With all of the concerns as to the high costs associated with
heating the greenhouse it is hard to believe that I would be trying to
convince growers to use more heat. Well, I'm not. I am telling you if
you use the heated air you have more efficiently you can cut down on the
amount of disease you see, particularly in the early stages of
production.
Many of the diseases associated with seeds and seedlings are enhanced
by cool temperatures. In many cases the pathogens that cause disease
are more active at cool temperatures and in cool temperatures the seeds
and seedlings are less active. This scenario has the potential to be a
real problem. One of the most common mistakes a grower makes is to put
seed or seeding flats on an unheated floor. The unheated the floor of
the greenhouse is the coolest place in the greenhouse. This is
particularly the case when if the floor is concrete or bare soil. The
temperature of this type of floor can be 40 or 60 F. Under these
conditions most seeds will take longer to germinate than they should and
they will be exposed to pathogenic fungi such as Pythium sp.
and Fusarium sp. Figure 1A Figure 1B. If the surface under the seed flats
were heated to the ideal temperature for seed germination the seed would
germinate sooner and be less likely to be affected by disease ( Figure 2 and Figure 3).
Even after the seeds germinate and emerge from the medium bottom heat
is a good idea. Seedings can be affected by post-emergence diseases
caused by Pythium sp., Rhizoctonia sp. and
Botrytis sp. ( Figure 4 and Figure 5).
What can a grower do besides cranking up the heat or installing
heated floors? Don't jack-up the heat, just get the flats up off the
floor. If you can put them on a bench. That would be best. That way
the flats are up in the heat zone of the greenhouse and warm air in
circulating under the flats. If a bench is out of the question then at
least remove the flats from direct contact with the floor. Use pressure
treated 4x4s, old wood pallets or some other inexpensive way to elevate
the flats. Six inches off the floor will make a world of difference.
This will allow some of the warm air in the greenhouse to circulate
under the flat and heat the media. The warmer the media the less likely
you'll see pre- and post-emergence diseases.
