Agriculture Reference
In-Depth Information
A.
PATHOGENIC PLANT DISEASES
Pathogens which cause plant disease are parasites which live and feed on
or in host plants. They can be passed from one plant to another. Three fac-
tors are required before a pathogenic disease can develop—a susceptible host
plant, a pathogenic agent, and an environment favorable for development of
the pathogen.
A pathogenic disease depends on the life cycle of the parasite. The envi-
ronment greatly affects this cycle. Temperature and moisture are especially
important. They affect the activity of the parasite, the ease with which a plant
becomes diseased, and the way the disease develops.
The disease process starts when the parasite arrives at a part of a plant
where infection can occur. If environmental conditions are favorable, the para-
site will begin to develop. If the parasite can get into the plant, the infection
starts. The plant is diseased when it responds to the parasite.
The three main ways a plant responds are:
•
overdevelopment of tissue, such as galls, swellings, and leaf curls
•
underdevelopment of tissue, such as stunting, lack of chlorophyll,
and incomplete development of organs
•
death of tissue, such as blights, leaf spots, wilting, and cankers.
The parasites which cause plant diseases may be spread by wind, rain, in-
sects, birds, snails, slugs, and earthworms, transplant soil, nursery grafts,
vegetative propagation (especially in strawberries, potatoes, and many flowers
and ornamentals), contaminated equipment and tools, infected seed stock, pol-
len, dust storms, irrigation water, and people.
Figure 3.8
illustrates the ef-
fects of microbial infection of various fruit trees.
1.
Fungi
Fungi are plants that lack chlorophyll and cannot make their own food.
They get food by living on other organisms. Some fungi live on dead or de-
caying organic matter. Most fungi are beneficial because they help release
nutrients from dead plants and animals and thus contribute to soil fertility.
These fungi are a pest problem when they rot or discolor wood. They can do
considerable damage to buildings and lumber which are improperly ventilated
or in contact with water or high humidity.
Most fungi which cause plant diseases are parasites on living plants.
They may attack plants and plant products both above and below the soil sur-
face. Some fungus pathogens attack many plant species, but others are re-
stricted to only one host species. See
Figures 3.8
and
3.9
.
