J. Michael Moore, Paul E. Sumner, David C. Jones and Paul F. Bertrand
Production Program for Bare Root Transplants
Plantbed Problems
Plantbed Fertilization
Tobacco Plantbed Insect Control
Plantbed Mechanization
Bed Clipping Practices in Relation to Seedling Diseases
Producing quality bare root transplants in plantbeds requires much effort and attention to detail on a daily basis. Following is an outline of management practices which has proven to be effective over the years. Attention should be given to each step.
Production of healthy transplants is the first step toward a successful and profitable crop. Poor quality transplants may cause a variety of early-season problems and may predispose the plants to season-long problems accentuated by disease and/or weather resulting in lower yields and leaf quality. Costs of production are increased by beds with poor stands, plants damaged by improper fertilization, inadequate cover management, poor moisture management, and replanting fields with poor stands. Adequate, but not excessive moisture is a must to ensure timely seed germination and proper growth of young transplants. Drainage, cover management, and irrigation all play a part in moisture management.
Locate beds in sites with good surface and internal drainage. Crown or raise beds to provide for natural removal of excess water. Install ditches or drains around beds to remove standing water. To avoid water soaked spots in beds the plastic tarp should remain covered until time for seeding. Rake or shape beds if necessary prior to seeding to remove low or uneven areas. Large amounts of water do not normally penetrate perforated plastic covers, but beds should be checked closely for wet spots after prolonged rainfall.
Perforated plastic covers are used by a majority of tobacco transplant growers in Georgia. Plastic used for covering fumigated beds is perforated by punching out 1/4 inch holes every 2 inches. Perforated plastic may eliminate the need for purchasing an additional cover for the beds. Plant growth may be more consistent and transplanting date more accurately predicted with plastic than with some synthetic or woven covers.
After germination, remove plastic covers from beds when air temperatures under the plastic reach 100 degrees F before the plants reach the size of a quarter. Older plants may tolerate temperatures up to 110-115 degrees F for short periods of time depending on existing humidity under the plastic. As a rule-of-thumb plastic covers should be removed after outside temperatures have reached 75 degrees F for two consecutive days and replaced when temperatures are expected to drop below 45 degrees F. A thermometer placed so that it may be read through the plastic provides a sure way of monitoring temperatures under the plastic. Transplant producers who have large numbers of beds, other farm enterprises, off-farm employment, or other time restraints may have difficulty properly managing perforated plastic covers.
Some growers choose to replace perforated plastic with synthetic or woven covers as the daily temperatures increase. Advantages of these covers include improved ventilation, no build up of radiant heat, and increased infiltration of rainfall or irrigation. Spray applications may be made without removing these covers. Covers need only be removed to provide access for clipping. Beds covered with synthetic covers have a tendancy to dry out in advance of plastic covered beds and need additional monitoring and irrigation.
Plantbeds should be irrigated following seeding to settle the seed and provide good seed to soil contact. Soil should be wet six to eight inches deep (1/2 to 3/4 inches of irrigation) before covering with plastic covers. Leaving the beds uncovered until it rains is risky as heavy rainfall may puddle seeds and result in poor plant distribution or washed out beds.
Tobacco seedlings require relatively high amounts of nutrients to insure the rapid growth desired for transplanting. However, the temptation to over fertilize plantbeds should be strongly avoided. Excessive rates of N and K fertilizers can decrease stands and damage young growth. Heavy N applications can cause plants to become too tender and succulent, and require more frequent clipping. Excessive succulent growth can increase the potential for disease under the plant canopy. Tender plants may be damaged when pulled and transplanted. Tender plants can be more easily damaged by sun scalding or by blowing sand once transplanted.
Select sites for plantbeds early in the fall. Have the soil tested for lime and fertilizer requirements. Be sure to identify the crop as a "tobacco plantbed." Apply needed lime as early as possible to allow sufficient time to react with the soil.
Apply a complete tobacco plantbed fertilizer that will supply at least 3 and no more than 6 pounds of N per 100 square yards. This may be obtained with 50 to 100 pounds of 6-12-6 per 100 square yards (2400 to 4800 pounds of fertilizer per acre). Use lower rate with plastic covers. Mix fertilizer with the upper 2-4 inches of soil prior to seeding. Irrigate to wet soil to a depth of 6 to 8 inches (1/2 inch of irrigation) after seeding but before covering to prevent fertilizer salt injury and promote uniform germination. Since much of the growth will occur in cool soils which have been fumigated, at least 50% of the applied N should be in the nitrate form.
Mineral sources of fertilizers usually provide more consistent results than organic sources. Organic sources can encourage buildup of some insect pests, and can provide a favorable environment for damping-off diseases, both of which reduce the stand. Natural organic materials have however, been touted as slow release forms of N that will continue to provide N over a long period of time, and offer protection against leaching.
Studies with these materials indeed show an initial slow availability of organic N; it takes about three to four weeks for unusable organic N to convert to inorganic forms which the plant can use. Of the organic N which becomes available over a 90 day period (about 30-50% of the total N), essentially all is converted during these first few weeks. Thus little protection against leaching results. The uncertainty of this conversion process, and the variability of natural organic compounds further complicate use of these materials. In view of the disadvantages and additional costs of organic sources, mineral sources of fertilizers are strongly recommended for plantbeds.
Excessive rainfall can leach nutrients out of the root zone, resulting in yellow, nutrient deficient plants. Tissue analysis is the only sure way to know the cause of the yellowing, but leaf symptoms can sometimes provide a clue. If the plants show yellowing of leaves and stems on the lowest leaves first, the problem is usually N deficiency. Additional nitrate-N may be applied once plants are dime-sized to maintain desired color and promote rapid growth. Limit application to 0.5 to 1 pound nitrate nitrogen (3 to 6 pounds of nitrate of soda or calcium nitrate) per 100 square yards which is equivalent to 25 to 50 pounds of N (150 to 300 pounds nitrate of soda or calcium nitrate) per acre. If color does not improve within a few days, N deficiency is not the problem. If the whole plant is yellow, sulfur may be deficient, and extra N will not correct the problem. Magnesium deficiency symptoms are similar to those of nitrogen in that the lower leaves turn yellow first; however, the veins of Mg-deficient plants remain green. Magnesium and sulfur deficiencies may be corrected by applying potassium-magnesium-sulfate (K-Mag or Sul-Po-Mag) at a rate of 3 pounds per 100 square yards (150 pounds per acre) or Epsom Salts at a rate of 5 pounds per 100 square yards (250 pounds per acre).
Do not apply fertilizers to wet foliage. Irrigate after application to wash fertilizer material off the leaves. Pelletized materials are less likely to cause leaf burn, but are somewhat slower to dissolve than pulverized materials. When applied properly, either source should be safe and effective.
TMV is spread mechanically by handling plants, cultivation, etc. It is favored by excess nitrogen. Symptoms can be quite variable and often disappear in mid Summer. Control is achieved through crop rotation, root and stalk destruction, and careful handling during transplanting as outlined below.
Note: These precautions are important. TMV may not show up until after transplanting at which time precautions are too late. As soon as possible after completion of transplanting, destroy the plantbeds.
Table 1. Plantbed Fumigation and Disease Control
| Chemical and Formulation | Rate | Remarks and Precautions |
| Weeds and Nematodes | ||
| Methyl Bromide, 98% | 9 lbs/100 sq yd | Apply Methyl Bromide When Soil Temperature Is above 50
Degrees F. Cans: Release gas under plastic covering. Plastic should be raised in the center and sealed around the edges with soil. Leave covered 24 hours. Remove cover 72 hours prior to seeding. |
| Methyl Bromide, 68.6% | 10-12 lb/100 sq yd | Cylinders: Inject into well prepared soil with chisel applicators. Cover immediately with plastic and seal all edges. Leave covered for 48 hours. Remove cover 72 hours prior to seeding. |
| Blue Mold and Pythium Damping-Off | ||
| Dithane DF | 1.4-2.4lb/Acre (0.7-1.2 oz./150 sq yd) |
Use as an over the top spray and continue on a 7-day schedule as long as conditions warrant. |
| Rhizoctonia Damping-Off and Target Spot | ||
| Carbamate WDG or Dithane DF |
3 lb/100 gal water
1.4-2.4 lb/Acre |
Apply when needed. Spray twice a week beginning when plants are dime sized - continue through transplanting. Thoroughly cover all plants. Use as a drench when damping-off is present in the bed. |
Insects can cause severe damage in tobacco plantbeds. The most common pests in tobacco plantbeds are vegetable weevils, tobacco flea beetles, cutworms, and mole crickets.
Vegetable weevil larvae chew small irregular holes in the leaves and often feed in the buds and on the stems. Tobacco flea beetles feed on the leaves, leaving numerous small punctures. Cutworms eat small half circles in the leaf margins and cut young plants off near the soil surface. Mole crickets burrow in the upper 2 inches of soil and feed on the roots and stems of plants, uprooting them.
Other pests which cause occasional problems include green June beetle larvae, slugs and snails, aphids, and tobacco budworms. Aphids and tobacco budworms, carried from the plantbed on transplants, can start early infestations in the field. Plantbeds should be checked once or twice each week for insects and their damage. Recognition of the insects present, their abundance, and their damage is essential in deciding whether or not to use an insecticide. Recommended insecticides for use on tobacco plantbeds are listed in Table 2.
| Table 2. Plantbed Insect Control | |||
| Insect | Chemical and Formulation 1 | Rate per 100 Sq. Yds. | Remarks and Precautions |
| Aphids (preventive control) |
disulfoton (Di-Syston 15G) |
9 ozs | Apply granules broadcast just before seeding or over top of small plants - water in immediately. Do not exceed recommended rate. |
| Aphids, Flea Beetles or Vegetable Weevils |
acephate (Orthene 75S) |
1 tbsp in 1 gal water (1 lb/Acre) |
Apply to foliage as needed |
| Budworms | acephate (Orthene 75S) |
1 tbsp in 1 gal water (1 lb/Acre) |
Apply to foliage as needed. |
| Cutworms | acephate (Orthene 75S) carbaryl |
1 tbsp in 1 gal water (l lb/Acre) 0.5-1 lb |
Apply to foliage in late afternoon or at
dusk.
|
| Green June Beetle Grubs | carbaryl (Sevin XLR Plus) |
5.5 oz. in 50 to 100 gals water | Apply only to uprooted areas of bed. |
| Mole Crickets | Orthene 75S and Sevin 5% B |
5.5 oz. in 50 to 100 gals water | See remarks under cutworms. |
| Slugs and Snails | Metaldehyde 5%B | 2 lbs | Scatter bait evenly at dusk around margins, walkways and open spaces in bed. Do not apply directly to plants. |
Direct seeding tobacco plantbeds has become popular in recent years. Some facts should be noted before one considers direct seeding tobacco beds.
The tobacco bed area should be prepared and fertilized normally. Prior to seeding, the bed area should be worked lightly to provide a firm seedbed. Make sure that the press wheel of the planter makes a 1/2 to 1 inch depression in the bed at seeding. This will cause the plastic cover to rest on the soil, not on germinated seeds. The depression may serve the same function as placing straw on the bed area. Coated seed must be placed in direct contact with the soil. A concerted effort should be made to interface the soil and seed as much as possible. The seeded bed should be irrigated as normal. Care should be taken not to used large volume sprinklers. These sprinklers will enhance the chance of seed being splashed out of the drill resulting in less germination. Use a combination of high pressure and small nozzle opening in the sprinkler head to produce small water droplets.
The tobacco seed are coated with a clay material so that they can be metered for a specific seed spacing. Normally, they are coated to a diameter of 6.5/64 to 8.5/64 inch. Commercial seed companies are marketing coated tobacco seed. Check with your local seed supplier for cost and availability.
The type of planter to use is one that will drop seed at a uniform rate. Large commercial plant growers are using the Stanhay Precision planter. This unit will drop seed in two rows per planter spaced two inches apart. Distance between planter units should be 10 to 12 inches. This will give a spacing between double rows of 8 to 10 inches.
Seed should be spaced 1/2" to 13/16" apart resulting in approximately 35 to 45 seed per square foot of bed. At this seeding rate the estimated plant population for a 100 square yard bed is 30,000 to 40,000 plants. Seeders or planters can be mounted on a tractor tool bar to insure uniform rows of seed.
Tobacco seedlings produced in traditional plantbeds vary greatly in size and suitability for transplanting. Because of plant variability, only 10 to 20 percent of the total seedlings in the plantbed are available for transplanting in any one pulling. Tobacco growers often adjust to this shortage in plant availability by allowing more plantbed yardage per acre of transplanted tobacco or by transplanting fewer acres per day.
Clipping seedlings increases the uniformity of seedling-height.
Most of the clipping research conducted in the United States has been aimed at increasing the uniformity of seedlings for once-over, mechanized removal from the plantbed. Although mechanized pulling is still in the future, the knowledge gained from this research is useful to growers now pulling plants by hand.
Research indicates that clipping increases the number of desirable seedlings by 10 percent. This increase can be translated into a 10 percent reduction in plantbed yardage required per acre of tobacco.
The biggest advantage of clipping is an increase in desirable seedlings available in a single pulling. Nearly twice as many desirable seedlings can be obtained from a clipped plantbed as compared to a non-clipped plantbed.
Tobacco growers using clipping should be able to increase the efficiency of seedling production. The 10 percent increase in the total number desirable seedlings coupled with a 100 percent increase in the number of seedlings obtained in one pulling should allow growers to reduce plantbed yardage from the traditional 80 to 100 sq. yard per acre to 60 to 80 sq. yard per acre, provided good overall management is maintained.
Tobacco growers are often faced with the need to delay transplanting. This delay can be caused by wet soil conditions, dry soil conditions, or the need to allow 21 days between fumigant application and transplanting. These delays are difficult because seedlings in plantbeds continue to grow rapidly.
Results from research in North Carolina indicate that clipping tobacco plantbeds two times at four to five day intervals can delay transplanting by 7 to 10 days. An additional clipping can be used to extend the delay 12 to 13 days.
Research conducted in Georgia indicates that seedlings can be clipped, pulled and stored in boxes for three days without any observable loss in yield. Storage allows for an extra delay in transplanting either when additional clipping is not practical or when transplanting must be delayed for a prolonged period.
Studies in North Carolina have been conducted comparing the field performance of plants from clipped and non-clipped plantbeds. These studies were conducted over several years first at research stations and later in 19 on-farm tests. Results from the research station experiments showed that yield from clipped plants was equal to and, in some cases, higher than plants from non-clipped plantbeds. On-farm tests show similar results. Quality of the cured leaf produced from clipped and non-clipped beds has been similar in all research and on-farm tests.
A high-suction lawn mower may be adjusted to clip at a height just above the buds of the largest plants. This removes portions of the larger leaves from the largest plants and allows sunlight to reach the smaller plants. Growth of the clipped plants is reduced, and the smaller, non-clipped plants can then catch up with the larger ones.
To get the maximum benefit from clipping, mowing should begin when the tips of the buds of plants are approximately four inches from the soil line. Use a ruler to measure the height to the tip of the bud of the largest plants, and adjust the mower to one-half to one inch above that height. Some additional adjustments may be needed for sinkage if the soil is wet. After the mower height has been adjusted, mow across the plantbed to be sure the clipping height stays above the buds.
If plantbed middles have been raised or crested to shed water, it is best to start mowing to the immediate left or right of the center. If the mower straddles the center of the raised bed seedlings located on the crest may be clipped too low.
Larger tractor mower rotary mowers may also be used to clip tobacco plantbeds. This is commonly used in vegetable plant production. If a tractor mounted unit is to be used, care must be taken to maintain a uniform height of the cutting deck. Uniform beds are a must when using a tractor mounted mower. These mowers must be modified by adding gauge wheels which stabilize the mower at the proper clipping height. Plantbeds clipped with tractor-mounted mowers should be elevated slightly, and flat. Growers with large acreage may wish to consider tractor-mounted mower due to the time and labor savings involved. Sanitation is particularly important, if the mower is also used for general purpose mowing in which case it may come in contact with weeds such as horse nettle which serves as an alternative host for tobacco mosaic virus.
Harvest can be accomplished by hand pulling or utilizing a plantbed undercutter. An undercutter has been used by vegetable growers for many years. It consists of a long, flat, steel blade mounted on each end to a frame. Once undercut, plants can be harvested easily by hand.
There have not been any disease problems associated with clipping plantbeds. There are two practices which will help minimize chances of future disease problems.
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