The overall objective in any disease management program is to ensure that no disease develops to damaging levels. With this objective in mind, it's helpful to know the three factors that cause diseases to develop:
The susceptibility of the crop
cultivar -- also referred to as a
host,
The presence of a pathogen
(species of fungi, bacteria,
viruses, and nematodes),
Environmental conditions
favorable for infection and
disease spread (air and soil
temperatures, soil moisture,
rainfall frequency, relative
humidity, soil type, and
Environment soil fertility).

All three factors must occur at the same time for disease to develop.

Unless all three of these factors occur at the same time, the disease will not develop. Whether a disease becomes severe or not also depends on the length of time that all factors are present.

What you can do
The management practices aimed at eliminating, reducing or altering one or more of the three disease factors typically include:
Crop rotation
Use of resistant or tolerant crop
varieties
Timely planting
Insect management
Weed control
Adequate soil fertility
Use of fungicides, if needed
Tillage (when unavoidable)

The effectiveness of each management practice depends primarily on the type of disease. For example, crop rotation is effective for controlling leaf diseases in corn but is relatively poor for controlling systemic (stem-related) diseases. Consequently, as with weed and insect management, a combination of practices or integrated system is the best bet.

Guidelines by crop
Here are some general guidelines, by crop, that can help reduce the incidence and severity of various types of diseases (supplied by Purdue University, West Lafayette, Indiana):

Treated seed can be a first line of defense against potential pests.

For assistance in scouting, pest identification, and management options, contact a viable source such as your farm supply dealer, crop consultant, or the Cooperative Extension Service.

Corn

Seed Rots and Seedling Blights
Select high-quality, vigorous,
disease-free seed of an adapted
hybrid.
Plant at proper depth and
spacing when soil temperatures
are higher than 54 degrees F.
Use seed corn treated with a
fungicide by the seed producer.

Stalk Rots
Select hybrids that have good
stalk strength and good
resistance to the disease. (Full-
season hybrids are generally
more resistant to stalk rots
than short season hybrids.)
Rotate crops (to crops other
than grain sorghum).
Do not exceed plant populations
recommended for the hybrid
used.
Control insects such as root
worms, corn borers, etc.
Harvest promptly (Where
significant stalk rots are
present, harvest as early as
possible to reduce lodging
losses.)

Foliar Diseases
Rotate crops, especially if
conservation tillage is used in
conjunction with continuous
corn.
Select hybrids with genetic
resistance to disease.
Maintain adequate, balanced soil
fertility levels based on soil tests.

Ear Diseases
Select adapted hybrids that have
genetic resistance to the disease.
Rotate crops, especially in fields
that had significant ear rots or
stalk rots the previous year.
Control insects such as corn ear
worms, etc.
Harvest promptly

Virus Diseases
Select adapted hybrids with
genetic resistance to the virus.
Control rhizome johnsongrass
and other perennial grass hosts
of viruses.