Midwest Vegetable Guide - 2022 - 25

Transplant Production
Damping-off may occur before or after seedlings emerge
from the soil. Preemergence damping-off occurs when fungi
infect seeds as they germinate. As infections progress, seeds
rot and eventually disintegrate. Poor stands become apparent
after several days or weeks.
Postemergence damping-off is usually observed in seed flats
or among transplants. Fungi infect stems at or near the soil
surface. The affected area of the stem takes on a watersoaked
appearance and sometimes becomes constricted.
Eventually, the stems are unable to maintain the structural
support of seedlings, which usually collapse and die within
24 to 48 hours.
Several soilborne fungi cause damping-off on vegetables.
Fusarium, Phytophthora, Pythium, and Rhizoctonia species
are well known causal agents of pre- and postemergence
damping-off. Control measures to prevent damping-off
diseases include:
* Using uncontaminated soil mix. Use a commercially
prepared soilless growing mix sold in 3 to 4 cubic foot
bales or bags. A common mistake is to open a bag of
" clean " soil mix and place it on a dirty floor or some
other unclean surface prior to planting. Remember that
your soil is only as clean as the dirtiest surface it has
contacted.
* Planting seeds shallow and in warm soil.
* Using soil mixes that drain well.
Seedborne and residueborne diseases affect most vegetable
crops. The pathogens (disease-causing microorganisms)
survive in or on seeds or plant residues, not in soil mixes.
Outbreaks of these diseases often show up as clusters of
diseased plants, and symptoms often include brown lesions
with yellow halos on leaves. By contrast, environmentally
induced problems often occur uniformly throughout the
seedlings or only in one location (for example, close to an
outside wall).
Several different fungal, bacterial, or viral pathogens may be
introduced into a transplant facility via contaminated seed or
transplants (Common Seedborne Diseases of Vegetable
Crops Frequently Grown as Transplants).
Once introduced, these pathogens may continue to cause
problems year after year if proper precautions are not taken.
Common Seedborne Diseases of
Vegetable Crops Frequently Grown
as Transplants
Vegetable Crop Disease
cabbage
cantaloupe
cucumber
pepper
squash
tomato
watermelon
Alternaria leaf spot, black rot
anthracnose, gummy stem blight
angular leaf spot
bacterial spot
squash mosaic (squash mosaic virus)
bacterial canker, bacterial speck, bacterial spot
anthracnose, gummy stem blight, bacterial fruit botch
Several measures should be taken to minimize or prevent
introducing seedborne or residueborne pathogens into a
transplant facility:
* Avoid saving seed unless you are specifically trained and
equipped for seed production.
* Inspect seedlings frequently while they are growing.
* Separate seedlots from one another. Save all information
regarding seed purchases.
* Irrigate in the morning to ensure soil and leaf surfaces
dry.
* Check fungicide and bactericide labels for specific
mentions of greenhouse use when treating transplants
(see the Conversions for Liquid Pesticides on Small
Areas table, in the Pesticide Application and Safety
section, for liquid pesticide conversions).
* Practice good sanitation. Plant pathogens often survive in
soil and plant residues. Therefore, sanitation is as
important for a greenhouse as it is for a kitchen.
Greenhouse floors should be as free of soil and residue
as possible; plastic or cloth floor coverings provide a
barrier between dirt floors and transplants. Transplant
trays and flats should be new or cleaned and disinfected
before each transplant generation.
More detailed information about disease prevention and
control in the greenhouse is available in Preventing Seedling
Diseases in the Greenhouse (Purdue Extension publication
BP-61-W), and Commercial Greenhouse and Nursery
Production: Sanitation for Disease and Pest Management,
available from the Purdue Extension Education Store (HO250-W),
edustore.purdue.edu.
A few chemicals are labeled for disease control in greenhouse
vegetable crops. Restricted use pesticides can only be used by
certified pesticide applicators who have the greenhouse
certification on their applicator licenses. Restricted use
pesticides are identified prominently on the label.
If a pesticide is not restricted use and is labeled for the crop
in question, check the label. If it does not mention greenhouse
use, then it may be used in greenhouses. Otherwise, the label
may explicitly prohibit greenhouse use. Thus, a specific label
for greenhouse use for some products is not required; but you
must carefully read each label to be certain the greenhouse
Midwest Veg Guide 2022
25
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Midwest Vegetable Guide - 2022

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