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It looks like tobacco growers will have to hold field-ready transplants for a few days or more this year. Often this coincides with periods of humid cloudy weather. Densely packed plants in a humid environment is a recipe for big problems. Below are some suggestions for holding plants until field conditions improve.
Maintain a low level of fertility: Reducing fertility levels to the 25 to 50 parts per million nitrogen range will slow growth and reduce the amount of rank green leaf material that may be more susceptible to bacterial rots.
At a lower level of fertility, plants may be more susceptible to target spot so appropriate controls for target spot must be kept in place. For best control of target spot, make sure that ventilation is optimal and apply a mancozeb fungicide (Dithane DF, Manzate ProStick, or Penncozeb DF) on a routine basis. Use a rate of 0.5 lb of product per 100 gallons of spray solution (1 teaspoon per gallon), and apply 5-7 gallons of mix per 1,000 square feet of bed (roughly 400 trays). Mancozeb should be applied every 5-7 days until plants are taken to the field.
Keep water levels up: The tops of the trays should be kept above the level of the boards in the bed. This will help to facilitate air movement across the tray surface and improve drying, important factors for managing foliar diseases.
Maintain regular clipping. Clipping plants down to no more than 1-inch above the bud will help to slow growth and hold plants at a reasonable height for planting. Regular clipping also improves air and light penetration into the tray surface, and this will be a great help in keeping diseases such as target spot and collar rot in check.
Facilitate good air movement. Along with proper clipping and keeping water at optimal levels in float bays, make every effort to keep air moving on plants in float beds. This means lowering side-curtains in greenhouses for as long as possible and running circulation fans if available.
Keep an eye out for Pythium root rot: The potential for severe Pythium root rot increases as the days become warmer. When float water reaches temperatures of 75 F and higher, disease spread is rapid and the degree of damage intensifies.
It’s not a bad idea to assume that Pythium will get into to even the most sanitary float system late in the season, so preventive Terramaster use is the best practice. In most cases, a single application of this fungicide at 0.7 to 1 fluid ounces per 100 gallons of float water, made at around three to four weeks after seeding, will protect plants until setting time.
However, if plants need to be held longer, a second application may be necessary – keeping in mind that the label prohibits use of Terramaster any later than eight weeks after seeding. If a second (or in some cases first) Terramaster treatment is needed, get it in the float water before the 8-week restriction comes into play. If disease is present before the fungicide goes in the water, use a higher rate (1.4 fluid ounces per 100 gallons of float water). Keep in mind that if disease is severe, there could be a loss of useable transplants despite the curative application of Terramaster.
Make sure to calculate the correct dose and mix the fungicide thoroughly to avoid severe injury. Even at lower rates, you may see root burn and slow growth on plants and these are more pronounced at higher rates, which may not be all bad if you are trying to hold plants.