Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
This is the time of the year when county agents are called to look at disorders in transplants being grown in greenhouses and when samples routinely come into our offices for diagnosis.
There are many diseases of vegetable transplants that can start in the greenhouse – fungal, bacterial, and viral. Diseases should be considered first when looking at transplants. Insects such as thrips, aphids, and whiteflies also can be a problem in greenhouses and should also be considered as causes of injury. They can cause direct damage and can be vectors of virus diseases.
However, many vegetable transplant disorders are not cause by pests. Some of the most common are:
Excessive Stretch and Leggy Plants
This is most commonly due to too high of temperature differential in growing houses (wide differences between day and night temperatures), excessive fertilization (especially with ammonium N fertilizers), and excessive watering.
Irregular Growth
This can have many causes including differences in seeding depth, differences in tray filling, differences in watering, differences in location in the greenhouse, irregular heating in the greenhouse (hot and cold spots), and differences in media to name a few.
Salt Injury
Plant desiccation and injury due to high salts occurs commonly when fertilizer rates are too high or when dumping occurs from slow release fertilizers at high temperatures.
Leaf Scorching
his can be due to salt injury also, but can occur when plants that are overcrowded are then spaced and exposed to full light or when very tender plants are put out to harden off in windy conditions.
Nutrient Deficiencies
Iron deficiencies are common if media pH rises above 6.3. Calcium and magnesium deficiencies are common if media pH drops below 5.2. Nitrogen deficiencies from under-fertilization are also common and also where initial nutrient charge in the media runs out.
Stunting
Poor plant growth or stunting most commonly is due to lack of nutrients in the media (media is missing initial nutrient charge). It also can be due to excessively cold greenhouse temperatures.
Ethylene Injury
Crops grown in greenhouses with propane or gas-fired unit heaters that are malfunctioning can be susceptible to ethylene injury. Ethylene (C2H4) is an odorless, colorless gas that acts as a plant hormone. Symptoms range from misshapen leaves and flowers, thickened stems, stunted growth, flower or leaf abortion to stem curling and wilting.