Horticulture Principles and Practices

7.5 DISEASES
Pathogen
An organism that causes
disease.
Saprophyte
An organism that derives
its nutrients from the dead
body or the nonliving
products of another plant
or animal.
Cultivated plants are usually more susceptible than their wild relatives to diseases. This
susceptibility is due in part to the fact that many different cultivars have uniform genetic
backgrounds as a result of the activities of plant breeders using the same breeding stock
in developing new cultivars. Further, modern cultural practices allow plants to be grown
in dense populations (as in monoculture), which facilitates the spread of disease.
Horticultural plants are attacked not only by insects but also by a large number of
microorganisms that cause infectious diseases. These parasitic pathogens (diseasecausing
organisms) can be placed into four categories—fungi, bacteria, viruses, and
mycoplasma-like organisms. Like all living things, these organisms have their own life
cycles. Since they are parasitic, they need a susceptible host on which to survive.
Plant diseases may be classified on the basis of the causal organism, such as fungi,
bacteria, viruses, and microplasma-like organisms.
7.5.1 FUNGI
An estimated 75 percent of all seed plant species live in some form of association with
fungi in their roots, which is known as mychorrizae. This association is called mutualism,
where both host and fungus benefit, and is similar to symbiosis, a mutually beneficial
plant-bacteria association found in legumes only. Mychorrhizal fungi are known to be
more efficient than plant roots in absorbing phosphorus. They are also essential for the
growth and development of forest trees and herbaceous species. Acid rain destroys
michorrhizae.
The reproductive structures from which fungi develop are called spores. Spores
occur in a tremendous variety of shapes, sizes, and colors. A few fungi have no spore
stage. When spores germinate, they produce hyphae, which grow and branch to form the
fungus body called mycelium. Certain spores are visible as mold growth on the leaf surface.
Fungal diseases such as Helminthosporium leaf spots, rusts, powdery mildew, and
cercospora leaf spots are examples of this mold growth form. Other fungi occur as tiny
dark fruiting bodies that are embedded in the tissue of the diseased plant. Examples of
fungi are Septoria and Ascochyta.
Fungal spores are transported in a variety of ways—by wind, water, birds, insects,
spiders, slugs, and mites. Some of them have a protective covering that enables them to
survive adverse environmental conditions. In order to infect, hyphae of fungi gain access
to the host through wounds (caused by equipment, pests, hail, ice, and so forth) or natural
openings such as stomata. Sometimes the pathogen penetrates the epidermal layer by
direct action on that layer.
Most of the infectious plant diseases are attributed to fungi. Fungi are either unicellular
or multicellular (mostly) plants that lack chlorophyll. More than 250,000 species
of fungi have been described, of which about 22,000 are known to cause plant disease.
Some of them can live only on dead tissue (saprophytic), such as organic matter, while
others live on living tissue (parasitic). Some of them are restricted (obligatory) to one
host type (dead or living), while others have flexibility (facultative). Those that feed on
dead and decaying matter are beneficial to plants because they aid in the decomposition
of organic matter or compost to release nutrients for plant use. In the lawn, they aid in
the decomposition of thatch (accumulated dead grass on the surface of the soil).
All fungi are not pathogenic. Many are useful to humans and plants. Penicillin
(from Penicillium) is one of the most important antibacterial drugs. Mushrooms used for
food are fungi; fermented beverages and foods (e.g., bread, wine, cheese, and beer)
depend on fungi (yeast) in their production.
Even though most plant diseases are caused by fungi, they are usually relatively
easy to control. Methods of disease control are described later in this chapter. Table 7–1
shows some important plant fungal diseases, their symptoms, causal organisms, hosts,
and methods of control.
230 Chapter 7 Biological Enemies of Horticultural Plants