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Plant Pathology Guidelines for Master Gardeners

Contact: Dr. Richard Raid

Parasitic Diseases & Plant Pathogens That Cause Them

The bulk of this program concentrates on those plant health problems that are caused by pathogenic microorganisms. These organisms include fungi, bacteria, and viruses.

About 85% of all plant diseases are caused by fungi. Therefore, on a statistical basis alone, you are likely to encounter fungal diseases much more often than those caused by other types of pathogens. We will now proceed to a description of the main characteristics of fungi.

Fungi include the molds and mildews that we are all familiar with in Florida.

At one time fungi were considered to be types of plants. Indeed, mycology, the scientific study of fungi, is still done today in botany departments. However, in modern biology, fungi are not considered plants. They are placed in their own Kingdom (Mycota, for the serious biologists out there), with equivalent status to the familiar Animal and Plant Kingdoms.

Sometimes, growth of fungi is so profuse that a large enough mass (mycelia - multicelled microscopic strands) will accumulate to be seen with the naked eye. A good example is the growth of the target spot fungus on this ripe tomato fruit.

Figure 7

Target spot on ripe tomato fruit.

Figure 7: Target spot

Most of the time, however, careful examination with a microscope is needed to see fungi and ultimately identify them. Fungi consist of multi-celled microscopic strands. Often, spores, or the reproductive structures of fungi are readily visible, as seen in this photomicrograph of the fungus Botrytis.

Figure 8

Figure 8: Botrytis spores

It is the peculiar size, shape, coloration, etc. of these Botrytis spores that are most useful in identification of fungi, including those that cause plant disease.

Figure 9

Sometimes, we encounter important pathogenic fungi that do not readily form spores. A good example is the root-infecting fungus, Rhizoctonia, shown here.

Figure 9: Rhizoctonia

We take note of the distinctive right-angle branching of the fungal threads (mycelia) in making an identification of Rhizoctonia.

Plant Pathogenic Bacteria

Our next group of pathogens is the bacteria. These are smaller than fungi. Though fungi cause more diseases than bacteria, bacterial diseases are generally more difficult to control. Again, bacteria are not plants. They are one-celled microorganisms, requiring good, powerful light microscopes to be seen. Though some bacteria produce resistant spores, no plant pathogenic bacteria do so. Details of bacteria are best seen at the very high magnifications of electron microscopes.

Figure 10

Note the whip-like appendages (flagella) of the bacteria in this photomicrograph.

Figure 10: flagella

Figure 10a

Bacteria depend on outside agents for dispersal. They do not spread on the wind as many fungi do. Splashing water is the number one means by which bacteria are disseminated, as in this picture showing overhead sprinkler irrigation of a farmer's tomato plants.

Figure 10a: agents of dispersal

Many bacterial diseases can be spread readily simply by touching an infected plant and then by touching a healthy plant. Like human bacteria, plant pathogenic bacteria are extremely contagious. Bacteria cannot penetrate the cuticle of plants but must enter the plant through a wound or natural opening.

Plant Viruses

Viruses are by far the smallest of the pathogens considered in this program. The term "organism" may not be appropriate for a virus. It is simply a strand of genetic material (DNA or RNA) that is enclosed in a coat or wrapping of protein. They must have a living host in order to reproduce (replicate).

Electron microscopes are needed to see viruses. Even the best light microscopes are not good enough to see such tiny particles.

Figure 11

This is a picture of the rod-shaped virus particles of the tobacco mosaic virus, the first plant-infecting virus discovered.

Figure 11: tobacco mosaic virus

Figure 12

Viruses usually are vectored or carried from infected to healthy plants by insects. Can you identify the insect in this picture?

  1. Hornworm
  2. Stink Bug
  3. Flying Ant
  4. Aphid

Figure 12: identify this insect
See Answer »


Figure 12

"d" is the correct answer. Aphids (and in recent years, whiteflies) are the most important vectors of viruses in Florida

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