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How to Manage Pests

UC Pest Management Guidelines


Field-grown bedding plants in full bloom.

Floriculture and Ornamental Nurseries

Management of Soilborne Pathogens

(Reviewed 3/09, updated 3/09)

In this Guideline:


Soil is a reservoir for many plant pathogens and plants are under constant attack by these soilborne organisms. If conditions become favorable for infection, plants will develop disease. Population levels of soilborne pathogens, including bacteria, fungi, nematodes, and some viruses can be reduced in the soil by appropriate treatments.

Learn which pathogens attack the crop to be grown. Examine the crop regularly, at least weekly, for symptoms of disease or signs of pathogens. To monitor for root diseases, select a few plants from different locations and remove plants from their containers or gently scrape or wash away soil. Examine roots and crowns for browning, softness, or other early indications of disease. Also look for discolored or wilted plants and fungal growths aboveground, which may indicate more advanced stages of disease. Test kits are available for detecting Pythium, Rhizoctonia, and Phytophthora fungi infecting greenhouse and container-grown nursery plants. However, keep in mind that some test kits do not specifically test for pathogenic species; in such cases, nonpathogens could result in a false positive result. Use test kits in combination with other information to make good pest management.

Understand the conditions and practices that promote disease and regularly examine for and remedy disease-promoting conditions and practices. Poor sanitation, inadequate drainage, and improper irrigation are the primary conditions that promote diseases of roots. Remove crop residue and old or low-quality plants that will not be marketed.

Soil Solarization. In warmer climatic areas, solarization has been effective for disinfesting containerized soil or growing medium and soil in cold frames, as well as soil in open fields. For soil in containers, monitor the temperatures of the growing medium closely to ensure that it is high enough to control pests by placing a soil thermometer into the center of the mass of the soil mix. Planting media can be solarized either in bags or flats covered with transparent plastic or in layers 3- to 9-inches wide sandwiched between two sheets of plastic. A double layer of plastic can increase soil temperature by up to 50°F. In warmer areas of California, soil inside black plastic sleeves can reach 158°F (70°C) during solarization, equivalent to target temperatures for soil disinfestation by aerated steam. At this temperature, soil is effectively solarized within 30 minutes. At 140°F (60°C), soil is solarized in 1 hour.

In open fields, soil is more easily covered with a single layer of plastic. Soil temperatures will be lower except at the surface, so plastic should be left in place for 4 to 6 weeks. For maximum effectiveness and treatment predictability, solarize open fields only in warmer climatic areas, unless previous testing has given consistently desirable results. Solarization is acceptable for organic production.

Heat. Heating the soil is very effective and has the advantage over chemical treatment in that the soil can be planted immediately after cooling. Many plant pathogens are killed by short exposures to high temperatures; however, experience has shown that the soil temperatures should be maintained for approximately 30 minutes. Most plant pathogens can be killed by temperatures of 140°F (60°C) for 30 minutes; however, some viruses (as well as weed seeds) may survive this treatment. (Where weed seeds are a problem, a higher treatment temperature is required, but the tobacco mosaic virus [TMV] and some weed seeds still will not be killed by the higher temperature.)

Although pure steam at sea level is at 212°F (100°C), the temperature at which steam is used to treat soil is usually about 180°F because of air that is present in the steam or in the soil being treated. If air is mixed with steam, the temperature of the steam‑air mixture can be closely controlled, depending on the ratio of air to steam. It has been demonstrated that some diseases, such as Rhizoctonia damping‑off, are much less severe in soil that has been treated at 140°F rather than at 180°F. Experience will tell the grower at just what temperature to treat soil. As a starting point try 140°F (60°C) for 30 minutes.

If a cement mixer is used to heat a bulk quantity of soil, generally it is not necessary to introduce air into the steam because a large amount of air is present in the mixer and the temperature can be controlled by simply regulating the flow of steam. Expensive air blowers are not required for this method.

Steam heating of containers filled with soil in vaults likewise may not require the introduction of air into the steam to control the temperature. However, circulation within the vault to insure even distribution of heat is important. Circulating fans can be located within or external to the vault and the steam can be introduced into the recirculating air. Leave space between the vaults and check temperatures throughout the vault to insure that there is good circulation of steam air.

Soil Fumigants. The most useful soil fumigants are methyl bromide and chloropicrin. (The 2007 Critical Use Exemption list allows use of methyl bromide for moderate-to-severe pathogen infestation.) Although registered for use, metam-sodium (Vapam), and dazomet (Basamid) are not very effective for controlling many soilborne pathogens, including Verticillium and Fusarium oxysporum. Fumigants such as methyl bromide are a source of volatile organic compounds (VOCs) but are not reactive with other air contaminants that form ozone; methyl bromide depletes ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available.

Methyl bromide is a gas at temperatures over 40°F. It escapes rapidly from soil if not applied under a gas‑proof cover. Polyethylene sheeting is commonly used to confine methyl bromide, although the gas does slowly escape through polyethylene. Methyl bromide is probably the most versatile of the soil fumigants because of its ability to diffuse rapidly through the soil and kill many kinds of organisms, weeds, and many seeds. Soil generally can be planted in just a few days after removal of plastic covers, although there are exceptions. A few plants such as Allium spp., carnations, and snapdragons are sensitive to and may be damaged by inorganic bromide that remains in the soil following fumigation. Leaching the soil with water before planting is helpful in reducing the amount of bromide in the rooting area. Methyl bromide alone usually does not kill all of the microsclerotia of Verticillium dahliae, the fungus that is the causal agent of Verticillium wilt. When this fungus occurs it is often necessary to include chloropicrin in the fumigant.

Methyl bromide may be injected by chisels if the soil is covered immediately by plastic. It is the only soil fumigant that needs to be applied under plastic covers. This is a restricted use material and requires a permit from the county agricultural commissioner to be purchased and/or applied. With the impending loss of methyl bromide as a registered funigant, research is being conducted to find viable alternatives because of ozone depletion concerns.

Chloropicrin (trichloronitromethane) must be injected into soil. It is the best fumigant for controlling Verticillium dahliae. It generally is combined with methyl bromide in various mixtures depending upon the organisms in the soil. If used alone, a water seal may be used to confine the gas; however, the gas is very objectionable and irritating (it is commonly known as tear gas) and, if not effectively confined, it may drift to inhabited areas. This is a restricted use material and requires a permit from the county agricultural commissioner to be purchased and/or applied.

Soil Fungicides. Some fungicides work best if incorporated before planting. Others may be incorporated or applied after sowing or planting. Some soil fungicides control a narrow range of organisms while others control a wide range of organisms. Some of the narrow range chemicals are the most effective in controlling a specific organism. Combinations are used to increase the number of organisms controlled.

Fosetyl-Al (Aliette) is active against Phytophthora species and some Pythium species. It is applied as a soil drench or as a foliar spray but is more effective as a foliar spray. It is absorbed by foliage and moves into roots. It is used as a drench at 0.8 to 1.6 lb a.i./1000 sq ft using 0.5 to 1.5 pt water/sq ft. As a foliar spray it is applied at 2 to 4 lb a.i./100 gal water.

The active ingredient in SoilGard is Gliocladium virens, a soilborne fungus that under certain conditions helps provide control of Pythium and Rhizoctonia fungi.

Iprodione (Chipco 26019) is used at 0.2 lb a.i./100 gal water applied as a drench (1–2 pt/sq ft) at seeding or transplanting. It is effective against Rhizoctonia damping‑off, Sclerotinia, and gray mold. Some iprodione is absorbed by plant parts.

Mefenoxam (Subdue Maxx) is active against Pythium, Phytophthora, and downy mildews but is not effective against pathogens outside of this group of organisms. This material replaces the fungicide metalaxyl. It is applied at planting as a drench and periodically thereafter as needed. Mefenoxam is also available in a granular formulation to use before planting. It is water-soluble and readily leached from soil. It is absorbed by plant parts including roots; movement in the plant is primarily in the xylem. Use of this material over a period of time may lead to resistance.

PCNB (Terraclor), also called quintozene, is very active against diseases caused by Rhizoctonia solani and Sclerotinia spp. and is the best available material for southern wilt caused by Sclerotium rolfsii. It is insoluble in water and must be thoroughly mixed with soil to reach its desired depth of control. It works through vapor action and has good residual action. It is inactive against Pythium pathogens. It is used at 0.5 to 1 lb a.i./1000 sq ft and mixed into the top 2 inches of soil for control of Rhizoctonia damping-off. Germination of some seeds may be inhibited and small plants may be stunted by this fungicide.

Streptomyces griseoviridis (Mycostop) is a biofungicide reported to help control seed rot, root and stem rot, and wilt caused by Alternaria and Phomopsis in container-grown ornamentals. In the greenhouse it may suppress Botrytis gray molds and root rots of Pythium, Phytophthora, and Rhizoctonia.

Thiophanate-methyl (FungoFlo, Cleary's 3336, Zyban, Systec, etc.) is generally applied after sowing. It helps to control Rhizoctonia diseases, cottony rot, Thielaviopsis rots, and some Cylindrocladium diseases. It is not effective against Pythium spp., Phytophthora spp., Sclerotium rolfsii, Botrytis spp., or Fusarium spp. It is used at 0.5 lb a.i. or less/100 gal water and applied as a drench or heavy spray (1-2 pt/sq ft). Thiophanate-methyl is absorbed by plant parts exposed to the chemical. Roots may absorb the fungicide (or its breakdown product carbendazim), which moves in the xylem to transpiring leaves.

Trichoderma spp. (Garden Solutions, Root Guardian) is a biological fungicide reported to help provide control of root diseases caused by Pythium, Rhizoctonia, and Fusarium in nursery and greenhouse crops. It is formulated as a seed protectant, a soil drench, and as granules.

Triflumizole (TerraGuard) is a protectant fungicide used as a cutting soak, soil drench, foliar spray, or through chemigation for control of Cylindrocladium spp. Its use is restricted to enclosed commercial structures such as greenhouses and shade houses.

Seed Treatments. Streptomyces griseoviridis (Mycostop) is used as a seed treatment for damping-off and early root rots for ornamentals planted in fields or greenhouse. Captan and thiram are also seed treatments; they are generally applied at rates of 1 to 4 oz/100 lb seed, but they offer only a small degree of protection.

Materials used for bulb or corm dips include thiabendazole (Mertect), which controls Fusarium basal rot and Penicillium blue mold.

Treatment of Containers and Equipment. Debris, soil, and plant material cling to containers and equipment; thoroughly wash equipment to remove all soil or planting mix particles. Heat treatment is effective in killing the plant pathogens that adhere to containers or that are in the debris. Where steam is not available, hot water or solarization can be very effective. Most plastic can be treated with hot water at temperatures that cause minimal softening. The minimum water temperature should be 140°F (60°C) whenever possible. Treatment time can be as short as 1 minute. Longer treatment times are more reliable and the container or equipment must reach at least 140°F (60°C). For solarization, containers should be moistened, stacked, and placed beneath a double-layer tent. Incubate for 30 minutes at or above 158°F (70°C) or 1 hour at or above 140°F (60°C).

Sodium hypochlorite (the active ingredient in bleach) is effective in killing some types of fungal spores and bacteria. It penetrates clinging soil and plant material very poorly. It is effective only as a surface disinfectant, so containers, tools, etc. must be free of soil and plant material and clean before treatment. Sodium hypochlorite is generally used as a surface disinfectant at 0.5%. To achieve this concentration of sodium hypochlorite, household bleach can be diluted 1 part bleach to 10 parts water. For known contaminated materials, a stronger solution diluted 1 part bleach to 5 parts water (1% sodium hypochlorite), may be more effective in killing pathogens. Allow the solution to be in contact with nonporous materials for a minimum of 5 to 10 minutes, then rinse well with clean water to remove bleach and avoid phytotoxicity. Bleach dilutions must be made fresh each day because once diluted, the effectiveness of the solution diminishes over time. Debris, potting mix, and other residues left over in bleach washes will also reduce bleach concentration.

Quaternary ammonia compounds are excellent bactericides and viricides, and are effective in killing some kinds of fungal spores. They penetrate plant material and soil poorly so containers, tools, etc. must be clean before treatment.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
S. T. Koike, UC Cooperative Extension Monterey County
C. A. Wilen, UC IPM Program, UC Cooperative Extension San Diego County
Acknowledgment for contributions to Diseases:
R. D. Raabe, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
A. H. McCain, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside

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