Diseases don't discriminate

Columns - Plant Health

The recent “buy local” message may leave consumers wondering about the quality of plants marketed on the shelves of national home improvement and chain stores.

April 23, 2010

Mary Hausbeck The recent “buy local” message may leave consumers wondering about the quality of plants marketed on the shelves of national home improvement and chain stores. My view from the plant pathology side of things is that diseases are a great equalizer among greenhouse growers.
Plant pathogens are equal opportunity pests. Powdery mildew, Thielaviopsis black root rot, Botrytis, Pythium and other diseases may be found as often in a small greenhouse attached to neighborhood florist shop as in a large greenhouse operation that supplies local or regional big box stores.

Greenhouse size doesn’t matter
Pathogens that reduce plant quality and cause death such as Pythium, Thielaviopsis, Phytophthora and Rhizoctonia can be residents in any size greenhouse. They are just waiting for the right combination of susceptible crop and favorable environment. They are uniquely adapted to survive and thrive in soils and/or root systems of susceptible plants. Ground cloth, soil on greenhouse floors, debris clinging to benches and containers can all harbor pathogens and provide them a safe harbor for months or even years.
Another reason why greenhouse size doesn’t matter is that airborne pathogens such as powdery mildew, downy mildew and Botrytis can hitch rides on or in cuttings that are taken from stock plants that are infected but may not show obvious symptoms. Likewise, pre-finished plants can also host pathogens and serve as a source of bringing problems into the greenhouse thereby exposing crops to new problems.
When cuttings or prefinished plants are shipped in sealed, humid boxes, the hitch-hiking pathogens can be activated and colonize plant tissue. Yet, it can take a few to several days for a disease to become evident. By then, the pathogen may have moved beyond the point where symptoms are evident and caused new infections on nearby healthy plants.

Disease management tips
  1. Whether you grow and sell plants in the same greenhouse or ship them to local or long distance retail outlets, here are some disease management tips to consider:Pathogens that survive well in a greenhouse require the most effective sanitation strategies. A greenhouse can never be too clean. If the head-house area is messy, diseased plants cannot be far away.
  2. Botrytis is ubiquitous, meaning it is a pathogen that may be part of the growing environment and can be everywhere. Some plants seem to be more prone to Botrytis more than others. Keeping these Botrytis-prone plants in the driest, best ventilated greenhouse can help to reduce this disease and reduce the need for frequent fungicide sprays. As plants grow and the foliage becomes denser, air flow is restricted favoring Botrytis infection.
  3. Vegetable seedlings growing alongside ornamentals can be at risk if the ornamentals got their start as cuttings from stock plants. In particular, impatiens necrotic spot virus (INSV) is common on some types of vegetatively propagated plants such as New Guinea impatiens. Many vegetables are susceptible to INSV and the closely related tomato spotted wilt virus. Thrips are able to vector the virus from infected New Guinea impatiens (or other infected crops) to vegetable seedlings. If vegetable seedlings become infected with INSV, they do not develop into full size plants. They become stunted with foliage that develops ring spots and/or bronzing. If fruit forms on these infected plants, it is often deformed and may develop mosaic symptoms.
  4. The longer that plants are grown in a greenhouse, the more likely they will acquire pathogens and serve as a reservoir of the pathogens. Unsold leftover plants or “pet” plants can be forgotten and not receive the same pesticide sprays or other treatments that may be needed to prevent these plants from becoming reservoirs of pathogens and insects. These plants can cause problems for future crops.
  5. If containers and trays are recycled, adhere to strict sanitation guidelines including:
    • Power wash all containers and trays that are to be sanitized to remove any visible debris.
    • Mix fresh disinfestant solutions just prior to treating used contain-ers.
    • Separate all containers so that the disinfestant can reach all surfaces and do not stack them.
    • Use a disinfestant soak of 20 minutes or more rather than a dip.
    • Refresh the disinfestant solution every four hours.
    • Store all treated containers inside an enclosed structure away from the growing environment and outside soil and dust.
  6. Cuttings offer a convenient way for pathogens to be introduced to any size greenhouse. Purchase the best quality cuttings available. Poor quality cuttings develop into inferior plants and may introduce disease organisms into the greenhouse that can cause problems for other plants.
  7. When new plants, plugs and cuttings are received, keep them separated from the other plants in the greenhouse for at least seven days and monitor them closely for any evidence of diseases or insects. This should especially be done with plants that can carry thrips, fungus gnats and shore flies that can vector disease pathogens. Maintaining a separate quarantine area for incoming plant material can be a challenge for smaller greenhouse operations where space is at a premium. Using plastic barriers on the bench as a physical means of separating new plant material from the resident greenhouse plants is a simple way to maintain a quarantine area. Sticky traps placed among new plant material can help determine if insects are present.
  8. Know your plant suppliers and their pest management programs. If a particular plant is prone to a specific pathogen the supplier should be willing to share with you its cultural program so that you can be assured the plants you receive are clean and healthy. Work only with reputable suppliers that are able to discuss openly with you any potential problems and concerns that may come up regarding their plants.
  9. Diagnose a plant’s problem quickly by using a diagnostic clinic at a nearby university. The nominal fee the clinic may charge for a diagnosis is money well spent to determine what management tactics are needed to protect your crop and prevent problems in the future. Let the clinic know that you are submitting a sample and send it in early in the week so that it arrives in good shape.

Mary Hausbeck is professor at Michigan State University, Department of Plant Pathology, www.plantpathology.msu.edu.