Getting The Most From Your Fungicides Part 3: Diagnosis & Application
Are you still guessing which products to use on a disease? Are you even sure you are treating diseases and not environmental, nutritional or even phytotoxicity look-a-likes? I have spent years trying to convince growers they can actually save money if they work with a lab to make sure what they think is a disease and which one it is.
In this article I will, hopefully, get you to think more about using a rifle approach to disease control and not the popular shotgun approach. It can save you a lot of money if you use only the best product for the actual problem at hand.
The first step is always to know what the actual problem is. A lab diagnosis is the best way to determine the problem you are facing. This must be done before the fungicide program is chosen. This time, I will attempt to show you why making the wrong choice will cost you money.
In Table 1, I give an example of a leaf spot, various fungicides and the results you can expect to see. You might think it’s easy to tell the difference between bacterial leaf spot, downy mildew and virus, but I assure you people who do not make a living at diagnosis often confuse symptoms. I have chosen a broad-spectrum product (copper) that can give moderate control of many foliar diseases. It is always the most effective in controlling bacterial leaf spots (Pseudomonas and Xanthomonas) but will not work on a virus disease.
I have included a very narrow spectrum product that works really well on downy mildew (Stature SC) versus a strobilurin (such as Heritage, Compass O or Insignia) that is broader in spectrum. So, choosing poorly based on failure to get a diagnosis can lead to applying a fungicide on a virus disease. Nothing good would come of it, and you would be paying for a pointless application that might even cause damage to the crop and result in further quality loss.
Second, do you know what the strengths and weaknesses of your fungicides are? Using broad-spectrum fungicides, like copper or mancozeb, all the time would certainly give you some control of most diseases. On the other hand, you will be spraying more often and getting less control overall for your costs than if you chose the best product for a specific disease. You would be unlikely to stop disease from developing but perhaps would slow it down. In these times of shrinking margins and the ongoing importance of sustainability, using the best fungicide you can afford for a specific problem is one way to save money.
Chase Horticultural Research publishes results of its fungicide control trials every couple years in a wall chart. We hope this has helped clarify which products work best on specific diseases. The current chart has been printed courtesy of Syngenta. Some of the most commonly used fungicides belong to one of the following chemical classes: sterol inhibitors, strobilurins, coppers and phosphonates. Table 2 lists the class, active ingredient, some of the specific products in each class and some of the best pathogen targets for that product or class. You should see that not all products work equally on all diseases.
Third, it is critical that you apply the fungicide to the correct site. If you have a root disease, you should apply the product to the roots. If you have a foliar disease, it is always best to apply the fungicide to the leaves. There are very few exceptions to this rule. There are some products with systemic characteristics, such as phosphonates, some strobilurins (Heritage) and thiophanate methyl (3336 and OHP 6672). Phosphonates are systemic in both directions while the others mentioned are systemic upwardly only. That is, you can apply them to roots and they will move through the xylem into the stems and leaves. The key is to understand that many times, they will not move in a quantity sufficient to control the disease. For the purposes of achieving optimal control, you should always apply the product to the site of the disease.
Another aspect of applying the product to the right site is to understand when you can direct the product to the upper leaf surface, the lower leaf surface, the crowns or the roots (see Table 3). Unlike many insects and mites, the fungi and bacteria do not move around the leaf surface, and you must contact them directly to achieve any real control.
In addition, these are not hard-and-fast rules. Some soil-borne pathogens are often in the crown area but can also cause root rot. If you check roots and they look good, you can concentrate applications for those listed in the “crown” row with a sprench or crown application. In contrast, root diseases caused by Pythium or Thielaviopsis (the cause of black root rot) are nearly always best controlled with a drench that contacts the entire root system.
Try to follow these steps to obtain the most effective disease control as often as possible.
1. Know the most common diseases that attack your crops and what they look like.
2. Know which chemical classes work best on the pathogen. Pick two or three to rotate.
3. Pick the specific product in each class by labeling on your crop, safety to your crop, REI, cost or availability in your state.
4. Try to apply the product to the best site. Use foliar sprays for foliar diseases and drenches for root rot.
5. Finally, keep good records. If you try to remember which fungicide you used from one season to the next, you will have a tough time. Make notes somewhere on the results of your efforts. As the saying goes: If you forget history you are doomed to repeat it.