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Managing Diseases and Insects in the Greenhouse
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The Greenhouse: A Plant Pest “Factory”
• Most GH plants are susceptible to one or more diseases and pests
• Humidity tends to be high in the greenhouse
• Greenhouse temperatures tend to favor one or more pathogens
• Contaminated material can be brought in from outside sources
• Plants may share the same water (sometimes recirculated)
• Pesticide options are limited
• Plants are handled frequently
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Sanitation• Sound sanitary practices make a difference!
• Goal is to eliminate carryover or introduction of pests and disease-causing organisms
• Before planting:– Remove all plant debris & weeds inside structure– Discard trash (inside AND outside)– Sanitize benches, work surfaces, and tools with 10% bleach or
quaternary ammonia (Quattro, Greenshield)– If possible, maintain a “weed-free” zone around greenhouse to
eliminate refuges for pathogens and insects
• Don’t allow field soil into the greenhouse– Don’t store trays & pots on bare soil– Grow on raised benches if possible– Clean shoes before entering structure– Install concrete or gravel walkways; cover soil under benches with
gravel or landscape cloth
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Sanitation
• Keep greenhouse tidy during the growing season
• Wash hands regularly and avoid using all tobacco products in the greenhouse (spread of tobacco mosaic virus)
• Avoid handling plants if foliage is wet
• End of season:– Remove all plant debris & weeds inside structure– Discard trash (inside AND outside)– Sanitize benches, work surfaces, and tools with 10% bleach or
quaternary ammonia products (Quattro, Greenshield)– Close greenhouse for several weeks to promote high
temperatures – this will help kill off weeds & insects
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Sanitation
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Exclusion• Minimize the movement of pathogens into the greenhouse…
• Control insects and weeds in and around greenhouse– Potential sources of plant pathogens
• Don’t use water from ponds, creeks, and rivers– Source of Pythium and other pathogens
• Pathogen-free planting material– Grow transplants from seed if possible– Buy seeds from reputable source– Don’t save seed OR treat to minimize threat from fungal,
bacterial, and viral pathogens
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Insect Screens
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Weeds• Get rid of them, both Inside AND Outside
the greenhouse– Sources of viruses– Reservoir of bugs and diseases– Usually go unnoticed because people ‘get
used to them’
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Manipulating the Environment• Goal is to make the greenhouse environment less favorable to
pathogens
• Temperature:– Avoid temperature extremes– Minimizes stress & makes plants less susceptible to disease
• Humidity:– Goal is to keep relative humidity below 90%– Minimize long periods of leaf wetness– Avoid tight plant spacing– Provide good ventilation & airflow (vents, side-curtains, fans)
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Greenhouse Pests
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Greenhouse Pests• Thrips• Whiteflies• Fungus gnats• Shore flies• Spider mites• Leaf miners• Aphids• Mealybugs
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Thrips
• Feathery wings• Adults winged• Nymphs wingless• Rasping mouthparts
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Thrips
• Direct damage to flowers when adults and immatures feed
• Vectors impatiens necrotic spot and other viruses
• Many host plants• Things to look for,
feeding injury ‘scrapes’, larvae in flower buds
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Thrips Damage
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Impatiens Necrotic Spot
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Thrips Control
• Screen vents• Eliminate reservoir
plants (weeds, old plants etc.)
• Monitor with sticky cards
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Whiteflies
• Produce honeydew and cause sooty mold problem
• Flying white adults can be unsightly
• Problem on Poinsettias and many other crops
• Can vector diseases
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Whiteflies
• Adult is the mobile phase
• Eggs, nymphs and pupae transported on plants.
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Whitefly Control
• Inspection of new plant material
• Sanitation• Monitor plants and yellow
sticky cards• Biological Controls:
– Encarsia formosa a parasitoid wasp
• Imidacloprid gives long lasting control but must use other materials to prevent resistance
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Whiteflies (Black = Parasitized)
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Fungus Gnats• Larvae (maggots) feed on
fungus in soil and nip at roots, and tunnel in stem.
• High populations of maggots can injure plants and spread soil borne pathogens like Pythium
• Thrive in moist soil• Monitor larvae by placing
a potato slice on the soil and changing weekly
• Monitor adults with sticky cards
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Fungus Gnat Control• Keep soil covered in potting area. Adults lay
eggs in moist media.• Keep greenhouse weed-free to prevent moist
conditions suitable to fungal growth.• Moderate watering to keep plants dry• Biological control: Bacillus thuringiensis
israeliensis(Gnatrol)
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Greenhouse Diseases
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Damping-off/Root Rot Pythium spp., Phytophthora spp., Rhizoctonia solani
• Symptoms– Stem rots near the soil
surface; seedling falls over
– Seed decays in the soil before or after germination
– Roots rot after plant is germinated and growing (stunting first, then plant dies)
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• Pathogens are soil- or water-borne– Spread in irrigation systems, splash, contaminated soil & media,
soil on old trays & containers
• Will affect most greenhouse crops
• Favored by high soil moisture (saturation)
• Sanitation is the key to control
• Fungicides are available for certain crops
Damping-off/Root RotDevelopment and Spread
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Powdery Mildew
• Common fungal disease on a wide range of plants
• Dusty powder on leaves• Early detection is key to
control
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Powdery Mildew
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Botrytis Gray MoldBotrytis cinerea
Sporulation of the fungus on old leaf tissue
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Botrytis Gray MoldBotrytis cinerea
‘Ghost spots’ on fruit Fruit rot
Tomato
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• Use quality, pathogen-free seed
• Employ good sanitation in the greenhouse
• Ensure adequate ventilation / air circulation
• Eradicate weeds and volunteers around greenhouse
• Use adequate fertilizer to encourage vigorous, but not rank, growth
• If irrigating overhead, allow for time to plants to dry
• Apply fungicides if needed
Management of Foliage Diseases
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Questions?