The development of effective
disease-control systems in the greenhouse involves knowledge of production
systems, water, growing media, cultural techniques, etc. as well as
the discipline of plant pathology.
It is very important to take time to know the organization one is
working. In a very real sense, one works with people as well as plants.
One needs to know who has the responsibility for what, and where in
the organizational structure critical decisions are made and programs
actually implemented. One of the major advantages of establishing
long-term relationships is the opportunity to view operations in detail
over an extended period of time. Many times it is not the obvious
gross errors that result in the development of an important problem.
Often problems that arise are the result of less-obvious minor slip-ups
that are not seen until one has established an atmosphere of trust
with personnel at various levels of responsibility. Insight and communication
with people are extremely important. After all, it does not do much
good to diagnose problems and develop effective control programs if
they are not properly implemented.
The levels of tolerance for various pathogens in transplants varies.
For instance the tolerance for Acidovorax avenae subsp. Citrulli (bacterial
fruit blotch) on watermelon, Xanthomonas campestris pv. campestris
(black rot) on crucifers (Cruciferae), and lettuce mosaic virus in
lettuce are zero. Meeting these tolerances requires very precise and
restrictive measures. On the other hand some infection with downy
mildew on crucifers and lettuce transplants (caused by Peronospora
parasitica and Bremia lactucae, respectively) is tolerated. Satisfying
these less-stringent requirements can sometimes be easier. Techniques
for the production of plug transplants in greenhouses are especially
conducive to the introduction and rapid spread of pathogens. Plant
populations are very dense, and humidity is high. Overhead watering
systems keep the foliage wet for long periods of time, and ebb and
flow irrigation systems can spread many root-invading pathogens rapidly.
Even limited amounts of inoculum introduced under these conditions
can result in serious plant losses to disease in a short period of
time.
The detection of a disease in the greenhouse in its very early stages
of development is of utmost importance. For some diseases such as
bacterial fruit blotch in melons or black rot in cauliflower and brussels
sprouts (Brassica oleracea var. gemmiferae) the very presence of these
diseases in the growing crop of transplants may make them unmarketable
for use in the field. With other diseases, frequent rigorously timed
fungicide and bactericide applications may be helpful in suppressing
serious spread. Regulatory restrictions, however, have limited the
number of pesticides that can be used in many situations. It is important
to read pesticide labels carefully and to be aware of any changing
regulations on application procedures.
The best way to control diseases is to keep them out in the first
place. In this respect the use of disease-free seeds is very important.
Fungicide/ Bactericide, hot water soak and sodium hypochlorite treatments
of seeds each have a place in disinfecting seeds of certain pathogens
such as Xanthomonas campestris pv. campestris of cruciferous vegetables
and Septoria appi (septoria blight) of celery. With certain crops
such as watermelon and lettuce, seed testing systems (both public
and privately operated) are in place that provide very good information
on the health of particular seed lots that purchasers of seed can
use to assist in making intelligent decisions.
We have found that the location of greenhouse systems can be very
important. Some considerations which can influence the introduction
of pathogens and insects that serve as a vector of certain pathogens
are surrounding vegetation both cultivated and native, prevailing
wind direction and speed, and the potential for windblown dust from
adjacent cultivated fields. For instance, lettuce mosaic virus and
one of the principle aphids that vectors the disease, green peach
aphid (Myzus persicae), can survive in wild lettuce (Lactuca serriola).
Sclerotinia sclerotiorum has many cultivated and native hosts, and
windblown ascospores can readily infect greenhouse-grown crops. Unpublished
information by the writer indicates that Plasmodiophora brassicae
(the cause of club root disease of cruciferous plants) can be moved
via windblown dust into the greenhouse.
Some greenhouse managers go to the extent of severely restricting
the entry of growers and other field personnel into the greenhouse
for fear that they may be carrying important pathogens on their clothing
or hands.
Most overhead watering systems are designed to distribute water through
flat fan nozzles. High water pressure can result in water soaking
of the leave of sensitive plants during irrigation, aiding in bacterial
infection and spread. It is important to monitor the water pressure
and to adjust it to as low a level as is consistent with even water
distribution over the surface area. Experience suggests that the pressure
should be no more than 20 lb/ inch2 (138 kPa) at the boom level. In
some cases it may be necessary to modify the irrigation boom to achieve
this objective. High water pressure also contribute to the spread
of fine water droplets for extended distances leading to the rapid
spread of pathogens from very small infection centers.
Fungus gnats (e.g., Bradysia spp.) and shore flies (e.g., Scatella
stagnalis) are very troublesome insects in greenhouse systems. It
has been demonstrated that these insects are capable of carrying certain
bacterial and fungal diseases in an on their bodies, leading to spread
of pathogens for short distances. These insects are very difficult
to control and almost impossible to eradicate, but it is important
to make every effort to do so. In addition to registered insecticides,
there are several biocontrol agents for fungus gnats available such
as the Bacillus thuringiensis product Gnatrol (Valent BioSciences
Corp., Libertyville, Ill.), the parasitic nematode Steirnernema feltiae,
and the predatory mite Hypoaspis miles. However, the cost and the
specific application requirements of the biocontrol agents preclude
their widespread use in vegetable transplant systems in the western
U.S. at this time.
Plant nutrition can play a role in the development and spread of some
pathogens. For instance it has been shown in unpublished limited trials
that low nitrogen levels can result in severe downy mildew infection
on cauliflower whereas adequate levels retard the progress of infection.
We have also observed that infection of romaine lettuce by Botrytis
cinerea (gray mold) has been especially severe in some plantings where
low nitrogen levels have resulted in yellowing and senescence of the
cotyledons leading to tissue that is susceptible to infection by this
organism.
Sterilization of reusable containers such as plug trays is very important.
Plastic containers often trap bits and pieces of roots in the bottom
of the plant cells. Dried leaves and stems can also adhere to the
surface. Styrofoam containers present an additional problem in that
roots grow into the Styrofoam. In either case, the best surface sterilization
techniques (i.e., high pressure chlorine washes) are inadequate to
completely kill all of the potential inoculum residing in this difficult
to remove plant tissue. Moist heat is the most effective way of killing
this inoculum. Growers who have experienced repeated problems have
converted to steam sterilization of containers and have achieved good
results. With only a few exceptions, most plant pathogenic bacteria,
fungi and viruses are eliminated when exposed to moist heat at temperatures
of 140 to 160 °F (60.0 to 71.1 °C).
It is important also to frequently sterilize greenhouse surfaces such
as benches, floors, walls, planting areas etc. Chlorine bleach, alcohol,
quaternary ammonium chloride salts, hydrogen dioxide/ peroxyacetic
acid and other compounds are useful for this purpose. Consult federal
and state pesticide regulations governing the use of any pesticidal
compound before applying.
The concentration of the same or very similar types of plants or the
serial planting of these in large contiguous areas increases the possibility
of rapid spread of disease. Separating large groups of plantings from
one another can be very helpful in reducing high inoculum pressure
that override otherwise effective control measures.
I have found that in designing workable and successful disease-control
programs or in diagnosing problems there is the need to look repeatedly
at production systems in detail. One often misses something, some
little detail that is vital, the first few times one goes over a system.
That small detail may be the key to many other things. Repeatedly
reviewing these systems and perhaps even talking to or observing the
personnel doing the actual work often reveals a key point that leads
to a solution to the problem.
There are many other considerations in the production of pathogen
free plug transplants. There is a great deal of literature available
that gives valuable guidance to those involved. One of these is a
recently published, comprehensive, integrated pest management (IPM)
manual available from the University of California as part of the
Statewide Integrated Pest Management Project. This is a comprehensive
manual that covers many aspects of greenhouse production besides pathology.
As most growers realize, the production of healthy planting stock
is not an easy process, but the persistent adherence to detail is
the pathway to success.
