Best Management Practices to Prevent or Suppress Pythium -Caused Losses in Greenhouses

Create a clean environment for your greenhousePythium can remain in a dormant state in dry soil for years and then become active when soil moisture and temperature levels are adequate for its activity (7).

Equipment used inside the greenhouse should not be used outdoors because it may pick up soil contaminated with Pythium. If outdoor use is required, use a high pressure washer to remove deposits of soil and plant debris. Only after that material is removed, thoroughly wet the formerly soiled parts of the equipment with a disinfectant and keep it wet with that product for the recommended length of time (usually 10 minutes, but check the label) before allowing it back onto greenhouse production floors. Workers who come and go from the greenhouse should clean the soles of their shoes by thoroughly brushing off soil and debris. It may be helpful to install floor mats (Figure 1) that can be soaked with disinfectant where workers must walk before entering production areas. The floor mat should be rinsed and recharged with freshly prepared disinfectant twice daily; more often if there is heavy traffic or the mat accumulates a lot of soil and debris.

Figure 1. Note the placement of the crate and brightly colored bucket such that people are directed to the mat rather than being able to readily go around them.

Grow plants on surfaces that are easy to clean (14,15,3)Grow crops at least several inches off the ground to reduce the opportunity of contamination by soil from the greenhouse floor. Benches should be high enough off the floor to make it inconvenient for anyone to rest a foot on while talking — the bottom of that shoe is dirty! Wooden benching can harbor Pythium and is difficult to thoroughly disinfect. Cement or plastic benching is more easily cleaned and disinfected. Metal benches are the easiest to clean and disinfect. Disinfectants are effective on smooth, impermeable, soil-free and debris-free surfaces.

Ponding of water on benches must be avoided, especially on propagation benches. Ebb and flow benches must allow water to drain away completely and should be constructed of a material that moisture will evaporate from promptly.

Floors should be of a material and design that does not retain puddles of water where algae can grow and shorefly or fungus gnat population development is encouraged. Flood floors should be graded to drain very quickly without ponding. There should be as few obstructions as possible within a flood floor, so that the organic debris can be thoroughly vacuumed between crops. During crop production, traffic on flood floors should be kept to a minimum

and only accessed after tires, workers’ shoes, etc. have first been cleaned and passed across mats soaked in disinfestant. One alternative to shoe dips is the use of disposable shoe covers. Workers should be discouraged from walking across ebb and flood production floors (those seen in figure 1, bounded by the rubber bumper) unless it is unavoidable.

Where possible, steam clean surfaces between crop cycles (Figure 2).

Figure 2. Steaming cement ebb and flood benches.

Unidirectional flow policyAlways schedule work to begin in the cleanest area of the greenhouse facility and proceed toward the dirtiest area. People who visit many greenhouse facilities or sections during the day, particularly IPM scouts, Extension workers, and sales people MUST follow this directive (Figure 3). Your plants are at risk to the introduction of or movement of plant pathogens. Insist on compliance with your policies. Plan their visit such that, once an area where a disease outbreak has occurred is visited, that person does not enter a clean area until after first washing their hands and disinfesting their shoes. If that person is leaving your facility, specifically offer them the opportunity to clean up. If such visits are frequent or routine, consider purchasing long white lab coats in a variety of sizes that can be borrowed by the visitor. Also consider having disposable gloves available for visitors who will be handling plants.

Figure 3. Scouts, crop consultants, extension educators and others who visit a greenhouse must be aware that they may move plant pathogens, mites, and insects from place to place on their clothing, hands, and any tools used for

examining plants. Yellow and light blue colored clothing in particular is like a magnets for insects and should not be worn by anyone entering a greenhouse.

Avoid high fertilizer concentrationsOver-fertilization can predispose most plants to Pythium (10).

Avoid using excessive amounts of fertilizer. Use the minimum nitrogen rate that will result in high quality plants. Be especially careful with slow-release fertilizers: if soluble salts build up while using these materials, it is difficult to make adjustments by leaching. Programs using solely soluble fertilizers are more easily adjusted. Regularly monitor the soluble salts level of the irrigation water, and also test the fertilizer solution to detect malfunctioning proportioners, fertilizer injectors, fertilizer miscalculations and incorrect measuring of fertilizer. To do this, purchase and use a solubridge or conductivity meter designed for greenhouse use. Periodically examine roots of the growing crops to ensure that they are growing. Send samples of the growing medium to a laboratory for nutrient analysis periodically. Some labs have a separate test for soilless media as opposed to soil-containing media.

Avoid water stressGrowers have observed that plants allowed to dry to the point of wilting and then watered seem to be very susceptible to Pythium.

Do not let the root systems of your plants to come under stress from lack of moisture: irrigation schedules should be based on plant needs and will vary with weather, crop, season, plant size, pot size, and growing medium. Root damage is likely when the growing medium for plants on high fertility regimes is allowed to dry out excessively. Monitor the temperature of well water in winter: cold water irrigation can stress plants, slow their growth and, particularly in the case of seedlings extend the time that they are susceptible to damping-off. Excessive soil moisture greatly favors Pythium diseases while careful management of soil moisture can suppress Pythium (4).

Apply fungicides only when necessary Certain fungicides, insecticides, and plant growth regulators actually place a stress on plants, particularly young plants, and may make plants more susceptible to Pythium.

Blanket applications of chemical fungicides to protect all crops against Pythium should not be necessary unless Pythium has been detected in a crop or has been a chronic problem in the facility. Biocontrols, if they are to be used, should be used as a blanket application to protect crops for which they are registered and should be used as early in the cropping cycle as possible. Note that biocontrols may need to be applied repeatedly. Read the product label carefully. Biocontrols can be used in conjunction with chemical fungicides. The rule of thumb is to wait 10 days after applying a biocontrol agent before you apply a chemical. Or, wait 10 days after applying a chemical before applying a biocontrol agent. Read the label of the products to fully assess the appropriateness of combining their use.

Manage fungicide resistanceFungicides with different modes of action should be used in alternation or as a mixture to slow the selection of fungicide resistance in a plant pathogen population. Using different materials alternately or as a mixture is a good strategy because the pathogen is not likely to be resistant to both modes of action. Currently, it is known that some isolates of Pythium in greenhouse production have resistance to one or more fungicides (13). Chemicals with different FRAC (Fungicide Resistance Action Committee) numbers have different modes of action. If several products have the same FRAC number, they may have different active ingredients but they all have the same mode of action and Pythium 'sees' them as the same and not different from one another.

Growing mediaGrowing media should have good water retention and a high amount of air space (at least 25% pore space). Compaction from having very fine particles, over-mixing, manual compression of media when planting, or repeated high-impact over head irrigation is to be avoided. Poor aeration favors Pythium (9).

Water sourceClean water sources are needed to avoid problems with Pythium. Clean sources include rainwater, municipal water, and well water (provided that the well head is protected from infiltration by surface water runoff). Pond, stream or river water present a risk of harboring Pythium (6,16,8,11,12). If water is found to contain plant pathogenic species of Pythium, it should be filtered to remove particulates and treated to eliminate Pythium prior to use on a greenhouse

crop. The most appropriate or best treatment to select varies with the quality of the water supply, the amount of water used during each irrigation cycle and the amount of time, resources, and training the grower is willing to devote to a treatment system. Seek guidance from Cooperative Extension in determining which option is the best for your operation.

NOTE that there may be many species of Pythium in a water supply but that most of the species are not important plant pathogens. Test kits currently available to detect the presence of Pythium do not identify exactly what species is present. Such a test by itself is valuable if it consistently does NOT detect Pythium. If Pythium is detected, further work must be done to determine exactly which species is present. Only spend the money required to eliminate Pythium if you know that the species present is definitely a plant pathogen. Plant pathogenic species of Pythium include: P. aphanidermatum, P. irregulare, P. cryptoirregulare, P. dissotocum, P. helicoides, P. myriotylum, and P. ultimum. Pythium species including P. diclinum, p. lutarium, P. aquatile, P. pachycaule, and P. coloratum are often not plant pathogenic or are very weak pathogens but are frequently isolated from water. These species appear to be residents of water and truly aquatic in nature.

Figure 4. Coarse particulate removal systems.

Recirculating systemsPlant pathogen management is more challenging in a recirculating system than a system in which excess water is run to waste (6,16). Return water should pass through a coarse filter to remove particulates (Figure 2). The tanks and plumbing should be designed with the possibility in mind that a serious Pythium problem will someday develop that will require a thorough cleaning. For this reason, you should be able to completely drain holding tanks and be able to scrub them thoroughly, and wash them down with a disinfestant or steam them.

Hose ends and other toolsHose ends and other tools that fall on the floor, especially dirt floors, are likely to pick up soil. If that soil harbors Pythium, Pythium will be sprayed over the plants at the next watering. Hang up hose ends to keep them off the floor. If they fall on the floor, rinse and disinfest the hose end.

Figure 5. Hang up hose ends. Disinfest hose ends that fall on the floor.

Remove organic debrisPythium can survive within organic debris. Following every crop, collect and remove all plant debris and stray potting mix. Clear away any unsold plants and sweep, rake or vacuum up remaining organic matter. Avoid redistributing small particles of debris by adding moisture before sweeping. Put lids on containers or seal bags used to collect the debris: do not carry loose debris through the greenhouse or leave open piles of debris for later disposal.

After the coarse debris has been removed, follow with a high-pressure water cleaning of bench surfaces to remove smaller particles and then disinfest surfaces that will come into contact with the plants, pots, or potting media.

Greenhouse DisinfectantsAfter surfaces are free of soil and plant debris, utilize a greenhouse disinfectant according to manufacturer’s specifications. Disinfectant treatments will give additional cleaning benefit but will not take the place of physical removal of debris.

MATERIAL (*Trade Name) APPLICATION

STEAM or DRY HEAT Heat materials to 180-200 °F for 30 min. under a cover to contain the heat

70% ALCOHOL (grain, rubbing, or wood alcohol)

Dip or swab the object and let dry. Do not rinse. Note that great caution is needed when using alcohol. It should be used in small volumes and NOT used where a spark or flame might ignited it. Alcohol burns vigorously WITHOUT an obvious, visible flame. Have something readily available to throw over burning alcohol to deny it oxygen.

SODIUM HYPOCHLORITE (Clorox* EPA reg. no. 5813-50)

Mix 1 gal. in 9 gal. of water. Dip, spray, or brush on and let drain. Rinse metal objects to prevent corrosion. Wash floors, benches, cutting knives and other impervious surfaces.

HYDRODEN DIOXIDE(ZeroTol* EPA reg. no. 70299-1, TerraClean* EPA reg. no. 70299-5; GreenClean PRO* EPA reg. no. 70299-15)

Mix as directed by the label. Dip, spray, or brush on or soak tool. Rinsing is not required to prevent corrosion of metal. Wash floors, benches, cutting knives and other impervious surfaces.

HYDROGEN PEROXIDEPEROXYACETIC ACIDSaniDate 12.0* EPA reg. no. 70299-8)

Mix as directed by the label. Dip, spray, or brush on or soak tool. Rinsing is not required to prevent corrosion of metal. Wash floors, benches, cutting knives and other impervious surfaces.

QUATERNARY AMMONIUM(Green-Shield* EPA reg. no. 499-368; Physan*)

Wash floors, benches, cutting knives and other impervious surfaces.

The effectiveness of a disinfestant depends, in part, on the composition of the item being treated and the target pathogen (14,15,3).

Vector ManagementIf there are areas of the greenhouse floor that tend to remain wet for long periods of time, this will increase the likelihood that insect vectors of Pythium including fungus gnat larvae and shorefly adults (5,1). Use yellow sticky cards to monitor for adult populations of fungus gnats and shoreflies in the greenhouse. This is particularly important several weeks prior to introducing new cuttings that are very vulnerable to damage by fungus gnat larvae, either directly or as vectors of Pythium. If adults are present, use insect growth regulators or other labeled materials

to control the larval stage of fungus gnats on the other crops in a greenhouse several weeks before introducing the cuttings of crops prone to injury from the combination of fungus gnats and Pythium.

Figure 6. Shorefly,whitefly, and thrips (left to right).

Figure 7. Fungus gnat larval feeding damage. A fungus gnat larva has a black head capsule.

Make it easy for workers to wash hands and tools used on plantsHand-wash stations with disposable toweling should be readily available for the frequent use of workers who handle crop material. Disposable gloves may be used instead. Note that some people have or develop sensitivity to latex gloves and even nitrile gloves. Symptoms usually include skin reddening and itchiness. Fabric glove liners can be purchased and worn under disposable gloves to prevent direct skin contact with the gloves. An advantage of using a glove liner is that it will absorb perspiration and make wearing gloves for long periods of time less uncomfortable.

Figure 8. Hand and small tool washing station.

Receiving plant materialAlways keep track of the origins of all plant material brought into your greenhouse so that suppliers can be notified of any Pythium problems that develop later. Newly-introduced plants should be held in an isolated area until they have been inspected carefully for symptoms and signs of Pythium.

Early detection of Pythium while plants are in a temporary quarantine area will allow a number of timely management actions, including the following:

diseased plants can be rejected (Generally, you can't cure a plant once it is infected.) visibly diseased portions of the shipment may be discarded before proceeding to grow the remainder samples can be sent to a lab for confirmation of an on-site diagnosis the source greenhouses can be consulted for the treatment history of the plants so that plants can be treated

with an appropriate material before transplanting future cultural, biological, and chemical management of Pythium can be planned in advance, rather than

having to be developed in a hurry for a late-detected outbreak

Request information from the supplier about the treatment history: plants treated recently at the source greenhouse with a chemical known to be at risk to fungicide resistance selection, for example, should not be immediately retreated with the same material.

Remember that it is possible for insects, mites, and pathogens to move into a new shipment of plants fairly quickly from other crops or weeds already in a greenhouse. Be cautious when assigning blame to a supplier. If symptoms or signs of problems are seen on plants as they arrive or within 5-7 days of arrival, it is highly likely that the problem arrived with the new plants, and the source should be notified and consulted before planning a pest management scheme.

Do not recycle any water used to irrigate newly arrived plants. Pythium may enter irrigation water from infected plants and be dispersed to other plants exposed to that water. This is especially true in hydroponic culture and in systems that recycle water routinely (6,16).

Planting practicesIt is best to not reuse pots, flats, or pot labels. If these are reused, they must first be cleaned of debris and disinfested (14,15,3). Hands, tools and machinery should be periodically washed during planting operations, especially between varieties of major crops or species of minor crops.

Figure 9. Disinfesting relatively clean flats.

Your employees are key to your successAn effort should be made to foster, among all employees, a sense of pride in the production of healthy, high quality plants. Employees should be educated to understand why certain practices are important, such as avoiding the spread of plant pathogens by:

• washing hands between different groups of plants• working from the cleanest areas to the dirtiest each day• using shoe dip stations when entering the production area• routinely picking up plant and potting mix debris and properly disposing of it • cleaning hands after handling diseased plants or floor debris• calling the attention of their supervisors to plants with symptoms or signs of diseases, insects, and mites

Be familiar with symptoms of Pythium diseasesSome species of Pythium result in stunting and plants will not develop more severe symptoms. Some will cause yellowing of the lower leaves, indicating nutrient deficiency due to poor root health. Some plants will wilt without yellowing. Some species of Pythium cause decay in the roots that may extend up into the stem, causing it to turn dark brown or black for an inch or more. If you see any of these symptoms, obtain a diagnosis and management recommendation from your Cooperative Extension educator, state plant inspector, or plant disease/insect clinic.

Figure 10. Damping-off of seedlings

Figure 11. These plants are the same age. The oneon the left is infected with Pythium while the one

on the right is healthy.

Figure 12. Wilting during bright, sunny daysfollowed by recovery during the nightindicates Pythium root rot is occurring.

Figure 13. Cutting rot.

Figure 14. Root rot.

Figure 15. Stem rot.

Scouting practices (2)One or two people should have the specific task of scouting for Pythium and other diseases, insects, mites, and weeds. Problems related to plant nutrition should also be checked. They should be given specific, regularly scheduled time to do this each week during the growing season. A floor plan of the greenhouse along with a scouting report form designed specifically or your operation should be kept on a clipboard. The scout should be equipped with a good magnifying glass, large and small plastic bags for collecting samples to be further examined onsite or sent to a plant disease/insect clinic or soil test lab, a marker to label the bags, flagging to mark problem areas to be treated or revisited a the next scouting, and a brightly colored bucket, tackle box, or shoulder bag to carry these items while scouting. It would also be very helpful to have a small digital camera or smart phone to record any symptoms or signs observed as well as record the growth stage and general health of the crop at the time of scouting. It is extremely helpful to have greenhouse bays, sections, floor areas, and benches numbered or labeled in a manner that can be shown on the overall floor plan and used by the scout to specifically record where problems are occurring and where treatments should be made.

The scout should keep records of when scouting was done, what was found, what was done with any samples, and any clinic or soil test lab reports received that are related to the scouting report.

As the scout gains experience and confidence, the experienced scout should begin to train all others who work with the plants to watch for Pythium symptoms as well as other diseases, insects, mites, weeds, and nutrient problems. This will foster a sense of shared responsibility for crop health among the workers and increase the number of eyes that are critically examining the crop each day.

Practices during non-cropping periodsLiving things in a dormant state are very resistant to heat, cold, and dryness. They are sensitive to these conditions if they are actively growing, Therefore if there is a period of time when there are no plants in a greenhouse and you wish to ‘bake’ out weeds, insects, mites, or plant pathogens, it is best to cycle

moderate temperature/moist conditions with hot/dry conditions. The moderate temperature/moist period will stimulate Pythium, weed seeds, insect, and mite activity, bringing them out of dormancy. The hot/dry conditions will kill the active organisms. Those that are not killed will resume dormancy in order to survive. By cycling favorable and unfavorable conditions multiple times, pest populations will be reduced. Most of the insects and mites that damage plants or move plant pathogens from plant to plant REQUIRE a live plant on which to feed. If you deny them a foot source, they will die. For that reason, it is important to eliminate ALL green plants, including 'pet plants', from the greenhouse in addition to eliminating weeds.

References

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