§      Recognizing and controlling oedema in greenhouse crops
§      Aphids, thrips and pathogens
§      Beware of over-watering chilled plants
§      Greenhouse *A* Syst

Recognizing and controlling oedema in greenhouse crops
Tom Dudek
District Horticulture Agent

During the winter months the environmental conditions in Michigan greenhouses are apt to be cool and humid with generally low light levels. With the high fuel prices, some growers are running cooler temperatures to save fuel expenses. These same environmental conditions also favor the development of oedema (edema). When the growing media remains moist and the plant roots absorb water at a faster rate than is lost through leaf cells, the leaves rupture. This rupturing of the leaf epidermis and the inner cells causes the raised, crusty appearance on the underside of the leaf. Plants like ivy geranium can have oedema all year long, but it is most common from December to February. Ivy geranium cultivars vary in their susceptibility to oedema. Experienced growers will select varieties less susceptible to the problem. Other greenhouse crops susceptible to oedema include: begonia, cacti, ferns, palms, pansy and cole crop vegetables like broccoli, cabbage and cauliflower.

With the recent influx of new vegetative cutting materials, we've seen symptoms of oedema on specific cultivars of cleome and sweet potato vine. Symptoms of oedema appear as bumps or blisters initially on the undersides of lower or older leaves on a plant. They may then turn brownish or tan and become corky. Severely affected leaves will turn yellow and fall off the plant.

To reduce the levels of oedema during the winter months: use a well drained growing media, avoid over watering, and keep plants on the "dry side" during extended periods of low light and cool temperature. Ventilate whenever possible to lower humidity and use horizontal air flow (HAF) fans to hasten air movement and maximize plant transpiration. Increase light intensity. Space plants farther apart. Avoid over-fertilizing plants, and avoid cultivars that are highly susceptible to oedema in your greenhouse.

Note: Thrips feeding damage on the undersides of ivy geranium leaves can often appear like oedema. Be sure to check for thrips with a hand lens. If thrips are present, use a recommended insecticide to control them.

Aphids, thrips and pathogens
Jeanne Himmelein
District Horticulture Agent

Aphids, thrips and pathogens are being seen in commercial greenhouses across the state. However, the insect pests that I have observed most often are fungus gnats and shore flies. The vacation spot for these pesky creatures is often a propagation house. The high moisture environment for rooting and germinating plant material presents an ideal condition for fungus gnats and shore flies to frolic amongst media and plant material. Environmental management strategies to control these pests include reducing water, which is usually not an option during propagation. Therefore, you have two options: biological control or chemical control. Both of these strategies have been effective if properly used. If you are using biological control, such as releasing mites or nematodes, and you already have a substantial population, then you're too late. You need to plan ahead and begin releasing as soon as you put water to the media.

If you missed the opportunity to release biological control agents and choose to control with a chemical, Dr. Dave Smitley (MSU entomologist) suggests a few options. Your best method of control is to focus on the immature larvae in the media by using the following products as a drench: a neem product such as Azatin XL, or an insect growth regulator like Adept or Distance. Mesurol and Marathon are also options, but you should take into consideration that Marathon can only be applied to a crop once. Thus, you may want to wait if you need the control that Marathon provides until later in the production cycle.

The cold temperatures two weeks ago can still influence your plants' growth. Chilling occurs when plants are exposed to temperatures below 50°. While there is usually no visible injury, plant growth slows or stops for a time after temperatures return to normal. New Guinea Impatiens and other warm crops are commonly affected but even pansies, which are very cold tolerant when they are mature, can be injured as young, rapidly growing transplants. Plants in the colder parts of the house such as on the outside edges or under a unit heater can be damaged while plants in the center of the house have no problems. Production schedules can be set back but probably the greatest danger is over-watering because you don't recognize the slower growth rate. This is most difficult to handle where an entire bench is on an automatic watering system and only the plants on the coldest end are affected. 

Greenhouse *A* Syst
Tom Dudek
District Horticulture Agent

Greenhouse *A* Syst is a self-evaluation tool to assist greenhouse operations in evaluating their impact on Michigan's surface and groundwater. Greenhouse production in Michigan generates more than $270 million in products annually. Floriculture requires advanced technology and precise use of pesticides, fertilizers, light, temperature and water to produce bedding and potted plants within a limited area. Like other agricultural enterprises, floriculture involves a number of processes that can adversely affect Michigan's surface and groundwater. Although many improvements have been made to reduce surface and groundwater pollution, more needs to be done. The Greenhouse *A* Syst publication was developed to encourage greenhouse growers to make needed improvements to protect water resources and to help them become aware of federal, state and local environmental quality regulations.

Greenhouse *A* Syst is part of the Michigan Groundwater Stewardship Program (MGSP) and the Michigan Agriculture Environmental Assurance Program. The MGSP program is funded through fees assessed on sales of pesticides and nitrogen fertilizers across the State of Michigan. Growers who complete a Greenhouse *A* Syst and any other relevant Farm *A* Syst worksheets are eligible for cost sharing dollars for items to specifically reduce the risk of groundwater contamination associated with pesticide and nitrogen fertilizer use. The Greenhouse *A* Syst program is voluntary and your farm's results are not shared with anyone else. No regulatory action will result from any practices that you're doing that are high risk. The goal of the program is to help you identify the high-risk practices you have and provide ways to reduce those risks. If you are interested in doing a Greenhouse *A* Syst contact your local MSU Greenhouse Extension Agent for details.