Managing Diseases in the Greenhouse

Kenny SeeboldUniversity of Kentucky

Plant Pathology Department

The Greenhouse: A Plant Disease “Factory”

• Most GH vegetables are susceptible to one or more diseases

• 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 share virtually the same space – sometimes in tight quarters

• Plants may share the same water (sometimes recirculated)

• Pesticide options are limited

• Plants are handled frequently

• Two of three elements of the disease triangle are always present– Susceptible host– Favorable environment

Successful management of greenhouse diseasesbegins with prevention…

• Host• Pathogen• Environment

Sanitation• Sound sanitary practices make a difference!

• Goal is to eliminate carryover or introduction of 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 30-ft “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– If field soil is to be used, sterilize by heating to 160-170F for at least 30

min

Sanitation

• Keep greenhouse tidy during the growing season

• Wash hands regularly and avoid using tobacco products in the greenhouse

• 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 (Quattro, Greenshield)– Close greenhouse for several weeks to promote high

temperatures – this will help kill off weeds & insects

Exclusion• Minimize the movement of pathogens into the greenhouse…

• Sanitation helps address the issue of exclusion

• Don’t grow ornamentals & vegetables in the same greenhouse– If this must be done, keep separated on different benches or

parts of 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

Hot Water Treatment• Can be used with many vegetables (see ID-36)

– Not safe for legumes or cucurbits other than cucumber– Will inactivate most pathogens except those that have become associated with

the embryo

• Temperature and soaking time depends on crop– Critical factors -> poor efficacy or crop damage can occur…– Invest in a quality thermometer

• Must agitate water to maintain uniform temperature– Use weighted mesh bag to hold seed– Dip in cold water to cool quickly

• Dry completely before storage / plantingImages – S.A. Miller, Ohio State U.

Vegetable crop Water temperature (°F) Soaking time (minutes) Broccoli 122 20-25 Brussels sprout 122 25 Cabbage 122 25 Carrot 122 15-20 Cauliflower 122 20 Celery 122 25 Chinese cabbage 122 20 Collard 122 20 Cucumber 122 20 Eggplant 122 25 Garlic 120 20 Kale, Kohlrabi 122 20 Lettuce 118 30 Mint 112 10 Mustard, Cress, Radish 122 15 Onion (sets) 115 60 Pepper 125 30 Rape, Rutabaga 122 20 Shallot 115 60 Spinach 122 25 Sweet potato (roots) 115 65 Sweet potato (cuttings, sprouts) 120 10 Tomato 122 25 Turnip 122 20 Yam (tubers) 112 30 Treatment of crops other than those listed may cause serious injury to seed.

Recommended Temperatures and Soaking Times for Hot-Water Disinfection of Selected Vegetable Seeds

Chemical Soaks

• Bleach– Use 1 qt commercial-grade bleach to 3-4 qt of water– Add 1-2 drops of dish detergent to decrease surface tension– Use ~1 gallon of solution per lb. of seed -> soak time is 1 minute– Rinse thoroughly afterward; spread to dry

• Trisodium phosphate (TSP)– Concentration of solution = 10% (1 part TSP, 9 parts water)– Use ~1 gallon of solution per lb. of seed -> soak time is 15 minutes– Rinse thoroughly afterward; spread to dry

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)– In cool weather, flush humid air in the early morning & late

afternoon (will help reduce dew formation & condensation)

Manipulating the Environment• Watering & irrigation are critical

• Don’t over- or under-water– Excess moisture can promote root and foliage diseases

• If watering overhead, do so early to allow for rapid drying of foliage– Apply gently to minimize splash

• Avoid the use of “float” systems– Promotes disease– No chemicals labeled to control diseases of vegetables grown

in these types of systems

Suppressing Pathogens

• Sanitation, exclusion, & environmental control are important

• Resistant varieties:– Along with cultural controls, can significantly reduce disease in

the greenhouse– Resistance available varies by crop & pathogen

• Solarization

• Soil covered with clear plastic mulch

• Heated by sunlight

• Max 45-52ºC at 4-6 in soil depth

• Shown to suppress numerous soilborne diseases

Solarization

Limitations of Solarization

• Not practical in northerly climates (fewer “sunshine” days than in southern areas

• Unproductive during summer

• Disposal of plastic

Chemicals for Disease Control As with other practices, don’t rely solely on chemicals to manage disease!

Timely applications are critical Apply before symptoms appear OR at first signs Maintain a regular schedule Proper calibration & nozzles

Safety & protective clothing

Re-Entry Interval after spraying Follow REI guidelines on product labels Post warnings on entrance to greenhouse

Choices of active ingredient are limited…

Soil Treatments• Methyl bromide not longer available

• Basamid G (9-13 oz/100 sq. ft)– Incorporated into soil– Allow time for fumigant to dissipate (> 4 weeks is ideal)

• Oxidate / Terracide (hydrogen peroxide)– Results variable

• Microbial amendments– Gliocladium (SoilGard) ; Trichoderma (T-22); Bacillus subtilis (Subtilex);

Streptomyces (Mycostop)– Results may be variable

• Composts– Results variable for control of damping-off

Greenhouse Fungicides & Bactericides for Vegetables

• Pesticides that do not specifically prohibit greenhouse use can be used if labeled for the crop in question.

• Chlorothalonil: Exotherm Termil– No longer cleared for use in the greenhouse on vegetables

• Copper compounds*: bactericide / broad-spectrum fungicide (protectant)– Kocide, Champ, Cuprofix, Nu-Cop– Numerous crops labeled (not lettuce)

• EBDC’s (Mancozeb / maneb): broad-spectrum fungicide (protectant)– Dithane DF / Maneb– Numerous crops labeled

• Sulfur*: powdery mildew / miticide (protectant)– Dusting sulfur (various brands)– Wettable sulfur (various brands)– Numerous crops labeled (not lettuce)

Greenhouse Fungicides & Bactericides for Vegetables

• Botran 75W (Control of Botrytis)– Cucumbers, lettuce, tomatoes

• Decree: (Control of Botrytis)– Fruiting vegetables (tomatoes, peppers), cucurbits, leafy

greens

• Previcur Flex (Pythium, Phytophthora damping-off)– Tomatoes, peppers, cucurbits, lettuce– Drench or drip-applied

• Scala: (Control of Alternaria & Botrytis)– Tomatoes

• Terramaster EC: (Control of Pythium)– Tomatoes

Greenhouse Fungicides & Bactericides for Vegetables

• PCNB: (damping-off, root/stem rots caused by Rhizoctonia)– Applied to media or soil– Turfcide 10G / Terraclor 75W– For cabbage, cauliflower, broccoli, Brussels sprouts, pepper,

and tomato plants grown in containers

Greenhouse Fungicides & Bactericides for Vegetables

• Bacillus subtilis: broad-spectrum fungicide– Serenade Max– Approved for most greenhouse vegetables

• Peroxides (hydrogen): broad-spectrum fungicide bactericide– Oxidate– Approved for most greenhouse vegetables– Low residual activity (frequent applications needed)

• Potassium bicarbonate: powdery mildew– Kaligreen– EcoMate Armicarb “O”– Milstop– Cleared for use on most greenhouse vegetables– Low residual activity

Fungicides for Greenhouse Use‘Soft’ Chemistries / Organic-approved

Diseases Caused by Fungi

Photomicrograph: NCSU Plant Pathology

Damping-offPythium spp., Phytophthora spp., Rhizoctonia solani

Tomato

Pepper

• 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-offDevelopment and Spread

Powdery MildewOidium spp.

Tomato

Cucurbit

Botrytis Gray MoldBotrytis cinereaStem canker

Leaf blight

‘Ghost spots’ on fruit

All images:Plant Management Network

Fruit rot

Tomato

Botrytis Gray MoldBotrytis cinerea

Lettuce

Early BlightAlternaria solani

(Gray) Leaf MoldFulva fulva

Tomato

Timber RotSclerotinia sclerotiorum

Stem rot + sclerotia (black seed-like structures)

• 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 (see labels for timing)

Foliar BlightsManagement

Diseases Caused by Bacteria

Bacterial SpotXanthomonas campestris pv. vesicatoria

Tomato Pepper

Bacterial CankerClavibacter michigensis subsp. michiganensis

Foliar symptoms on greenhouse-grown transplants

Bacterial Canker

Wilting

Pith necrosis

Stem canker

Pith necrosis

Bacterial Spot & CankerDevelopment and Spread

• Plants susceptible at all stages

• Seed- and transplant-borne

• Overwinters on plant debris, stakes, etc.

• Easily spread by mechanical contact & water splash

• Can be carried to the field – serious outbreaks can result

Management of BacterialSpot & Canker

• Prevention is the primary tool to manage bacterial diseases

– Use pathogen-free seeds and transplants• Avoid saving seed from previous crops• Heirloom growers: hot-water or bleach soaks (ID-36)• Resistant varieties: pepper only

– Sanitation & prevention of spread• Remove and destroy diseased plants• Keep heirlooms in an “isolated” spot• Clean benches, tools, stakes, etc.• Don’t handle plants if foliage is wet

– If watering overhead, do so early; don’t over-water

– Apply streptomycin on a 5-7 day interval

– Fixed copper + mancozeb/maneb (1.5-3 lb/A): 5– 7 day intervals

Diseases Caused by Viruses

Tobacco MosaicTobacco mosaic virus (TMV)

- TMV is seedborne & survives well on surfaces & plant debris- Easily transmitted by mechanical means

Typical mosaic symptoms on foliage Fruit symptoms

TSWV & INSV

Thrips - vector

Tomato spotted wilt virus & Impatiens necrotic spot virus (TSWV) vectored by thripsand can occur together in the greenhouse

INS

TSW

Tomato Yellow Leaf CurlTomato Yellow Leaf Curl Virus (TSWV)

Uncommon in KY, but has occurred in 2005 and 2008

Vectored by whiteflies

• There are no chemical options

• Tomato seed can be treated with trisodium phosphate to kill TMV

• Cultural:– Use resistant varieties if available (TMV)– Don’t plant ornamentals and vegetables in close proximity– Avoid handling tobacco products; wash hands frequently

• Clear weeds around fields and greenhouses

• Control insect vectors in the greenhouse

Viral DiseasesManagement

Greenhouse Resources• County Extension Offices

– ANR and Hort. Agents– Short Courses, demonstrations, Master Gardener programs– Publications– Video Tapes & DVDs– Soil Tests– Plant Disease Diagnostic Labs (Lex. & Princeton)

• Disease, Insect, Weed ID and control

• Web Resources– KY Pest News

• http://www.uky.edu/Ag/kpn/kpnhome.htm– ID-36 (2006-2007 Veg. Production Guide for Commercial Growers)

• http://www.ca.uky.edu/agc/pubs/id/id36/id36.htm– Plant Pathology Dept.

• http://www.ca.uky.edu/agcollege/plantpathology/index.html– Hort. Dept.

• http://www.ukyu.edu/Ag/Horticulture

Questions?