MSU Extension Michigan Fruit Management Guide

Most nurseries in Michigan produce fruit propagation material in greenhouses at some stage in the production cycle. Growers may also produce their own planting stock in a greenhouse or tunnel. In addition, the acreage of commercial fruit production under high tunnels is increasing. Tunnels allow extension of the growing season and production of high-quality fruit by protecting crops from rain.

Many diseases, particularly those that need rain or dew for infection, are absent or considerably reduced under covered production. However, powdery mildew may be favored by the higher temperatures and a lack of rain. Plants in propagation beds may suffer from soil-borne pathogens such as Rhizoctonia, Cylindrocladium and Pythium, which are favored by the continuous availability of moisture. Furthermore, the year-round presence of green leaf tissue in greenhouses may enhance survival and reproduction of rusts and other fungi.

Because of the risk of spreading diseases from nurseries to production fields, disease control is imperative to keep planting stock healthy. Disease control may also be needed to control diseases that are favored by the growing conditions in tunnels. The first step should be to use cultural methods and sanitation to reduce disease pressure. For example:

• Avoid overwatering and adjust watering schedules and misting regimes to allow the plants to dry out.
• Open vents to reduce relative humidity.
• Reduce plant density.
• Sanitize pots and benches to reduce carry-over of pathogen inoculum.
• Sterilize/pasteurize growth media.
• Have a plant-free period to break pathogen life cycles.

At times, fungicides will be needed, either on a preventive basis or for corrective action. It is important to obtain a correct disease diagnosis before applying fungicides and to verify that the fungicide in question is effective against the disease you are trying to control. As in field situations, it is important to alternate chemistries to reduce the risk of fungicide resistance buildup. This is especially important when numerous sprays are applied per year because of the extended growing season. Biological control agents may be good options as rotation partners and tend to work better under covered conditions than in the field. It is important to note that the Environmental Protection Agency has changed its approach to the use of pesticides in greenhouses. The current interpretation of the rules is that the fungicide has to be labeled for the crop in question and should not be prohibited from use in the greenhouse.

As an example, the fungicide Switch can be used on blueberry plants in the greenhouse because blueberries are on the label and there are no restrictions for greenhouse use. The fungicide Pristine, however, is prohibited from greenhouse use even though it is labeled for blueberries because the label says, “Pristine is not for use in greenhouse or transplant production.” This last statement also applies to transplants grown outdoors. Some fungicides are prohibited from nursery use due to fungicide resistance concerns.
Be sure to read the fungicide label before use because the label is the law.