Controlling the environment is a key component of preventing diseases on edible crops.
As an increasing number of growers start growing edible crops in controlled environment structures they may be facing some diseases that they haven’t encountered before. For ornamental plant growers who are adding edible crops, they will not have as many or as effective chemical controls as they have access to with their ornamental crops.
“Growers currently don’t have a good complement of products for edible crops being grown in greenhouses or other controlled environment structures,” said Michigan State University plant pathologist Mary Hausbeck. “And if there are chemical controls available, growers may want to spray all of their herbs together. For instance, if the rosemary needs to be sprayed to protect against Botrytis, it’s wise to make sure that other nearby herbs are also listed on the fungicide label. Otherwise, growers need to make sure that their employees who are going to be spraying know which herbs they can and cannot spray with a particular product. Fungicides labels can be complicated.”
Hausbeck said growers should be cautious about using pesticides in greenhouses and other controlled environment structures like warehouses, vertical farms and plant factories.
“Previously, EPA required that use in greenhouses had to be specifically stated on a pesticide label in order for greenhouse growers to use it. But states varied in their interpretation and so usage was determined on a state-by-state situation. Several years ago EPA determined that if the label doesn’t restrict use in greenhouses, then a pesticide can be applied to a greenhouse-grown crop. In Michigan, our state agency views greenhouses and other controlled environment structures similarly. Just make sure that the label doesn’t prohibit the use of a fungicide in a greenhouse.”
Optimizing environmental control
Because of the limited number of chemicals available to greenhouse growers of edible crops, Hausbeck said it is critical for growers to use environmental control to limit disease.
“Environmental control has to be part of the equation,” she said. “Powdery mildew, downy mildew and Botrytis have a tremendous capability to reproduce in large numbers via spores. A microscope is needed to see one spore, but when there are many spores, the eye can see the fuzziness of the mildew or grey mold on the plants. The spores are moved around the greenhouse via air currents.
“Powdery mildew and downy mildew tend to be specific as to what plants they will infect. Powdery mildew is a problem on tomatoes, cucurbits and peppers. Downy mildew can be a problem on basil, cucumber and lettuce. Botrytis has a much broader host range.”
Botrytis is in a lot of growing environments because it can take advantage of plant tissue whether it is alive or dead. Downy mildew and powdery mildew won’t colonize dead plant tissue that is lying on the floor or sitting in a trash pile. Botrytis will infect plant parts that are still attached or have been discarded.
Powdery mildew, downy mildew and Botrytis prefer cooler temperatures.
“Occasionally I have seen Botrytis sporulate in coolers,” Hausbeck said. “Older leaves that are mature and are senescing may be in direct contact with moist growing media and may lead to Botrytis infection. Plants growing in a moist environment with high humidity and extended periods of leaf wetness are at particular risk of Botrytis infection. For instance, Botrytis can cause stem blight on rosemary and other herbs leading to significant losses.”
Hausbeck said Botrytis has a fairly broad temperature range in which it can infect plants.
“Growers who effectively use environmental control to keep conditions dry and unfavorable for Botrytis can get by without fungicides,” she said. “Botrytis needs leaf wetness to infect plants. Moisture is key.
“At 80-85 percent relative humidity even a small drop in temperature can lead to Botrytis issues. If the relative humidity is maintained at a low level, Botrytis won’t be successful in becoming established. Growers should do everything possible to keep Botrytis at low levels.”
Botrytis can also come in as a secondary problem. It may become a problem as a result of the plants undergoing an environmental stress that results in some leaf browning. Botrytis can take advantage of this dead tissue.
Hausbeck said Botrytis can be an issue with rosemary propagated by vegetative cuttings.
“Growers should try to lower the humidity immediately following taking cuttings from rosemary stock plants for 24 to 48 hours,” she said. “Botrytis infection can occur on the wounded stems of stocks plants leading to dieback. Growers can raise the temperature in the greenhouse by a degree or two to reduce the relative humidity. Growers should try to drop the humidity below 70 percent or as low as possible depending on the circumstances of the greenhouse. They should also provide good air movement around the plants. Having good airflow movement around the plants helps to prevent all types of diseases, including Botrytis, bacterial diseases, powdery mildew and downy mildew.
“It doesn’t take a lot of expensive technology to keep the relative humidity low and the plants dry. Avoid overwatering and keep the leaves dry by watering at a time of day when they can dry rapidly. Keep the air in the greenhouse moving so that there aren’t pockets of high humidity. Ensure there is good drainage so there is no standing water on the greenhouse floors. In areas of the greenhouse where there is standing water, the relative humidity in these areas can be increased by as much 15 percent.”
Correctly identifying the disease
Hausbeck has encountered times when some of the problems growers have reported were caused by environmental stress.
“In some cases growers have been able to send me really good pictures and I can make a diagnosis based on the photos,” she said. “If it appears the plants have a potential disease I’ll recommend that growers send samples to a diagnostic lab. There are some symptoms that don’t look like a pathogen at all. I ask the growers about their growing systems and to describe when the symptoms first appeared and the pattern they are seeing across the crop.”
Having good airflow movement around plants applies to preventing all types of diseases, including basil downy mildew.
Photo courtesy of Mary Hausbeck, Mich. St. Univ.
Hausbeck said as more edible crops are being grown, she expects to see some “oddball” diseases showing up that she hasn’t seen before.
“Some of these are fungal pathogens,” she said. “I have seen Septoria, which is a fungal leaf blight, on a couple of herbs. As herbs are being grown in greater volume, it’s likely that we are going to see more problems develop just by the sheer amount of plant material being grown. It’s important for growers to recognize in some situations this is still a relatively new arena for plant disease.
“We don’t know everything about the range of crops that can be impacted and what the disease symptoms might look like. If growers see a problem with a crop they really should connect with a diagnostic lab sooner than later. In the case of a disease problem, the recommendation may be to toss the plants and not try to save them because there isn’t a good complement of fungicides. Growers may sit on a crop trying to nurse it along with different growing regimes, not recognizing that it is a pathogen that is destroying their crop. This could result in costing them more time and more money.”
More control options available
Hausbeck and her laboratory at Michigan State have developed a reference chart for “Registered Products for Common Greenhouse Diseases on Vegetables and Herbs.”
“We have compiled this “cheat sheet” for products that can be used on edible crops,” she said. “We have grouped the crops according to brassica, cucurbit, leafy greens, fruiting and herbs. It is only meant to be used as a guide. Growers must read the pesticide labels carefully.
“What we have learned is that there is not a blanket product that can be used on all herbs. If a product can be used on herbs, it may or may not include basil. The situation is getting better with these crops and registered products in terms of having more options.”
For more: Mary Hausbeck, Michigan State University, Plant Biology Laboratory; firstname.lastname@example.org; https://veggies.msu.edu.
Hort Americas works with a wide variety of vendors that help controlled environment gorwers manage the climate inside their greenhouses and vertical farms. Learn more about fan, ventilation equipment and other products available from Hort Americas.
This article is property of Hort Americas and was written by David Kuack, a freelance technical writer in Fort Worth, TX.