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.

Leave a Reply

More From Disease Control...

March 4, 2015

Nexus Corporation’s Cheryl Longtin Encourages Women To Seek Volunteer Leadership Opportunities

When Cheryl Longtin came to the horticulture business in 1994, she applied her experience in the automotive industry to promote the adoption of more technology in greenhouse production. Longtin says horticulture, with its rich family tradition, has long promoted women in the industry compared to other industries, but women in horticulture must continue to seek out opportunities to provide volunteer leadership in organizations that shape the future of the business.

Read More

March 4, 2015

Second Annual GreenhouseConnect Will Bring Growers and Suppliers Together in San Diego This October

Following a successful inaugural event in Tampa last fall, Greenhouse Grower has announced the dates of its second annual GreenhouseConnect: October 26-29, 2015. Representatives of an expected two dozen leading greenhouse operations from across the U.S. will join senior-level suppliers at Rancho Bernardo Inn in San Diego for several days of one-on-one strategic meetings, a growers-only roundtable, informational sessions and a variety of networking events.

Read More
cultivate'15 logo

March 4, 2015

Cultivate’15: AmericanHort Announces What’s New

In an industry that has seen major changes occurring at a fast pace, many industry professionals leave Cultivate with their heads spinning and no clear idea of how to regroup and strategize. Cultivate’15 is “Changing the Game.” As this year’s focus, Changing the Game will call your attention to the ways in which our industry has changed and your opportunities to compete successfully.

Read More
Latest Stories
Rose Rosette on Knockout rose, May 2013. Photo credit: Alan Windham, University of Tennessee

March 2, 2015

Rose Rosette Disease Fight Gets A Boost From Government…

In 2014, $4.6 million was awarded through the Farm Bill to tackle rose rosette disease, a devastating pathogen that affects one of the industry’s most important crops.

Read More
Fig 1 Leafy Gall On Leucanthemum Becky

March 2, 2015

How To Prevent Leafy Gall Before You Lose Plants

Leafy gall is a nasty disease that can go undetected until plant damage is done. Take these steps to protect your crops from infection.

Read More

October 6, 2014

EPA Registration Granted To Stockton’s Timorex Go…

Timorex Gold, a broad spectrum fungicide, has received EPA registration in the U.S. for disease control on organic and conventional crops.

Read More

August 5, 2014

Prevention Measures For Impatiens Downy Mildew Start At…

Impatiens downy mildew is a fast-moving disease that can quickly go from bad to worse if conditions are right. In case you missed it, here are some of the highlights from Ann Chase's (Agricultural Consulting) downy mildew update at Cultivate'14.

Read More

July 22, 2014

Spray Coverage Key To Uniform Pest And Disease Control …

Greenhouse growers need to understand proper spray application coverage when applying pesticides and growth regulators to ensure successful treatment results.

Read More
Jeff Rich

July 18, 2014

Removing The Mask Of Phytophthora

Phytophthora is the number one disease of floriculture and nursery crops nationwide. Here are some effective measures growers can take to reduce the occurance of this pathogen, known as "the plant destroyer."

Read More

June 27, 2014

Biocontrols Can Be Highly Effective With Serious Commit…

Biocontrols can be very effective when the greenhouse operator makes a serious commitment to using them for integrated pest control. See how Parkway Gardens has successfully used biocontrols for the last nine years.

Read More

May 1, 2014

Be On The Lookout For Botrytis Blight

Greenhouse growers will be challenged by weather forecasts for cloudy and rainy conditions favoring Botrytis blight. Remembering cultural practices and correct fungicides will help until we get more sunshine.

Read More

April 18, 2014

Growth Products Introduces Improved Packaging For Compa…

Following the suggestion of a user of Companion Biological Fungicide, Growth Products has made improvements to the product's packaging to make it easier to handle and more durable.

Read More

April 8, 2014

Switching To Organic Fertilizer

Growers of short-term vegetable or ornamental crops should have an easier time switching from conventional mineral fertilizers to organic fertilizers.

Read More
Edema om Ivy Geranium. Photo credit: SHS Griffin

March 27, 2014

Geranium Leaf Spots: Is it Rust or Edema?

Damp conditions can lead to both rust and edema in geranium. Here's some advice on scouting and treating each.

Read More

March 21, 2014

Dümmen Clarifies Growers’ Options Regarding The K…

The Dümmen Group has released a statement to clarify growers' options regarding regulations the Kansas Department of Agriculture (KDA) placed on petunia cuttings sourced from Dümmen's Las Mercedes, El Salvador farm during weeks 51 through 7.

Read More
Impatiens downy mildew, Photo Credit: Ian Maguire, Biological Scientist, University of Florida

March 19, 2014

Impatiens walleriana Makes Possible Comeback In 2014 Se…

Despite efforts to thwart downy mildew, growers, researchers and landscape experts are predicting a continued impact on Impatiens walleriana supply issues for the 2014 season.

Read More

March 18, 2014

Exclusive: Kansas Department Of Agriculture’s Jef…

Greenhouse Grower talked to the Kansas Department of Agriculture’s Plant Protection and Weed Control Program Manager Jeff Vogel to learn about the Kansas Pest Freedom Standards and how the state is regulating tobacco mosaic virus (TMV) on petunias this season.

Read More

March 18, 2014

Kansas Department of Agriculture Taking Measures On Pet…

The Kansas Department of Agriculture (KDA) has ordered growers and retailers to destroy all petunias received from the Dümmen Group's Las Mercedes, El Salvador farm.

Read More
Leaf Distortion

March 18, 2014

Kansas Department of Agriculture BMPs For Scouting TMV …

The Kansas Department of Agriculture is asking growers and retailers to follow regulatory action and recommendations in scouting petunias for symptoms of tobacco mosaic virus (TMV).

Read More

March 13, 2014

Two BioSafe Systems Products Proven To Reduce Fusarium

In a recent study, SaniDate 12.0 and ZeroTol 2.0 from BioSafe Systems were evaluated as sanitizing products to control fusarium in irrigation water.

Read More

February 26, 2014

BioSafe’s OxiPhos And ZeroTol Are Effective For C…

A recent study has determined the effectiveness of OxiPhos and ZeroTol 2.0 in controlling impatiens downy mildew.

Read More