Fungicides: How Controls Impact Efficacy

Fungicides: How Controls Impact Efficacy

Chemical control of plant disease remains a popular and often over-used mainstay of our industry. Even the best fungicides and bactericides work better when cultural control methods are used, as well. In fact, if you try to put the entire weight of disease control on the fungicides you apply after a disease appears, you will be out of business quite soon. You cannot afford to waste your money on fungicides if the deck is stacked against them working. Using an integrated disease management program is the only way to make sure you get the most out of your fungicides and bactericides.

There are many factors in cultural control of plant diseases, but a few stand out in their effect on fungicide efficacy. Some factors, like new pots and sanitation, may be important in reducing disease but have relatively low impact on the ability of a fungicide to control the disease. Other methods, like using the right amount of water or keeping the leaves dry, can make all the difference in the world on whether or not a fungicide (or bactericide) has a chance to work.

If the conditions are too much in favor of a disease, the disease will not go away even when very effective fungicides are used. Anything that stresses the plant or promotes the pathogen makes the disease worse and, therefore, the fungicide must work harder. There is no such thing as a fungicide that can overcome a disease under all growing conditions.


Use only new or thoroughly cleaned pots, flats and other containers. If you must re-use containers, wash and disinfest them using products containing quaternary ammonium, chlorine, peroxides or other disinfestants. I performed some simple tests in cooperation with a nursery operation to determine the actual need for cleaning if a quaternary ammonium soak was used on recycled plug flats. The best control was a thorough washing followed by a five-minute soak in a quaternary ammonium at labeled rates.

Even higher rates used much longer were not as effective as when flats were washed before treating. Other research has shown steaming flats can be very effective if the plastic will withstand it. This has been especially effective in reducing contamination of flats with the black root rot pathogen, Thielaviopsis basicola.

Use new potting media whenever possible. Do not add native soil to any potting medium without steaming or treating with a product like methyl bromide. Consider how potting medium gets into a compost or dump pile. The plants fail to grow and are not salable (signaling the possibility of a disease), or they are simply overage.

Even if plants are all overage, the quality of the potting medium when it has already been through a growing cycle is reduced, and it now contains roots and at least lower stems of the crop. Further, there usually are pesticides and fertilizers that remain active. All of these things make the “new” crop a challenge to produce because the consistency from one crop to another is gone.

How can you determine the rate of fertilizer to add when you do not know what is already in the potting medium? If you reuse contaminated potting media, you are stacking the deck against your crop and even the best fungicides do not work if the disease pressure is too high.

Don’t Dip Cuttings

Avoid dipping cuttings. This is an excellent way to spread bacterial and fungal pathogens, including Xanthomonas, Erwinia, Fusarium and Cylindrocladium. Even when effective fungicides or bactericides are used, the spores will spread throughout the entire batch of dipped cuttings. If you suspect a pathogen, a post-sticking sprench will be the most effective way to apply a fungicide.

In other cases, spraying the stock plants the day before cuttings are made can be an effective way to reduce losses from pathogens like Cylindrocladium. I have been involved in several situations where a Fusarium or Cylindrocladium cutting rot was controlled simply by stopping the pre-stick fungicide drench.

If you recycle water, consider a water treatment. This water has the same concerns as reused potting media. Fertilizer, pathogens and pesticides may wreak havoc in your propagation and throughout production of the crop. The most common pathogens spread this way are the water-molds: Pythium and Phytophthora.

Additionally, do not water crops from overhead. Wet leaves are ideal targets for many plant pathogens. Splashing rain water or overhead irrigation spreads spores for bacteria and many fungi.

Keep in mind that rescue treatments with fungicides do not work when disease pressure is too high.

Table 1. Relative impact of cultural controls on disease and fungicide efficacy. 

Cultural Control Strategy Effect On Disease Effect On Fungicide Efficacy
Sanitation Low to medium Low
Disinfesting tools and equipment Low to medium Low
New pots, flats Medium (soil-borne mainly) Low to medium (can be high if a resistant strain is carried over)
New potting media Very high, easiest way to have root and crown disease Very high
Clean seeds, plugs, cuttings and liners Very high, easiest way to have root and crown disease Medium
Dipping cuttings Very high, best way I know to spread disease to all cuttings at once High (adding effective fungicides to a dip is often ineffective due to extreme disease pressure)
Remove diseased plants Medium, stops spread Medium
Keep leaves dry Very high, can control many foliage diseases with dry leaves alone High, disease pressure is low when leaves are dry
Use the right amount of water Very high High, over-watering shortens longevity of fungicid

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