Diagnosing Foliar Diseases of Vegetables

This article by Tom Isakeit appeared in "Valley Vegetable Notes," July-August 1998.

Often, a grower will refer to photographs in books and bulletins for guidance in determining the cause of abnormalities on leaves. However, such materials depict more common symptoms of infectious diseases, and usually the variation in symptom appearance caused by the pathogen is not depicted, nor are many common and uncommon non-infectious causes of similar symptoms. Thus, such material does not depict many commonly encountered symptoms. They should, however, be considered a useful starting point for disease diagnosis.

Although it would be preferable to know the cause of a symptom, if the symptom isn’t getting worse over time (i.e., more of the leaf affected or appearance on other foliage), for practical management purposes, it is sufficient to rule out an infectious cause.

In this article, I will present some general points for deciding whether a symptom is caused by an infectious agent or something else. In making generalizations, I will use watermelons as examples, although these generalizations will also apply to most other vegetable crops.

Generally, leaf-spots caused by fungi tend to be circular (reflecting the pattern of fungal growth) and the affected tissue tends to be dead. Such tissue is either black or brown in color. Furthermore, the discolored tissue is not collapsed in relation to surrounding tissue. This is in contrast to physical or chemical causes of leaf injury, which tend to cause affected areas to become dried up and shrunken.

Sometimes, leaf-spots caused by fungi have very distinctive symptoms, and a photograph is sufficient for making a diagnosis. An example is Cercospora leaf-spot of watermelon. In contrast, photographs of leaf-spots caused by Alternaria or by the gummy stem-blight fungus are just not distinctive enough for diagnostic purposes. Additionally, photographs of advanced symptoms of downy mildew, gummy stem blight, and anthracnose that depict extensive leaf blight are not useful either.

To determine if these particular diseases are present, I recommend that growers remove suspicious leaves and place them in a “moist chamber” at room temperature for 12 to 24 hours. The moist chamber is a small, sealed container (or even a sealed plastic bag) in which a wet paper towel is present to generate a humid atmosphere. Under these conditions, spots will expand, or at least, pathogenic fungi will produce spores on them. The presence of spores can be determined with a good hand lens or a “pocket” microscope. Of course, the identification of these spores does require previous training, but for the purposes of a watermelon grower, that training need not be extensive. I have done such training by simply having infected plant tissue on hand and taking five minutes to show the grower what to look for with a hand lens. The results of a moist-chamber incubation are usually conclusive for diagnosing a fungus disease or ruling it out.

Additional diagnostic clues can be determined by the pattern of symptom appearance in the field. Uniformity suggests a non-infectious cause; for example, leaf-spots that occur over most of a field can be caused by foliar fertilizer burn. Infectious diseases tend to have a clustered distribution in a field. The timing of symptom appearance can also offer a clue. The sudden appearance of similar symptoms suggest a non-infectious cause. If the advanced symptom of an infectious disease is found in a portion of a field, earlier symptoms can also be found. For example, if downy mildew is the cause of a blighted leaf (i.e., dead), leaves with yellow spots - an early symptom - will be found on leaves in close proximity. Finally, the pattern of symptom appearance should be compared to recent weather patterns. A leaf blight occurring when day temperatures are in the 90s with no rain occurring is not caused by downy mildew or gummy stem blight. In fact, leaf-miner insects can be the cause of such blight in different areas of Texas, and they can easily be diagnosed by the presence of serpentine, hollowed-out trails on non-blighted leaves.

Diseases caused by bacteria and viruses will also tend to occur in clusters in the field, and result in a range of symptom stages on plants in that area. So, if the distribution of symptomatic plants is clustered and fungi are ruled out with a moist chamber, these other infectious agents need to be checked out. Unfortunately, this is not always a “do-it-yourself” proposition for growers. There are some diagnostic kits for specific bacterial and viral diseases that are “grower-friendly.” For example, STA Laboratories has an excellent bacterial fruit-blotch testing kit, while Agdia has numerous kits for bacterial and viral diseases on various crops. Another option is to bring samples to a plant-disease diagnostic clinic.

One other consideration is that wilting and yellowing symptoms on the leaves may be caused by infectious agents working on the roots. Be ready to dig up and examine the whole plant.

The grower should strive to quickly identify the presence of foliage-infecting fungi if they occur on the crop. Under the right weather conditions (i.e., moist conditions), these fungi can cause significant yield losses, but if they are diagnosed quickly enough, they can be readily controlled with fungicides.