all and not overwatering the plant root system. These are things that are important when growing ornamentals and are helpful when growing vegetables. It’s moisture more than temperature. When the humidity is 85 percent and higher, growers should do what they can to reduce the moisture in the growing environment and be vigilant for moisture-loving diseases such as Botrytis to develop.”

Hausbeck said unlike ornamental crops, root rots on greenhouse vegetables are usually not major diseases that wipe out a crop. She said some ornamental crops can suffer considerable losses from the root pathogens Pythium and Phytophthora.

“Diseases that have tended to be major problems on vegetables in the greenhouse include the foliar pathogens downy mildew on cucumbers and cladosporium leaf mold on tomatoes,” she said. “Leaf mold can overwinter in plant debris and soil.

Diseases that tend to be major problems on greenhouse vegetables include cladosporium leaf mold on tomatoes.
Photos courtesy of Mary Hausbeck, Mich. St. Univ.

“Vegetable pathogens tend to be crop specific. At the recent 29th Annual Tomato Disease Workshop I attended, most of the conversations centered around bacterial diseases on tomatoes. Those disease pathogens tend to be introduced via the seed. One disease that tomato producers talked about a lot is bacterial canker. This pathogen (Clavibacter michiganensis subsp. michiganensis) can move through hydroponic systems and from plant to plant through root grafting.”

Insect-vectored viruses

When it comes to viruses, ornamentals and vegetables are susceptible to some of the same viruses, including impatiens necrotic spot virus (INSV), tobacco mosaic virus (TMV), cucumber mosaic virus (CMV) and tomato spotted wilt virus (TSWV).

“These viruses have caused sporadic problems for ornamental growers and they can move onto vegetable crops with some devastating results,” Hausbeck said. “If thrips are a problem and there is a reservoir of INSV or TSWV in infected plants somewhere in the greenhouse, ornamental growers transitioning to vegetables need to know the symptoms on susceptible ornamental crops. They need to be especially vigilant since ornamentals can be infected without obvious symptoms.

“In the greenhouse, vegetables are susceptible to some of the same viruses that have caused problems with ornamental crops, including TMV. This means applying the same precautions such as employees not smoking and washing their hands after cigarette breaks. Ornamental growers need to be aware that they don’t get to walk away from these viruses just because they have switched to vegetables.”

Downy Mildew
Downy mildew is a foliar pathogen that can be a major problem on cucumbers.

Hausbeck said some of the viruses (INSV and TSWV) that attack ornamentals can be moved quite readily to food crops like lettuce via thrips.

“Pathologists don’t like to see the mixing of vegetables with greenhouse ornamentals,” she said. “That’s because INSV and TSWV can be brought into a greenhouse via infected cuttings or prefinished ornamentals. The virus can then be moved to vegetable crops by thrips. Growers need to monitor for thrips. If there are ornamentals growing in one part of the greenhouse, there is a risk that a virus may be moved from ornamentals to the vegetables.

“A virus such as TMV can also be introduced to greenhouse vegetables through infected ornamentals. The virus is easily spread among plants through physical contact such as grower handling.”

Chemical control limitations

Hausbeck said as ornamental growers move into food crop production they may not recognize that there are more limitations on fungicides that can be used on vegetables.

“Over years of production, ornamental growers are used to managing Pythium and Botrytis,” she said. “Those are some of the diseases that can also occur on greenhouse vegetable and herb crops. Growers may think that they can grab the same chemicals they are using on ornamentals like geraniums and poinsettias and use them on food crops. Depending on the product, this may not be legal.”

Hausbeck said on some ornamental fungicide labels the list of plants is very broad.

“Since these chemical labels may be broad in their application to ornamental plants, growers may mistakenly believe that the same product can be used on vegetables,” she said.

Bacterial Canker
Clavibacter michinganesis subsp. michiganensis, which causes bacterial canker on tomatoes, can move through hydroponic systems and from plant to plant through root grafting.

Hausbeck said it is important for ornamental growers who are expanding into vegetables to recognize that they are growing food.

“Food is tightly regulated,” she said. “Growers cannot use a product that isn’t expressly allowed by label to be used on specific vegetable crops. If a crop is on the label, then the grower has to determine if the label allows the use of the product within as greenhouse setting. If there is no mention of a greenhouse on the label, then the product can be used as long as its use in the greenhouse is not restricted. This should be verified through state agencies or through the product registrant.

“Vegetable growers need to read and reread the label before applying any chemical. Growers can’t assume that they can use some of their trusted products to combat a disease like powdery mildew when it occurs on a food crop. And even though ornamental growers can use Truban for the control of Pythium, the fungicide is not labeled for greenhouse vegetable crops.”

Hausbeck said even though some of the pathogens will be similar on ornamentals and vegetables, greenhouse growers need to become educated about the products needed to manage diseases on food crops.

“Growers need to know the key disease threats for the specific crops that they want to produce,” she said. “Manage the greenhouse environment to keep it as clean and dry as possible. When a fungicide is needed, know which products are legal to use on vegetable crops within the greenhouse and know which tools work best.”

For more: Mary Hausbeck, Michigan State University, Department of Plant, Soil and Microbial Sciences; (517) 355-4534;;

David Kuack is a freelance technical writer in Fort Worth, Texas;