The 2022 greenhouse season is underway with a cool and cloudy start. At this time, all greenhouses have been seeded. Overall, seedling stands in greenhouses should provide adequate transplants for this year’s crop. As the seed germinates and seedlings grow, pests, tray and greenhouse conditions, fertility, and contaminations due to drifting of pesticides may arise and affect the crop.
Based on the farm calls received and the greenhouses that the CTRF has visited, seedlings between farms do vary in size depending on the type of production method being used and the time of seeding. Thus far, there have been a variety of issues occurring in the greenhouses, including dry cells, slow seed germination, uneven seed germination and seedling growth and algae formation on the surface of media interfering with seedling emergence and growth.
1-Dry cells
Styrofoam dry cells were observed in some float greenhouses when some tray cells showed dryness after floating (fig 1). The exact cause of cell dryness is not clear, but it can be associated with debris in the soil mix or issues with soil wetting agent and filling up cells loosely. These factors may prevent water absorption and soaking of the growth media, which will result in slow seed germination. To mitigate cell dryness some growers mist the trays’ surface using a handheld hose equipped with a mist nozzle.
2- Slow seed germination
Soil media in trays is essential for seed germination since it sustains seed germination and growth soon after seeding. If the soil media conditions are not adequate to ensure proper aeration, by not filling the trays too tight or too loose, media wetness, soil and water (float trays) and overall greenhouse temperatures, slow seed germination and seedling growth will occur. Greenhouse temperature is crucial for seed germination and growth. Cooler temperatures during seed germination will extend the number of days necessary to reach maximum germination and to decrease uniformity in the size of seedlings, and after germination, they can exacerbate seedlings’ chilling injury. A temperature of 24°C or 75°F is recommended for optimal seed germination and seedling growth and a temperature of at least 15°C or 60°F or higher be maintained during the first 14 days. However, some growers will maintain at least 21°C or 70°F for the first 14 days to maximize the speed of germination and plant uniformity. After the first 14 days, good rates of seedling growth can be achieved with an overnight minimum temperature of 10°C or 50°F without any injury to plants.
3- Algae
Algae may develop on the surface of either dry or float trays, especially under cool and dim conditions. Several distinct types of algae may form on trays depending upon what types were in the substrate used to fill the trays or which types were present upon the trays from previous seasons. Blue-Green Algae are the most common algae in greenhouses, and they affect tobacco seedlings by causing oxygen levels in the media to fall, thus inhibiting root growth, and competing with the plants for fertilizer. Two situations are usually seen in cases of algae infection, one where the seedlings are not able to emerge through the algae and the other being where the seedlings emerge but cannot establish themselves before succumbing to the algae and dying.
There are no pesticides registered for algae control in tobacco greenhouses. Algae are best managed by providing conditions as favorable as possible for seed germination and early seedling growth, so that the seedlings will establish quickly and outpace the formation of the algae on the media surface. Following good tray fertilization practices is also essential since it encourages algae growth. Therefore, fertilizer should not be added to float tray water until the plants have reached the two-leaf stage. For dry trays, fertilizer applications should not start until three weeks after seeding. As seedlings grow, the canopy will close over and shade the media surface, therefore, reducing the negative effects of algae.
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