Soil-borne diseases are caused
by plant pathogens (mostly fungi and nematodes) that can survive in
the soil indefinitely. Indeed, soil is their natural habitat. Typically
soil-borne pathogens cause root rot and foliage blight. Most plant
pathogens that cause disease of the foliage, such as alternaria, and
septoria, cannot survive in the soil. Despite a few exceptions to
these generalizations, it is useful to categorize pathogens as either
soil-borne, or not soil-borne, because it helps us understand how
to manage them more effectively.
Soil-borne pathogens are difficult to control because they have the
ability to survive for many years in the soil. Plowing crops into
the ground after the growing season does not help reduce soil-borne
pathogens; its like throwing brier rabbit into the briar patch. The
most important practice is to avoid planting susceptible crops into
contaminated soil.
Crop rotation, along with other cultural practices, and fungicide
applications, are important tools for managing plant diseases. An
integration of management practices is the most effective approach.
This article addresses practices that can be targeted to specific
soil-borne diseases. Knowing exactly what diseases you are dealing
with is the most important first step in developing an IPM program.
Make sure your diseases are accurately diagnosed.
Oomycetes pythium, phytophthora, and downy mildews are often referred
to as “water-molds or primitive fungi.” They are not “true
fungi” but we refer to them as fungi because they look like
and behave like fungi. They are more properly known as oomycetes.
Oomycetes survive for many years in the soil by producing specialized
resistant spores called oospores. These thick-walled structures germinate
only when root secretions from a susceptible host are present, and
soil moisture is abundant.
Pythium is a common inhabitant of all field soils
and has a very wide host range. Though it is a weak pathogen in the
field, and rarely a cause for concern, it often causes damping-off
in the greenhouse. This is because pythium does not compete well with
other soil-inhabiting fungi and bacteria, which are generally absent
in soil-less growing media used in greenhouses. When starting transplants
in the greenhouse choose a soilless growing medium that drains well.
Avoid contaminating the medium with soiled hands, hose-ends and tools.
Avoid overwatering and overfertilizing. Pythium occasionally causes
disease to seedlings in the field when soil is excessively wet and
cool. Also, transplants are more susceptible during the first week
or two after setting out.
Phytophthora is closely related to pythium but is
different in several ways. Phytophthora is much more pathogenic but
unlike pythium it is not widely distributed. Also, phytophthora species
that attack vegetable crops generally have relatively narrow host
ranges.
Phytophthora infestans attacks only tomato and potato.
It behaves like a downy mildew in that it causes blights of the foliage,
and spores are easily blown throughout the field. It generally does
not take up residence in the soil because it needs two different mating
types (similar to male and female) to produce the oospores that allow
long-term survival. For this reason we don’t see this disease
repeating itself in the same field year after year. Typically it is
brought in on seed potatos, and its spores can be blown for a mile
or so in a good storm. Phytophthora infestans can survive in potato
cull piles. Diseased plants should be plowed into the ground to prevent
further spread. It is especially important to bury cull piles of potatoes
and tomatoes. There are several fungicides registered for use on potatoes
and tomatoes but they may not perform well when disease pressure is
significant.
Phytophthora capsici causes blight of cucurbits,
tomatoes, peppers, and eggplant. Crown rot and fruit rot are the most
common symptoms. Often, a powdery or mealy bloom of whitish growth
appears on the surface of infected fruit. This phytopthora species
can survive for very long periods of time in the soil. Plowing infected
plant debris into the ground will not help, and may increase the level
of soil contamination. However, chisel plowing may improve drainage,
and that can help reduce phytophthora blight. There may be some benefit
gained from removing the diseased fruit from the field. However, do
not dispose it on agricultural land. Diseased fruits should be landfilled
or place where cucurbits, pepper, and tomatoes will never be grown.
Phytophthora capsici can contaminate farm ponds if fields with diseased
plants drain into the pond. Avoid planting susceptible crops on land
known to be contaminated with phytophthora capsici. Avoid bringing
in contaminated soil to clean fields with plows, etc. Plow and disc
contaminated lands last and spray the soil off equipment before moving
to new fields.
Downy mildews are caused by a number of different
species. Most have a narrow host range, attacking a single genus or
family of plants. They cause blights of the foliage rather than root
and crown rot. Downy mildews of crucifers are caused by peronospora
parasitica. Most crucifers,
including weeds are susceptible. Disease occurs when temperatures
are between 50 - 60 F and plants are wet for 12 to 24 hours. The pathogen
can survive in the soil by forming oospores. Rotate away from crucifers
for at least two years. Some broccoli cultivars are resistant; see
your seed catalog to check for resistance of a particular variety.
True fungi rhizoctonia causes damping off, crown
rot and root rot of a variety of vegetable crops. As is the case with
pythium, rhyzoctonia is mostly a problem in the greenhouse. Several
fungicides are available to protect seedlings. Occasionally, head
rot of cabbage, “soil rot” of squash, or stem rot of crucifers
will occur in the field. Rhizoctonia is generally not a problem in
field soil and fungicide applications are usually not warranted.
Sclerotinia blight affects a wide range of vegetable
crops but not corn or grasses. Sclerotinia sclerotiorum survives in
the soil by producing sclerotia, hard black structures that are 1/8
to 1/2 inch in length. They are usually embedded in the fruit and
stems of the plant. Sclerotia that are within one to two inches of
the soil surface germinate when the soil has been saturated for about
a week and temperatures are between 50 - 70 F. They may directly infect
stems, or produce tiny mushrooms, which disperse millions of spores.
The spores do not have enough energy to germinate and infect healthy
tissue. However, spores that land on dying flowers germinate and develop
enough growth to cause infection once the flower drops to a stem or
fruit. If conditions for diseases are present, fungicides should be
used when the plants come into flower. Once the disease has become
established there is no point in using fungicides because there are
no secondary disease cycles. Sclerotia that develop after infection
cannot germinate until the following year.
If practical, remove diseased plants and fruit from the field. A single
head of cabbage can have thousands of sclerotia. If removal is not
practical, it is better to plow the infected plants into the soil
rather than disking them in. Plowing under the diseased plants will
bury the sclerotia to a depth that will inhibit germination, while
disking will tend to place sclerotia where they can germinate and
come in contact with roots and so forth.
Fusarium enters the host plants through roots and
grows up through the water-conducting cells of the plant into the
stem. Symptoms of fusarium wilt include yellowing and wilting, often
on one side of the plant. A cut through the stem often show discoloration
of the vascular system. This fungus is highly host specific. For example,
fusarium wilt of basil occurs by a specific strain that can only cause
wilt in basil. Fusaium wilt of tomato only affects tomato. Fungicides
are of no value, rotate with another crop for at least four years.
Some cultivars have been bred to be resistant to fusarium oxysporum.
Verticillium wilt is caused by verticillium dahliae.
Verticillium infects plants in the same way that fusarium oxysporum
does. Yellowing, scorching and wilting follow infection. Discoloration
of the vascular system may be evident. This fungus has a very wide
host range but some specificity exists. For example, strains that
infect maple trees may be weak pathogens of vegetable crops. A variety
of vegetable crops can be infected by the same strain. Eggplant is
particularly susceptible. A combination of lesion nematodes and verticillium
causes “early dying” in potato. Fungicides are of no value.
Avoid planting susceptible crops in contaminated fields. Some resistant
cultivars are available.
There are many species of plant parasitic nematodes,
and most of them have wide host ranges. Every handful of soil has
a few plant parasitic nematodes but they are not a problem unless
they build up to high numbers.
