Managing Diseases and Insects in the Greenhouse

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

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

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

Sanitation

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

Insect Screens

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’

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)

Greenhouse Pests

Greenhouse Pests• Thrips• Whiteflies• Fungus gnats• Shore flies• Spider mites• Leaf miners• Aphids• Mealybugs

Thrips

• Feathery wings• Adults winged• Nymphs wingless• Rasping mouthparts

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

Thrips Damage

Impatiens Necrotic Spot

Begonia with tomato spotted wilt virus

Thrips Control

• Screen vents• Eliminate reservoir

plants (weeds, old plants etc.)

• Monitor with sticky cards

Whiteflies

• Produce honeydew and cause sooty mold problem

• Flying white adults can be unsightly

• Problem on Poinsettias and many other crops

• Can vector diseases

Whiteflies

• Adult is the mobile phase

• Eggs, nymphs and pupae transported on plants.

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

Whiteflies (Black = Parasitized)

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

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)

Greenhouse Diseases

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)

• 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

Powdery Mildew

• Common fungal disease on a wide range of plants

• Dusty powder on leaves• Early detection is key to

control

Powdery Mildew

Botrytis Gray MoldBotrytis cinerea

Sporulation of the fungus on old leaf tissue

Botrytis Gray MoldBotrytis cinerea

‘Ghost spots’ on fruit Fruit rot

Tomato

• 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

Questions?