Watch For Early Seedling Diseases

Every year greenhouse growers experience disease problems on their bedding plant seedlings. Here is a quick review of the three most common diseases:

Pythium damping off is caused by many species of the fungus Pythium. The fungus is present in soil, soilless medium and water. Untreated water could be a major of Pythium. Waterlogged conditions, lower oxygen levels in the root zone, and damaged and injured roots favour the development of this disease. Immediately after transplanting the seedlings are very susceptible to this fungus. The pathogen attacks the fine root hairs causing water stress symptoms in the seedlings. Leaves will appear dark green and seedlings will be stunted. The fungus can also attack the basal portions of the stems and crown causing a keeling over of the plants.

To protect seedlings from infection by this pathogen:

avoid damage to root hair during transplanting
use ready-made plugs
if using transplanting mix - pre-wet the mix
water thoroughly immediately after transplanting
keep the growing medium moist
don't follow wet-dry regime
avoid waterlogged conditions
make adjustments for cloudy or sunny weather conditions
select a well-draining growing medium
a highly decomposed medium will water-log easily and will not dry out properly
avoid high sodium water, it destroys the texture of the medium and leads to waterlogged conditions
treat water with bleach or hydrogen peroxide to reduce the pathogen population
control fungus gnats, the insect is known to spread the pathogen
after accurate diagnosis of the disease apply recommended fungicides according to the label instructions.

Botrytis gray mold is caused by Botrytis cinerea, can be a very devastating disease of seedlings in plug or germination trays. The pathogen grows well on dead plant material. At high plant densities a microclimate of high humidity is created in the trays which is the most important environmental condition for the development of this disease. High humidity combined with cooler night temperature can be disastrous for seedlings. The pathogen under these conditions produces billions of spores, which become airborne, germinate and start many new infections resulting in a fast spread of the disease. The main strategy to control this fungus is to keep the air moving around the trays and avoid a sudden drop in air temperature. Use registered fungicides, if needed.

Powdery mildews are caused by several different pathogens. These are obligate parasites (live only on living plant material). The pathogens overwinter by producing thick-walled, sexual surviving structures. Although a high relative humidity is required for initiation of this disease, once the disease has started, it can flourish under low humidity conditions. Under these conditions, powdery mildew can appear on cotyledonary leaves of seedlings. Best strategy for control this disease is prevention. Keep the greenhouse surroundings free from weeds. Remove infected leaves to check the spread of the disease. Use recommended fungicides on timely basis.

P. Bains and M. Mirza, CDCN
Greenhouse Coverings - January 2002

For more information about the content of this document, contact Shelley Barkley.
This information published to the web on July 4, 2002.
Last Reviewed/Revised on June 20, 2008.