1997 CHEMICAL CONTROL OPTIONS FOR DISEASE CONTROL IN TOBACCO PLANT BEDS, GREENHOUSES AND FLOAT-SYSTEMS
by William Nesmith

Diseases can be a limiting factor in the production of tobacco transplants and infected seedlings can serve as a source of diseases for the field. This fact was very well illustrated in both 1995 and 1996 with blue mold epidemics, when infested transplants served key roles in driving the epidemics. Fortunately, both the incidence and severity of diseases in seedling production can be greatly reduced through fumigation, sanitation and preventive spray programs. Unfortunately, adequate labeled materials are not available for all disease problems, especially for the greenhouse and float systems. Especially lacking are labeled chemical options for the control of Pythium, Rhizoctonia, and Sclerotinia.

Some important changes have developed during 1996 in chemical control options. First, 1997 labels do not authorize use of Ridomil 2E or Ridomil Gold in plant beds. However, older formulations of Ridomil 2E product can still be used in plant beds, in the manner directed on the label. Secondly, application is being made to EPA requesting use of Acrobat MZ in transplant production systems, but approval is unlikely prior to May 1, 1997. Carry-over Acrobat MZ labels do not apply in 1997, they covered the product only from May 31, 1996 to September 30, 1996. The chemicals labeled in Kentucky on February 17, 1997 for blue mold control are adequate for disease control in tobacco transplant systems if used correctly in a total preventive approach.

The following chemical control options are recommended for disease control, in addition to sound cultural steps:

• FOR TRADITIONAL-TYPE OUTDOOR PLANT BEDS
  1. Fumigate the bed site to destroy disease organisms in the soil. Fumigant options include: gases, liquids, and granules.
     • Gases - include products containing methyl bromide or methyl bromide + chloropicrin. Gas should be used under a plastic tarp at the rates of 9 to 18 lbs/ 1000 sq feet of bed (see labels for specific rates on each product). Bed sites should be prepared as if ready to seed then fumigated for best results. Expose the site to fumigant for at least 24 hrs, plus 24-48 hrs of aeration prior to seeding. Soil temperatures should be above 55 F during the fumigation period and soil moisture should be sufficient to support germination.
     • Liquid-fumigants - include products containing SMDC (metham sodium) and marketed under such names as Vapam and Sectagon, but other products may also be available in some areas. They should be used at the rate of 1.5 gallons/1000 sq ft of bed, injected into the soil at least 4 inches or drenched into the soil with at least 40 gallons of water/1000 sq ft of bed area, then covered immediately with plastic tarps. The tarp should be left in place at least 24-48 hrs, but a long aeration period of 21 days is needed prior to seeding. Light tillage of the soil during the aeration period may be helpful to speed escape to the fumigant.
     • Granule fumigant - are available as dazomet, sold as Basamid, and used at 7.5 lbs/1000 sq ft of bed. The availability of this product is limited in Kentucky, currently, but it is an effective fumigant. Since it has a very long aeration requirement (14-50 days), its use in the spring in Kentucky is greatly limited. Granules should be spread evenly over the bed site and incorporated to a depth of 8 inches, then the site sealed completely with plastic. Leave the plastic in place at least 5 - 7 days, then aerate the site until it is safe to seed, as determined by use of a germination test.
  2. Apply Ridomil 2E (check to be sure your container is labeled for this use) prior to seeding the bed, using 4 teaspoons per 1000 sq ft of bed, applied either as a drench or spray. No additional applications are approved. This should give control of metalaxyl-sensitive strains of blue mold and Pythium, and assist in black shank control.
  3. Use Bordeaux mixture (bluestone-lime mixture) as a drench to the soil when the plants have emerged and again 10 days later. This treatment will control algae and aid in the control of diseases caused by bacteria (wild fire, angular leaf spot, and blackleg). Following the label EXACTLY as to mixing instructions, because bordeaux mixture can be toxic to tobacco seedlings. Do not apply this mixture to large seedlings. The target is actually the soil and not the tobacco plant.
  4. Sprays of Streptomycin are highly effective in control of most bacterial diseases of the bed, especially angular leaf spot, but streptomycin-resistant strains are present. Sprays can begin as early as the two-leaf stage and should be repeated weekly until transplanting for control of bacterial leaf spots, such as angular leaf spot. It may also slow or suppress blue mold under certain conditions, but cannot be relied on to control blue mold. The material should be applied as a 100 ppm solution (1 teaspoon of Streptomycin 17% WP to a gallon of water), using 3 to 5 gallons of material per 1000 sq ft of bed. On beds receiving bordeaux mixture, it is not necessary to start the streptomycin sprays at the two-leaf stage, rather they can be delayed until plants are about the size of a dime, unless bacterial leaf spots are observed earlier. Do not mix streptomycin with other spray materials. Since streptomycin is a local systemic, best results are achieved when it is applied under conditions of slow drying (such as just before dark).
  5. When plants are about the size of a dime, begin weekly sprays with fungicides: Carbamate WDG (ferbam) and other formulations of ferbam have national labels for outdoor beds and Dithane DF has a Ky label. Carbamate should be used at 1.5-3.0 lbs/100 gallons of water (3 to 5 tablespoons per gallon) and Dithane DF at 0.5-1.0 lbs/100 gallons (1 Tablespoon per gallon). Use 3 gallons of spray mixture per 1000 sq ft of bed while plants are dime-size, increasing to 6 gallons on large plants nearing transplanting size. Thorough coverage of the seedlings is very important. These fungicides are broad-spectrum and will control or suppress a range of fungal diseases associated with the bed, including metalaxyl-insensitive strains of blue mold. This weekly sprays should continue until transplanting time. Should a broad-spectrum fungicide be needed prior to the plants reaching dime size, use Carbamate. Dithane DF can cause serious damage to small seedings, but it has the better efficacy once seedlings are larger and the canopy is dense.
  6. On Tobacco Mosaic susceptible varieties, sprays of milk within 24 hours of pulling plants can greatly reduce spread of TMV during pulling of plants. Use 5 gallons of milk (dry, whole or skim) per 1000 sq ft of bed. Good sanitation practices are also needed, including washing hands frequently during pulling in a phosphate detergent and avoiding tobacco products by those handling plants. This is not needed with TMV resistant varieties. This treatment can also be used in greenhouse and float-systems, as it is generally regarded as safe and therefore exempt from the requirement of having residue tolerances.
• GREENHOUSE AND/OR FLOAT-BEDS - (Note: Outdoor floats are not the same site as traditional beds, rather they are modified greenhouses for pesticide considerations.)

  Before a pesticide can be used in greenhouse and float-beds production system, it must be specifically labeled for greenhouse and float systems. Although there is some disagreement on this issue, EPA's Regional Office in Atlanta, GA continues to advise that this requirement remains in effect. Therefore, only Carbamate and Dithane are labeled for use in these systems in Ky. The label on Ridomil 2E specifically prohibits greenhouse applications. Streptomycin is not specifically labeled for greenhouse use on tobacco, but does have some greenhouses uses labeled on other crops. Therefore, only treatments number 5 and 6 from above (under outdoor section) apply to these new systems in Ky.

• CARBAMATE WDG: For the control of blue mold and suppression of Botrytis and damping-off diseases, as soon as seedlings emerge or immediately after plugging, begin frequent foliar sprays with Carbamate WDG at 1.5 to 3.0 lbs/100 gallons ( 2 to 4 tablespoons/gallon). Use the lowest rate while seedlings are small. Apply as a fine spray to the point of run-off, using 3 gallons of spray material per 1000 sq ft when seedlings are small increasing gradually to 6 to 12 gallons as plants increase in size and the canopy increases. Repeat applications up to twice weekly, depending on the rate of growth and disease potential/pressure. The label has a specific restriction concerning contamination of the float-water, requiring the trays to be packed tightly together during application.
• DITHANE DF: Do not begin applications before the plants have reached plugging size (dime size) to avoid serious phytotoxicity to seedlings. For the control of blue mold, anthracnose, and damping-off diseases apply as a foliar spray to the point of run-off using Dithane DF at 0.5 lbs/100 gallons of water (one teaspoon/gallon). Note the rate is lower for greenhouse and floats than in the outdoor soil plant beds. Use 3 gallons of spray material while plants are small, but increase gradually to 6 to 12 gallons as plant size and canopy increase. Two label restriction apply: Do not make applications after transplanting to the field and avoid contamination of the float-water during applications.

  In Kentucky tests, Dithane DF has caused damage to seedlings under certain conditions (especially if the float-water becomes contaminated), but it clearly has provided superior control of blue mold in the outdoor float-beds compared to Carbamate. In greenhouse plantings, significant difference have not been observed in the control of blue mold given by Carbamate and Dithane, but Carbamate was superior to Dithane in Botrytis control at the concentrations labeled.

MILD HOLIDAY WEATHER MAY SPELL TROUBLE FOR FALL ALFALFA SEEDINGS
by Paul Vincelli

Overcast, humid weather with generally mild temperatures for much of the time around the holiday season was very favorable for activity of the fungus Sclerotinia trifoliorum. Extension agents will recognize this fungus as the one that causes Sclerotinia crown and stem rot of fall- seeded alfalfa. It can also attack red clover.

During that period of generally mild weather, we saw a dramatic rise in plant infection in our research plots with Sclerotinia, as high as 60% infected plants around New Year's Day (with even higher levels to be expected during the next mild spell). Farmers who happened to be closely inspecting there fall-seeded alfalfa also saw a great deal of white, cottony fungal growth on many of the plants. Most, if not all, of these infected plants will die and rot away before spring greenup.

What this means: It means that fall-seeded alfalfa fields may simply fail to green up if Sclerotinia was active. We've seen this many times before, where a lovely stand of fall-seeded alfalfa disappears during the winter. Yet this always seems shocking for someone who has never directly experienced it before. Often, the only evidence that is left are the "sclerotia", hard, black survival bodies that are up to 1/8", irregular in shape, and with a gray or white center.

Bottom line: If a high-quality stand of alfalfa seeded last summer or early autumn is gone by spring greenup, consider Sclerotinia crown and stem rot as the most likely cause.

1996 BT-CORN EVALUATION FOR EUROPEAN CORN BORER
by Aaron Anderson and Ric Bessin

Five Bt-corn hybrids were evaluated for control of European corn borer and yield at the UK REC substation in Princeton. The five hybrids (and the type of Bt technology) were as follows: NorthrupKing 6800BT (YieldGard), Mycogen 7959 (NatureGard), Mycogen 7559 (NatureGard), Ciba Maximizer 454 (Knockout), and Monsanto Ezra+ (Yieldgard). They were compared with Mycogen 7660, NorthrupKing 6800, Ciba 4494, and Monsanto Ezra-. In this study, half of the plots were infested with approximately 100 newly hatched European corn borer larvae in mid-June, and the again in mid-July. This was intended to simulated first and second generation corn borer attack. However, the data presented below will be an average for each hybrid across all plots.

Hybrid	Damaged plants at pollen shed %	Damaged plants at pollen shed + 4 weeks	Damaged plants harvest %	Live larvae at harvest (per plant)	Stalk tunneling (cm)	Shank tunneling (cm)	Yield (bu/A)
M 7660	11.8 a	6.8 ab	77.6 a	0.27 c	9.8 b	0.57 b	112.7 a
NK 6800	13.3 a	10.3 a	90.0 a	0.63 a	13.9 a	0.93 a	107.8 ab
Ciba 4494	14.3 a	9.9 a	85.6 a	0.53 ab	11.4 b	0.53 b	99.8 bc
Ezra-	12.1 a	9.9 a	83.6 a	0.46 b	13.4 a	0.60 b	94.7 c
Max 454	0.7 b	0.0 b	41.6 b	0.11 d	2.6 c	0.36 bc	110.0 ab
M 7959	0.0 b	0.2 b	35.2 b	0.05 d	1.4 cd	0.26 cd	110.2 ab
M 7559	0.2 b	0.8 b	33.2 b	0.10 d	2.4 c	0.23 cd	110.5 ab
Ezra+	0.3 b	0.2 b	20.0 bc	0.00 d	0.2 d	0.10 d	98.4 bc
NK 6800BT	0.5 b	0.2 b	4.4 c	0.00 d	0.1 d	0.05 d	118.0 a

In summary, the Bt technology works. This technology will greatly reduce damage by European corn borer, but it is not absolute control. For nearly all of the variables measured the Bt hybrids were able to significantly reduce damage by European corn borer. The percent damaged plants at pollen shed and pollen shed plus 4 weeks represent damage by the first generation. The percentage of damaged plants at harvest represents combined damage by both generations. There were no significant differences among Bt-hybrids except for stalk tunneling. With stalk tunneling, the YieldGard hybrids had significantly less tunneling than the other Bt hybrids.

Looking at yields, some of the non-Bt hybrids were able to out yield some of the Bt hybrids, even though the non-Bt hybrids had considerable damage. The important message is that having the gene to produce the Bt endotoxin does not necessarily mean that you will improve your yields. If you are interested in using this technology, you should select a hybrid that has a complete package of characteristics you need including yield potential, disease resistance, and local adaptability. In the absence of corn borers, your Bt-hybrid should yield as much as your standard hybrids that you are currently using.