How to spot and stop diseases on greenhouse tomato seedlings

Stop diseases now on tomato seedlings and produce healthy transplants for the field.

Problems can start early

Bacterial diseases can ramp up long before symptoms become obvious. Bacterial speck, bacterial spot, and bacterial canker are three pathogens that can wreak havoc in the greenhouse. Symptoms of both bacterial speck and bacterial spot appear as dark blotches or spots on the leaves. Sometimes these blotches or spots are nearly black in color but they can also be light brown. A yellow “halo” can surround these blotches or spots, but may not always be present. Bacterial canker looks different than other bacterial diseases. This disease causes a light browning along the mid-vein of the leaves and extends down the petiole. It is very easy to overlook bacterial canker infection because it can mimic other sorts of disorders including plant stress from drying. Bacterial canker symptoms do not appear to be “disease-like” compared to bacterial speck and spot and it is easy to overlook.

In general, bacteria are spread via splashing from plant to plant and require a plant wound or a natural plant opening in order to gain entrance into the plant. The plant wound can be small such as a broken plant hair or other microscopic abrasion. Natural plant openings are found along the leaf edge and the bacteria can move along the leaf surface and move into the leaf via these natural plant openings along the leaf edge. This can result in a dark fringing along the outer edge of the leaf.

Bacterial speck, bacterial spot, and bacterial canker can be introduced to a greenhouse tomato seedling via the seed. Bacteria can occur on the seed coat, but the bacterial canker bacterium can sometimes be inside the seed. When the seed coat stays attached to the cotyledons following emergence, any bacteria on the seed coat can move to the seedling. Once the bacterium moves to the seedling, it reproduces rapidly when conditions are warm and wet. Bacteria move from plant to plant by splash droplets. When vegetable transplants are watered overhead, the bacteria readily jump from diseased transplants to adjacent healthy transplants. When the humidity is especially high, these bacteria can become encapsulated within tiny airborne water droplets and may move within the greenhouse via water aerosols.

Bacteria can also be introduced to tomato seedlings via plant residue left in the greenhouse from a previous season’s diseased tomato plants. Perhaps old foliage has become tucked away under plant benches or packed down in the walkways where it may serve as a potential source of bacteria to infect a new group of seedlings.

Bacterial control is best begun even before there is a hint of disease. This starts with tomato seed that has been tested for bacterial pathogens and commercially treated. Growers may treat the seed themselves and that can include hot water treatment. Although hot water treatment can be effective, the high temperature may hurt germination. Because of this, hot water treatment is not used widely, but may be worth exploring for those who are looking for relief from bacteria but are growing transplants for the organic industry and have limited options.

Keeping the growing environment and foliage as dry as possible can help limit the spread and development of bacterial diseases. Conventional growers and organic growers can use sprays of certain copper fungicides to help limit bacterial diseases. Some of the bacterial pathogens may not respond to copper if they have mutated and developed resistance. Growers do not immediately know whether their particular bacterial problem will respond to copper. Based on our research trials, it is best to begin the copper spray program on tomato transplants just as soon as the true leaves emerge and re-apply every five days. To stay within label specifications of a seven-day application interval, copper-based products may need to be alternated. Streptomycin (AG Streptomycin and Agri-mycin 17 are examples) can also be used alone or in combination with copper products to limit bacterial disease on tomato transplants in the greenhouse. Note: Check with your state officials to ensure this product is registered in your state. Tanos is labeled to suppress bacterial diseases. “Suppression” often means that activity has been noted but may not always be adequate under heavy disease pressure.

Problems can develop as seedlings develop

Alternaria and Botrytis can cause problems as the seedlings grow and the lower leaves age, overlap, and develop a canopy. Alternaria is also known as early blight and first appears on the oldest leaves mimicking a bulls-eye target with concentric rings in the spots. Botrytis is everywhere within our environment and is best known as grey mold because of the fuzzy, grey-colored masses that are produced on diseased leaves and stems. It is not uncommon for Botrytis to infect an older tomato transplant via the cotyledons that are clinging to the stem; the pathogen then progresses into the stem and may constrict and girdle it.

Botrytis and Alternaria require that the leaves stay wet for at least four to six hours so that their spores (fungal seeds) can germinate and grow into the plant and cause disease. If the relative humidity is high during the day and the temperature drops even a degree or two, the moisture in the air can condense onto the foliage in a fine film and provide enough water for the spores to germinate and infect the plant. BotytisBotrytis becomes less of a problem. Alternaria is a common problem in the field where it can overwinter in tomato debris from previous crops but can also occur occasionally on seedlings in the greenhouse. Botrytis and Alternaria spores move on air currents within the greenhouse but since they are very common pathogens in the environment, the distance that they might travel is not considered to be of great importance. is primarily a greenhouse problem but may also occur in low and high tunnels if there is not good ventilation and air movement. Once the plants are moved outside, there is better air circulation and

Keeping the relative humidity below 85 percent and providing good air circulation limits disease. Water that accumulates under plant benches and tight plant spacing can contribute to pockets of high relative humidity that favors Alternaria and Botrytis. Preventive fungicides are nearly always applied to prevent Botrytisand Alternaria since they can be frequent problems. There are several formulations of the fungicide Dithane available that can protect greenhouse seedlings. Heritage is also available and is especially effective against Alternaria and can be used in alternation with Dithane. Tanos is a fungicide that can be used against Alternaria and can be alternated in a treatment program along with Dithane or Heritage to protect tomato seedlings. Decree is recommened when Botrytiscontrol is needed.

Late blight is caused by a water mold called Phytophthora infestans and is not considered a problem for tomato seedlings in Michigan greenhouses. The late blight pathogen typically overwinters in potato cull piles and is often introduced to production fields via potato seed pieces. Commercial potato growers are vigilant each year for late blight as it is a common problem for them when the weather is cool and wet. The sporangia (seeds) of the late blight pathogen can be easily dislodged from the plant’s surface and carried long distances from one field (or growing region) to another via air currents and storm systems. Weather that is overcast, wet, rainy, and humid allows the late blight sporangia to survive its travels so it can cause disease if it lands on the surface of an unprotected host plant (i.e. tomato, petunia, and weeds such as nightshade). When conditions are bright, sunny, and dry, the late blight sporangium cannot survive long because the sunlight breaks it down and the low relative humidity causes it to shrivel and die.

Control measures for late blight are similar to those recommended for the other tomato diseases and include keeping the foliage dry, providing good air ventilation, spacing plants, and heating when needed to dry out the greenhouse. Fortunately, tomato transplant growers can protect against late blight with the same fungicides they use for AlternariaBotrytis. In Michigan State University tomato field trials that I’ve run for the last several years (including 2009), the active ingredient in Dithane was excellent in protecting the tomato plants from late blight. The active ingredient in Heritage (azoxystrobin) was also very good. Revus (mandipropamid) is a new product (received a supplemental label that included tomato in August 2009) that has been outstanding against late blight in our outdoor field trials. The use of Revus is not prohibited in the greenhouse on tomato seedlings, but it may not be used on tomatoes for transplant production. Revus could be used in combination with one of the Alternaria/Botrytis products since Revus does not control BotrytisAlternaria. Revus could be rotated with other helpful late blight fungicides including Curzate, Ranman, and Tanos. All late blight specific fungicides could be used in combination with one of the Alternaria/Botrytis products. Late blight fungicides, combined with one of the Alternaria/Botrytis products, can eliminate the potential occurrence of late blight when used properly and preventively at the greenhouse level. and or

General guidelines

  • Dedicate operations for seedling and transplant production. Greenhouses that grow both tomato transplants and mature plants for fruit production are especially at risk of keeping diseases active in the greenhouse and available to infect new tomato seedlings.
  • Keep the relative humidity as low as possible (less than 85 percent) through heating and venting as appropriate.
  • Space plants to prevent pockets of high humidity from forming.
  • Use fans to move air and vent to exhaust moisture-laden air out of the greenhouse.
  • Scout seedlings two times each week to ensure that problems are detected early when corrective measures can be taken.
  • If disease symptoms are detected, remove affected plants including adjacent healthy-appearing plants.
  • Water at a time of day when plants can dry quickly.
  • Apply fungicide preventively when weather conditions are favorably for disease (i.e. wet, humid).

Tomato late blight: Fungicide Recommendations

Homeowners
Chorothalonil: Apply fungicides that list chlorothalonil as the active ingredient every seven days or as the label specifies.Copper-based products: Chlorothalonil can also be mixed with copper fungicides (each at the full labeled rate).

 

Organic growersYour local organic certifying agency can help you determine what products are approved.
Copper-based products: Some forms of copper hydroxide are approved for use in organic production. Spray every seven days or as the label specifies.Bacillus subtilis: Serenade, 4 to 8 lbs. every five to seven days (0 days PHI). Provides suppression. Biological control product that needs-be applied before disease development. Control may be limited under heavy disease pressure.Other fungicides labeled for late blight that are OMRI-listed include Sporatec, Sonata, and OxiDate.NOTE: In general, products available to organic growers are not likely to halt tomato late blight when the environment favors disease (cool and wet).

 

Growers using integrated pest management
New products are marked with two asterisks (**).Materials marked with one asterisk (*) are particularly recommended for problem infestations.Preharvest interval is in parentheses.** Fluopicolide:Presidio, 3-4 fl oz every 10 days (2 days).** Mandipropamid/Difenoconazole:Revus Top, 5.5-7 fl oz every 7-10 days (1 day).* Azoxystrobin/Chlorothalonil:Quadris Opti, 1.6 pt every 7-21 days (0 days).* Chlorothalonil:Bravo Ultrex, 1.3-1.8 lb every 7-10 days (foliage) or 1.8-2.6 lb every 7-14 days (fruit) (0 days). BravoWeather Stik or Echo 720 or Equus 720 SST, 1⅜-2 pt every 7-10 days (foliage) or 2-2¾ pt every 7-14 days (fruit) (0 days). Echo 90DF, 1.1-1.6 lb every 7-10 days (foliage) or 1.6-2.5 lb every 7-14 days (fruit) (0 days). Equus DF, 1.3-1.8 lb every 7-10 days (foliage) or 1.8-2.6 lb every 7-14 days (fruit).* Cyazofamid:Ranman, 2.1-2.75 fl oz every 7-10 days (5 days).* Cymoxanil:Curzate 60DF, 3.2-5 oz every 5-7 days (3 days).* Dimethomorph:Acrobat 50WP, 6.4 oz every 5-10 days. Forum, 6 fl oz every 5-10 days (maximum 2 applications before alternating with another mode of action) (0 days).* Mancozeb/Zoxamide:Gavel 75DF, 1.5-2 lb every 7-10 days (5 days).* Propamocarb:Previcur Flex, 0.7-1.5 pt every 7-10 days (5 days).* Pyraclostrobin:Cabrio, 8-12 oz (0 days). Apply in a strict one-to-one alternation program with fungicides having a different mode of action.Azoxystrobin:Amistar, 1.6-2 oz every 5-7 days (0 days). Quadris, 5-6.2 fl oz (0 days). Do not apply more than one foliar application of Amistar and Quadris or other strobilurin fungicide before alternating with a fungicide that has a different mode of action.Mancozeb/Copper Hydroxide: ManKocide, 2.5-5 lb every 3-10 days (5 days).Famoxadone/Cymoxanil: Tanos, 8 oz (3 days). Apply in a strict one-to-one alternation program with fungicides having a different mode of action.Fenamidone:Reason 500 SC, 5.5-8.2 fl oz every 5-10 days (14 days).Maneb:Maneb 75DF or Maneb 80WP, 1½-3 lb, every 7-10 days (5 days). Manex, 1.2-2.4 qt every 7-10 days (0 days).Mancozeb:Dithane DF Rainshield or Dithane M-45 or Dithane WSP 1½-3 lb every 7-10 days (5 days). Manzate 75DF 1½-3 lb every 3-7 days (5 days). Dithane F-45 Rainshield, 1.2-2.4 qt every 7-10 days (5 days). Penncozeb 4F, 0.6-2.4 qt every 3-7 days (5 days). Penncozeb 75DF or Penncozeb 80WP, 1½-3 lb every 3-7 days (5 days).Mancozeb/Copper Sulfate: Cuprofix MZ Disperss, 1.75-7.25 lb every 3-10 days (5 days).* Mefenoxam/Chlorothalonil:Ridomil Gold Bravo, 2 lb every 14 days (maximum 3 applications) (14 days).* Mefenoxam/Mancozeb:Ridomil Gold MZ, 2.5 lb every 14 days (maximum 3 applications) (14 days).* Mefenoxam/Copper Hydroxide: Ridomil Gold Copper, 2 lb every 14 days (maximum 3 applications) (14 days).Trifloxystrobin:Flint, 4 oz every 7-10 days (3 days).Phosphorous Acid Salts: Fosphite, 3-5 qt per 20 gal (ground) or 10 gal (aerial) of water every 14-21 days (0 days).

Seedling with roots just starting to show browning due to root rot.
Seedling with roots just starting to show browning due to root rot.

Bacterial canker on a tomato transplant.
Bacterial canker on a tomato transplant.

Bacterial speck on tomato transplants.
Bacterial speck on tomato transplants.

Bacterial spot on tomato transplants.
Bacterial spot on tomato transplants.

Botrytis blight on a seedling.
Botrytis blight on a seedling.

Early blight on tomato leaves.
Early blight on tomato leaves.

The dark spots along the tomato stems are a symptom of late blight.
The dark spots along the tomato stems are a symptom of late blight.

The white, fuzzy production of sporangia on the foliage is also part of late blight disease symptoms.
The white, fuzzy production of sporangia on the foliage is also part of late blight disease symptoms.

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