Key Takeaways
- When in doubt, consider sending in samples for diagnosis.
- Be sure to document plant health problems with photos, date, weather, etc., so that you can use these diagnostic clues if the issue arises again.
- Contact your state plant diagnostic lab to be sure you send in the right type of plant material needed for diagnostics of your specific plant health problem (which ensures accuracy in diagnostics) and for information on how to ship the samples.
- Misdiagnosing a plant health problem can lead to the wrong plant protectant product being used.
- When incorrect plant protectants are applied, it is both an unnecessary expense AND can result in a decline in plant health due to uninhibited growth of the actual disease-causing pathogen.
Introduction
Many plant diseases have overlapping symptoms, making them difficult to identify by just looking at them. To compound this issue, common names for some diseases are similar and may have indistinguishable symptoms, but they may not actually be caused by closely related pathogens. One example is the term "blight". A number of diseases have the word blight in them, but the pathogens that cause them may or may not actually be related. For example, early blight on tomatoes is caused by a true fungus, while late blight is a fungus-like organism (an oomycete). Even though these pathogens are not related, their symptoms overlap (Figures 1 & 2). This may seem like semantics, but accurate identification is crucial because many fungicides do not control all types of pathogens causing disease.
Why Accurate Diagnosis is Worth it
Experienced gardeners and producers can likely identify a lot of common diseases without the assistance of a diagnostic laboratory. However, if you are unsure of the identity of a plant health issue, it is a very good return on investment to have the samples diagnosed to ensure effective management tactics are used.
The current fee at the Plant and Insect Diagnostic Clinic at Iowa State University to identify a plant pathogen is $25 per sample. If we use a planting of 10 backyard tomato plants as an example, the value of those plants alone is similar to the cost of submitting a sample for diagnostics. Additionally, if you save around a half of a pound of tomatoes from each plant or grow an additional half a pound per plant due to better plant health management, the investment in sampling pays for itself.
That investment could also pay off in the future when you might be able to identify the disease on your own. When sending in samples for diagnosis, take copious notes and photos so that if you see the same issue in the future, you may not need to send the sample in for diagnosis and can compare the future pest issues to ones that you had diagnosed in the past.
The Case of Suspicious Tomato Spots
A large number of disease-causing pathogens can affect tomatoes. We already mentioned early blight and late blight, but there are many others. There are leaf diseases caused by bacteria, fungi, and fungal like organisms. In the case below, we have two very similar-looking diseases: bacterial speck and Septoria leaf spot. Differentiating them can be very challenging for home growers of tomatoes (Figure 3).
A common reaction to seeing blemishes on your plants is to purchase a plant protectant to help control the problem. But, how will you know which product to purchase if you don't know what kind of pathogen (if any) is to blame for the blemishes? There are some products that control multiple types of pathogens, but one product will never control all types of pathogens.
In our scenario let's say that you have four products available to purchase labeled to control tomato diseases. Products A, B, C, and D control Septoria. Only product D provides control for both Septoria leaf spot and bacterial speck (Figure 4). These are real examples of products that are used in tomato production, very few products control bacterial speck, or bacterial pathogens in general.
It makes sense to choose the product that controls both, but that isn't always an option. If you need to make more than one application, responsible pesticide application following integrated pest management strategies would suggest you should purchase at least two different products so you can rotate the active ingredient you're using, which helps mitigate pesticide resistance from developing in pathogen populations.
If you actually had bacterial speck but mistook those spots for Septoria leaf spot, only one of the four products would have any impact on controlling bacterial speck on your tomato plants. All the other products you had available to you, labeled for tomato disease control, would prove ineffective. Thus, the cost of getting your plant health problems identified will save you in a number of ways, including savings from the purchase of ineffective plant protectants. Additionally, unless you know what is causing the disease, you don't know what other cultural practices you can implement to prevent future incidences of the disease. Different diseases can be minimized with crop rotations, minimizing rain splashing from the soil to the leaves, watering practices, etc.
Conclusion
Every growing season, keeping good records and notes during both the picturesque and grotesque times will make you better equipped with information to aid in diagnostics when plant health problems arise. And always, when in doubt about what is causing plant health problems, send a sample for diagnostics. The cost of diagnostics will pay for itself in many ways.
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on May 6, 2025. The information contained within may not be the most current and accurate depending on when it is accessed.