Getting The Most From Your Fungicides: Application Rate, Interval & Timing

Getting The Most From Your Fungicides: Application Rate, Interval & Timing

I am sure you’re thinking the headline is somewhat dull, but if you do not pay attention to these aspects of disease control, you will be wasting a lot of money and wind up irritated. This article is part of the series to make the most of your fungicide/bactericide dollars in order to keep your profit margin up. I hope the thoughts I pass on will help some of you make better, more cost-effective decisions regarding fungicide/bactericide use.

Does Rate Really Matter?

I always ask which rates are being used to treat a disease before I try to suggest a control strategy. It is interesting to me how often the rates being used are too low to be effective. You might as well be spraying water if you use too low a rate. Remember, water is not neutral but something fungi and bacteria thrive on. So spraying very low rates can end up with more disease and not less.

Deciding The Right Rate

After conducting fungicide trials for more than 30 years, I have seen some cases in which a lower rate is more effective than a higher rate. One of the best examples in our trials has been the negative effect of using the highest labeled rate of Aliette for downy mildew.

When I first moved back to California, I heard how this fungicide was not effective on downy mildew–even when used at 5 pounds/100 gallons. I set out to find out the reason and discovered that after a couple years, the most effective rates were 1-2 pounds/100 gallons. If the rates were increased to 5 pounds/100 gallons, the degree of disease control was less. Only testing in your facility will tell you what the best rate is, so I always suggest starting in the middle of label rates when testing a new product.

Higher efficacy at a lower rate may be due to unseen or undetected phytotoxicity. Some pathogens, such as botrytis, take advantage of stressed plants and are worse when the plant is damaged. This is seen when copper fungicides are used when they cannot dry quickly. The resulting burn can end up infected with botrytis and actually more disease. Read the labels and stay in the middle of the road–at least the first time around.

In a rhizoctonia cutting rot trial, we saw one of the three fungicides tested was slightly more effective at the highest rate tested while the other two gave the same degree of control at all labeled rates tested. Rates on fungicide labels reflect the summation of many trials conducted all over the United States, and sometimes all over the world.

Rarely, I find that the rates are not what our trials have shown. Sometimes, they might be lower than our trials indicate are effective. At other times they are higher. The bottom line is you must use the rates on the labels, and if they are not working for you, switch fungicides.

Can I Reduce Rates When I Use A Tank Mix?

Nope! This is especially true if you are adding two mode of action groups to manage development of resistance. One of the ideal ways to develop resistance is to use a low rate of an active ingredient many times without rotation. This stresses the fungus or bacterium just enough to allow the strains with higher resistance to that active ingredient to develop. Because you probably do not want to promote fungicide resistance, try to avoid using lower-than-labeled rates. In other cases, tank mixes are designed to expand the range of the spray. For instance, if you are applying something for pythium and something different for rhizoctonia and you decrease both rates, you are once again applying water.

Stretching The Interval To The Breaking Point

This is another experiment commercial growers should not do. The result of applying products less frequently than suggested on the label is an outbreak of the disease. It is always harder to stop a disease that is active than to prevent one. Unfortunately, we have seen some situations, such as certain anthracnose diseases and fusarium crown rot, in which applying products too frequently makes the disease worse. This is due to a combination of the fact that every application adds water and, as mentioned earlier, fungi like water and phytotoxicity on the crop. Fusarium, botrytis and anthracnose fungi like phyllosticta take advantage of damaged leaves and stems. Phytotoxicity is always a function of rate and interval with a sensitive plant.

The interval must be based on how severe disease pressure is. The interval changes with the crop age and weather, and only experience can tell you when a one-week interval is needed instead of two to four weeks.

When Should I Start Spraying?

The old saying, “An ounce of prevention is worth a pound of cure,” has been around for a very long time because it is right. The key question is when will disease start? Knowing the weather that promotes certain diseases allows us to time treatments right before disease might occur.

You can avoid this guessing game somewhat if you are an excellent scout. If you can see the very first signs of disease, many times, that is the best time to start treatments. However, there are some diseases that cannot be cured no matter how fast you react, and they should always be prevented.

It is especially critical for crops like poinsettias that have a narrow target market time. I always suggest applying products when the particular disease is most likely to be present or at the final date you can apply an effective treatment. On poinsettias, rhizoctonia cutting and stem rot only causes significant losses during rooting and the first month of production.

Treating for this disease for the entire three-to-four-month production cycle is a costly and wholly unnecessary step. The late-season botrytis stem rot that is sometimes found is often confused for rhizoctonia stem rot. Once poinsettias have closed their canopies you cannot apply a spray to their stems for botrytis stem rot. So an application of something for this disease must occur before the canopy closes for best effect.


Deciding what to spray, how much, how often and when to start is a tough set of answers to come up with. It takes years of experience and constantly learning to be a great grower. Remember, if being a great grower was easy, even researchers could do it.

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