University of Florida

Plant Pathology Guidelines for Master Gardeners

Contact: Dr. Richard Raid

Signs Of The Pathogen

As you probably already are aware, symptoms associated with many different plant health problems can look quite similar. Indeed, professional plant pathologists can often struggle with a diagnosis based on symptoms, even after years of experience. Therefore, protocols for correct diagnosis are based on signs of the pathogen!

Signs are actual physical evidence of the occurrence of the pathogen in association with the unhealthy plant material. These include:

  • Mycelium or Mold Growth: under some conditions, readily visible to the naked eye
  • Conks and Mushrooms: the familiar structures of some fungi that are formed by some pathogenic fungi
  • Fruiting Bodies: reproductive structures of some fungi that are embedded in diseased tissue, often requiring a hand lens to see
  • Sclerotia: hard resistant structures of some fungi
  • Rusts
  • Bacterial Ooze and Specific Odors: associated with tissue macerations by certain pathogens

Rest assured, you must become familiar with signs of pathogens and the techniques needed to detect and identify them. If you try to diagnose plant diseases on the basis of symptoms alone, you will be wrong too many times to be of very much benefit to your clients.


Figure 39

Note the talcum-powder like growth on the underside of this yellow squash leaf. The top of the leaf is yellow. Yellowing of squash leaves is common and may be the result of quite a few causes. However, only one cause, the powdery mildew fungus, produces this characteristic growth on the under surface of the leaves. This is the actual fungus that you are observing. It is a sign of the pathogen.

Figure 39: yellowing of squash leaf: fungus


Figure 40

Note the sign of the pathogen seen here causing the disease of squash fruit called wet rot. This fungus growth on the tip of the squash fruit is most readily seen early in the morning or when dews and humidity are high.

There are lots of reasons why the tips of the young squash fruit become brown and withered. Most notable is a lack of sufficient bee activity to properly pollinate female flowers. However, the "whiskery" growth of the wet rot fungus is a sure sign that the browning and withering is indeed a wet rot disease problem.

Figure 40: wet rot fungus


Figure 41

There are many causes of leaf spots on tomatoes. However, if you look closely at the image, you will note an olive-green/gray growth of a fungus within a yellowish circular spot. What you are seeing is a sign of the fungal pathogen causing a disease known as leaf mold. There may be as many as 30 causes of yellow leaf spots of tomato leaves, but only one, the leaf mold fungus, produces this characteristic sign of olive green mold growth. So you see, by looking for the sign of the pathogen, you have gone from 30 possibilities to the correct diagnosis.

Figure 41: tomato leaf mold


Figure 42

This tomato stem was split open after noting that the plant was wilted. Note the hard black sclerotia inside the stem. These are a definitive sign of the pathogen causing this wilt, Sclerotinia sclerotinium. The disease is commonly called white mold. Many causes, including simply a lack of adequate irrigation, can cause tomato plants to wilt. But one, and only one, is associated with formation of these black sclerotia. You can see how important it is to see and recognize signs of pathogens when properly diagnosing diseases.

Figure 42: tomato: white mold


Figure 43

On this tomato fruit, note the signs of fungus growth and formation of very young sclerotia (the structures that look like seeds forming in the middle of the fungus mat) at the stem end. These sclerotia identify the fruit-rotting symptoms as caused by the southern blight fungus.

Figure 43: southern blight on tomato


Figure 44

You might recognize this disease as bean rust. The distinct yellow leaf spots with the brown centers are relatively easy to identify. But these brown centers are predominantly a mass of rust-colored spores of the rust fungus. These are observable as a sign of the pathogen when carefully examined with a hand lens. What you see through the hand lens is an actual massive accumulation of fungus spores. So what you see is a sign of the pathogen.

Figure 44: bean rust


Figure 45

This is a photomicrograph at 400} magnification of the spores taken from the rust lesions. We might do this for confirmation of the diagnosis in our labs, but you can do a fairly good job of hazarding a diagnosis with careful observation and a hand lens.

Figure 45: rust lesions under microscope


Figure 46

This used to be an ear of corn. Now what you see is this grotesque growth of the corn smut fungus which has taken over. What you are observing in the black interior is a mass of billions of smut spores. (By the way, this fungus growth is supposedly edible, with a typical mushroom flavor - but we are not tempted at all!)

Figure 46: corn smut fungus


Figure 47

This is a sign of a fungus pathogen. The sign is a mushroom or conk. The disease of turf grass it causes is called fairy ring because the conks are arranged more or less in a circle. Obviously, there are lots of reasons for lawns to decline, but only one decline is associated with this telltale sign of fairy ring.

Figure 47: mushroom fairy ring in lawn

Return to top