The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source
site maintained by the Florida
Cooperative Extension Service.
Copyright 2005, Board of Trustees, University
e' 0I0 DISEASE CONTROL BASICS ALTERNATIVES TO CHEMICALS
A. R. Chase
University of Florida, IFAS .
ARC-Apopka Research Report RH-1980-16
Unfortunately disease control is generally thought of in terms of"chemical
treatments applied to plants previously infected with pathogenic organisms, 'Howeve U:
disease control involves many practices.designed to avoid establishment of disease.--- -
supplemented by chemical therapy if disease occurs. Indeed, in the future, we will
have to depend more and more upon cultural means of disease control as chemical use
becomes more restricted, and fewer chemicals are available.
Development of disease depends upon three factors as illustrated by the disease
triangle (Figure 1). The pathogen, the susceptible plant host, and a favorable
environment are each necessary for disease to occur. If any of these factors is
significantly altered or eliminated, disease will not occur. Disease control methods
based on changing these factors can be divided into three categories.
There are many ways to eliminate or diminish the numbers of a pathogenic organism
which do not involve the use of a chemical.
A. Quarantine The most effective way of keeping a pathogen from infecting plants
is quarantine. This is usually done in situations in which the pathogen is very
destructive and is not present in the geographic area. Governmental mandated
quarantine has not been used widely with diseases of foliage plants since nearly all of
the important pathogens are already present in Florida. For this reason, the source of
disease in each case is important and growers should maintain their own quarantine for
Pathogens may be introduced in propagative material, potting media, pots,
equipment, insects and weeds, or soil under benches and in walkways.. The specific
method which will be most effective depends upon the source of the pathogen.
B. Plants- Pathogen-free plants are essential and should be chosen from stock
known to be healthy (chosen during the time of the season when symptoms ofdisease
normally appear) or tissue cultured plants proven healthy. If pathogen-free plants
are not used, all other methods of disease control may not be sufficient to eliminate
C. Potting media and pots Sterile potting media is equally important. Since
this has been discussed in detail in an earlier article (Knauss, 1979), it will not be
covered again here. Using clean or new pots and clean tool ;- necessary also since
many times a pathogen can survive on the sides of a pot o be carried on a hose nozzle
allowed to touch the ground. If a grower places pots on the ground, all of the work
applied toward producing healthy plants may be negated, thus, plants should be grown
on raised benches.
D. Removing diseased plants Recognition and removal of diseased plants can
aide in decreasing spread of disease between plants; be sure to include the potting
medium which can harbor pathogens.
E. .Controlinsects and weeds Some pathogens are introduced to plants by fe
and movement of ,insects or by ;living on weeds close to the.host plant. Keeping in! -
and weeds under strict control is necessary.since once infected, the only way to' ,:
eliminate or even decrease losses due to virus diseases is ,to destroy the plant. Thus,
keeping the pathogen from the plant is the only,adequate method of controlling virus
II. HOT PLANT "
A.. Resistant varieties Resistant varieties should be used if they are aval
therefore..allowingthe plant:to grow in the presence of the pathogen with a minimum of
loss to disease. However, resistance is not common, and substitution of p.lats.
depends'upon salability of the substitute. Another factor making the use of resistant
plants uqli.kely,:,is that many of ,ur most serious diseases occur on a wide variety of
plants ,and resistance to them,isi.traditionally difficult to breed.;
The next most successful method-.of disease control involving the host plant is
decreasing the susceptibility of the plant through cultural means. Maintaining plants
in a.healthy,.-nonys,.tressed state, is"very, important in combating disease; Fertilizer,
appropriate pottrirg.media, adequate light and humidity each, i influence: plant health.,
Imbalances in any of. these areas stress the plant and increase disease susceptibility.
B. Fertilizer level In some diseases high fertilizer rates result in rapidly
growing, succulent plants which,are more severely;affected by that disease.: 'Keeping
fertilizer levels at recommended rates does much to.:strengthen plant resistance.
.C. .Water level Water, stress through either too much or too little water can
resulte.in ,mere severe .infections. as is often seen in' root rot diseases which are more
serious, wban, plants aretover-ywatered. Roots die when kept too wet due to oxygen. :
starvation'. Once the.pathogen gains entrance through dead roots: t can spread:
throughout. the .root, system. ... .
0. Potting.medium The:potting medium should not retain water- for :orog periods
of time si nce even. the-right amount of. water will be too much in a heavy pottingf'mix.
E. : Rotatio, Another method used' to reduce losses from disease isi rotationdf
crops. This is helpful if the host, plant .can be grown in an area fre of the disease,
and a nonsusceptible plant grown in the original area. This may be most useful for
the grower with multiple locations, who has the opportunity to move stock plants
completely away from an area of disease loss.
The third and final area of disease control is manil t1dt.un of the environment
to diminish losses. In many diseases, this js the key, as some cultural practices
are sufficient to control diseases in the absence of other methods mentioned.
A. Physical barriers Placement of an effective barrier between the plant and
the pathogen is very successful and of the utmost importance. Growing plants on
raised benches removes them from a very common and extremely important source of
infection, the ground. Many root and stem rot pathogens r"e present in native soil
and move into ground beds and pots placed on the ground. ;iis can be devastating in
sterilized soil na-..se natural antagonists in the mix hat been eliminated and the
:pathogen has' no competitiono. :Growing plants in an enclosed structure i important for
much .the. same reaso(n.-- Free'wa'ter on leaves, airborne spores of pathogens and insects
carrying pathogens can..be controlled much'rore effectively in enclosed structures. In
.greenhouses, other methods'-cn be adopted which are aimed at creating 'an environment
unfavorable to the .pathogen development. '
F. Watering methods The mc:t important consideration is eliminationof
standing wai.r on plant foliage, since this is absolutely necessary ft.. infection to
occur in mar.y bacterial and fungal leafspots. Elimination of overhead sprinkler
irrigation; and ra.infa.il 's the best rmthod for control. .Adopting bther environmental.
changes is not as.reasy sirce the optimum for plant growth is'often tho optiuiim .for
pathogen growth. .
,C.. Plant spacing Cottrol ing humidity can be accomplished to, a-'liited extent
through plant spacing.- Plants..spaced close together create a' humid environment- in
their immediate vicinity which is not created when they are spaced more widely. This
humid environment is ideal for':pathogen development. .'' ''
D.. Sanitation Keeping weeds under control, aisles clean nd' free 6f plant
debris and removing diseases plants, (this can sometimes be extended toindclude
-removal of diseased-leaves), all contribute toward creating ian environment unfavorable
to the pathogen.
In summary, there are.n uerous methods available:'for control of:plant disease
which,:do not involve chemical applications.. In many tases', these methods redute
production costs, plant losses and eliminate or significantly reduce the need for
costly chemicals.- However, if a few key practices-are not followed, the need for
chemicals will be unden'iajlle. These. methods include: :1. eliminating overhead
watering; 2L growing plants'.on-raised benches in enclosed .structures; .3. using
pathogen-free plants;. and 4. using pathogen-free pots and media.i: In-fth future,
methods such as those discussed here will become more important as the'i6st of applying
chemicals increases every day and may become prohibitive. The current trend of the
.Environmental Protection Agency (EPA)'is toward removal of all dangerous. pesticides
from use.-: There are. very few currently available pesticides non-toxic tbohumar: and
the environment and registering new chemicals becomes more difficult each year. It
makes. sense to grow plants in the best way, possible starting.now, and to'begin to
loosen the hQl.d-we have on chemicals as much as-possible. '. '
* .r t :
C'.. *.' .;
-. ORGANISM. .' i
*'V IRO'NMiET -HOST PLANT