Using float bed systems has many advantages over growing transplants in outdoor beds. It requires less labor, and there’s less worry about the effects of weather. Also, greenhouse-grown transplants typically lead to a more uniform crop in the field.
This growing technique does have one major disadvantage, however. It can create near-perfect conditions for the growth and spread of Pythium spp., a fungus-like plant pathogen that uses the bay water to travel among the seedling roots, causing infection throughout the system.
Pythium root rot is considered the most important disease affecting tobacco seedlings—both burley and flue-cured—grown in greenhouse float bed systems. “Just about everyone has Pythium pressure,” says Emily Pfeufer, Extension assistant professor in plant pathology at the University of Kentucky.
Infection can lead to decreased transplant vigor and, if allowed to flourish, could delay or even destroy an entire season’s crop. Luckily, if growers take the recommended measures listed below, Pythium root rot can be controlled.
Pythium root rot starts around 25 days of growth. At this time, the roots have grown down into the bays, exposing them to any pathogens in the water. First, the seedlings’ leaves become yellowed.
From there, the plants begin to wilt, and the roots turn a light brown or gray color and become slimy. Eventually, the roots can fall off, and the transplants become unusable or, at the very least, poor-quality and weak.
In the field, these weak transplants will grow slowly and be more susceptible to transplant shock, heat, drought and disease.
Extension specialists recommend a two-pronged approach to preventing an outbreak of Pythium root rot. Avoiding contamination and using proper sanitizing techniques in the greenhouse are very important, but because this pathogen is so common in soils and water and is easily transported, preventive application of fungicide is recommended in most cases.
There are several ways to keep Pythium spp. out of float bed systems. Use municipal or well water to fill the bays instead of surface water from ponds or creeks because these waters are frequently contaminated with the pathogen.
Another source is the growing trays. Check trays for pitting or other damage, and, if possible, buy new trays each year. If reusing trays, properly sanitize them with either steam (hold at 165 degrees to 175 degrees for at least 30 minutes) or bleach (immerse cleaned trays in a 10 percent household bleach solution for several minutes and then rinse thoroughly).
If using bleach, keep in mind that it loses its effectiveness as it gets dirty, and workers need to protect their eyes and skin when using it. As trays age, they will become much harder to fully sanitize, so they should be replaced after a maximum of three seasons.
Other ways of preventing contamination include using soilless growth media instead of field soil in the trays; keeping trays clean during transplanting and off-season storage; and treating shoes, tools and equipment with a 10 percent bleach solution before entering the greenhouse. Some growers even keep clean greenhouse shoes near the door and never enter the greenhouse wearing field boots.
Because Pythium root rot can so quickly invade a float bed system, preventive fungicide use is recommended. Terramaster 4EC is registered for prevention and control of Pythium root rot in tobacco greenhouses and has been proved to be very effective.
It should be applied at preventive rates just as the roots emerge from the tray bottoms, usually two to three weeks after seeding. Typically, the initial application is all that is needed when using new or well-sanitized trays, but the fungicide may be applied every three weeks in higher-risk operations.
Terramaster 4EC is also labeled for curative applications, but using it as a preventive can stop the pathogen before widespread disease can take hold and is much less likely to cause plant injury. “Preventive Terramaster application is like insurance—and a fairly small price to pay for healthy transplants,” says Pfeufer.