Frequently Asked Questions

1. Q: What causes root rot and how can I prevent it?
  A: Root rot is usually caused by fungi such as Fusarium, Phytophthora, Pythium and Rhizoctonia. Water molds like Pythium and Phytophthora are worse when plants are over-watered or the potting medium does not drain well. Always use new pots and potting medium. Treat recycled irrigation water with chemical or physical techniques.

Use one of the following combination to prevent root rot:

1. Subdue Maxx and Medallion,

2. Aliette and Chipco 26019

3. Terrazole (or Truban) and 3336 (or Fungo or OHP6672).

These are broad-spectrum combinations that will prevent most soil-borne pathogens.


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2. Q: Where does disease come from?
  A: Some diseases (like powdery and downy mildews and Botrytis) are airborne and move from infected plants via wind and fans. Pythium, Phytophthora and Erwinia can be found in contaminated irrigation water. Viruses can be introduced by insects like western flower thrips (the vector of Tospo viruses).

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3. Q: Do I have to use fungicides for all diseases?
  A: No – some diseases, like powdery mildew on some plants can be eradicated. Some diseases can also be prevented using environmental modification. In environmentally monitored greenhouses, Botrytis blight can be controlled with fans and heat to reduce relative humidity. Pythium root rot can be controlled with use of high-quality well-drained potting medium and good water management.

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4. Q: How does disease spread?
  A: Some viruses are spread by insects like aphids, thrips and leaf hoppers. Spores of some fungi spread with wind and fans while others move best in water. By far, the most effective means of spread is through contaminated seeds, plugs or cuttings.

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5. Q: Why don’t we have more bactericides?
  A: Fungi cause more than 90% of plant diseases. Since bacteria are relatively unimportant in the big picture designing a product just for them is not financially feasible. The new bactericides are used for human and animal health. Since bacteria become resistant so quickly, they cannot be used on plants but are saved for human use.

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6. Q: Why are chemical controls easier to use than biological controls?
  A: Biological agents like Trichoderma and Bacillus are living organisms and require special conditions. It is more likely that a chemical applied to plants treated with a biocontrol agent will interfere with its activity than it would another chemical. Since chemicals are not alive they can be formulated to be very stable under wide range of environmental conditions for a long time.

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7. Q: What works best for Fusarium diseases?
  A: Use healthy bulbs, plugs and cuttings whenever possible. Keep stress from the environment, such as wilting, to a minimum. Watch for symptoms when plants start to flower (a form of stress). Treat with Heritage, Medallion or Terraguard at labeled rates and intervals.

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8. Q: What works best for downy mildew control?
  A: Try to use resistant cultivars. Keep records and eliminate cultivars that suffer consistently from this disease. Inspect all plugs and cuttings for signs of downy mildew infection upon receipt. Rotate at least two of any of the following fungicides: Aliette, Heritage and Stature. Never spray more than twice a week since this can be counter-productive and result in loss of downy mildew control.

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9. Q: Does the same downy mildew attack all of my plants?
  A: No – downy mildew fungi are host specific. That means the fungus that attacks rose will not attack alyssum. However, the alyssum downy mildew will attack other members of the crucifer family like wallflower, cabbage, broccoli and stock.

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10. Q: How can I avoid phytotoxicity?
  A: Read the label and follow directions for application method, rate and interval. Don’t tank mix unless you are sure all of the products are compatible. Do not apply products when temperatures are below 55 or above 90°F. Try new products on a small group of plants. Rotate to reduce exposure to a single product.

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11. Q: Why should I rotate fungicides?
  A: Rotating fungicides between different classes reduces the possibility of resistance development. Tank mixing products from different classes is also effective as long as the products have been tested for compatibility. Most labels on the newer fungicides have strict use recommendations to manage resistance.

12. Q: What is damping off?
  A: Damping-off occurs when a fungus (like Rhizoctonia, Fusarium, Pythium or Phytophthora) attacks seeds before or right after they emerge. Even Alternaria and Botrytis can cause damping-off under some conditions. Make sure you get an accurate diagnosis of the cause since these fungi require different fungicides for optimal control.

13. Q: What is the difference, in appearance, between downy mildew and powdery mildew?
  A: Downy and powdery mildew are very similar, causing a fuzzy white or purple covering on leaves, flowers and stems. Downy mildew tends to grow on leaf undersides while powdery mildew appears on upper surfaces of leaves and on flowers. Sometimes powdery mildew causes red or purple discoloration in affected leaves. Upper surfaces of leaves with downy mildew become pale, show angular yellow spots and sometimes wilt.

14. Q: What causes leaves to turn yellow but still have green veins?
  A: Yellowing of young leaves of many woody plants and trees is often caused by a deficiency in one or more micronutrients (often iron). Other minerals that are important for leaf color are zinc and manganese. A complete fertilizer with micronutrients included can prevent chlorosis.

15. Q: Why do bedding plants seem to get more diseases?
  A: Bedding plants are usually herbaceous annuals that have a short production cycle under intensive conditions that include high levels of water, fertilizer and temperature. These conditions are frequently exactly suited to pathogens, and disease abounds.

16. Q: Are the new, and more expensive, fungicides better?
  A: The new fungicides are better in some ways than the older ones. The first is that most of them are reduced risk and are safer for people to use and less damaging to the environment. They are also often narrow in their target and affect only a few diseases unlike some of the industry standards that can be effective on many diseases. They are not always more effective than older products.

17. Q: Why should I add a wetting agent or spreader-sticker to my chemical sprays?
  A: Wetting agents or surfactants decrease the surface tension between a spray droplet and the leaf. This allows the spray to spread out evenly and cover more area as well as penetrate fungal growth like rust pustules and downy mildew. Stickers allow fungicides to adhere better to leaves, giving them a longer lasting effect. Finally, some wetting agents give fungicides better mobility through the potting soil.

18. Q: What are viral diseases and how do I know that I have one?
  A: Impatiens necrotic spot virus (INSV) and rose viruses are fairly common diseases in the United States. Typical infections of INSV vary according to the plant host and can cause black stem streaks, ring spots or stunted leaves with black spots. Roses show wavy lines, vein clearing or oak leaf patterns on leaves. Roses can also show excess thorniness, wrinkled bright red leaves or very small leaves.

19. Q: How can I control viral diseases?
  A: There is no cure for viral infections so prevent them by using only healthy appearing plants and rootstocks for grafting and budding. Since some viruses are spread via insect vectors, control aphids, thrips and leaf hoppers. You can sterilize pruning tools between plants to stop a virus from spreading to healthy plants.

20. Q: What is the difference between a spray, sprench and drench?
  A: Sprays are applied to leaves and all above-ground portions of the plant usually to control diseases that occur above ground like leaf spots, powdery mildew and rust. Sprenches are used for diseases that attack the base of the plant or the crown and are applied at a lower rate than a spray or a drench. A sprench is directed to the stem or base of the plant with enough volume to wet the first inch or more of the potting medium. This will differ according to the fungicide and target. A drench should be applied to the potting medium and not over the top of the plant. Drenches are used for true root diseases usually with enough volume to thoroughly wet the entire container and allow a very small amount of drip through.

Chase Research Gardens, Inc. and GPN do not endorse any product listed in this technical information. Product recommendations are based on Chase Research Garden's experience in working with the products directly, observation or conversations with personnel within the greenhouse industry and academic community. Check your local or state regulatory agency for certain chemical and pesticide restrictions.