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Dying brassica seedlings

Submitted by Okanagan1 on Fri, 04/14/2023 - 14:19
Region
Okanagan

This week's gardening question is all about dying brassica seedlings.

Question

I have a brassica conundrum that I am hoping you can solve. 

I have, for years, struggled to grow brassica seedlings indoors. The seeds sprout just fine, get to about 2" high, then the leaves go pale, shrivel up and die. The stalk is fine, it is the leaves that die. 

Kale, broccoli, cauliflower and cabbage are all equally affected. This has been going on for at least 5 years. I've tried different soils, pots, lights, watering conditions, everything I can think of. 

Here's the kicker though - the moment I put seedlings out in the greenhouse they do fine. Arguably that location is hotter in the day, and cooler at night than inside my home (where I have the seedlings for now). So I can start brassicas from seed in the greenhouse in April, but not in my home in February. Could the missing link be UV light somehow?

I grow every other common garden plant - tomatoes, squash, peppers, lettuce, spinach, cucumbers - and start them all inside under the exact same conditions as the brassicas. Brassicas die, everything else does fine. 

Any ideas?! Because I can't start these inside, the plants are often growing into July, which I don't want. But because of this seedling issue I can't start them any earlier.

Answer

The stems don’t display the classic damping-off collapse at the soil line of the stem, but the yellowed and shriveled leaves may indicate some sort of vascular wilt. It may be that a spreading fungal pathogen is restricting the flow of water and nutrients into the leaves (starting with the lowest ones).

Brassicas are considered cool-season crops, while many other garden veggies (tomatoes, peppers, cucumber, squash, watermelon) are warm-season crops. As the name suggests, cool-season crops prefer cooler temperatures, with the plants generally being quite hardy to cold. And in the case of brassicas, there are certain fungal pathogens that tend to affect this family much more than warm-season veggies (and even other cool-season veggies outside the brassicas).

Here are a few things to try:

  • Try a fresh packaged seed-starting mix with plenty of perlite, smaller containers, and new hybrid seed with good disease resistance
  • Move the brassica seedlings to a cooler spot (16°-18°C) with some fresh air/ventilation as soon as they sprout
  • Use a stronger plant light, or position your plant light closer to the seedlings
  • Thin the seedlings out to only the strongest one per cell/pot once true leaves appear
  • Cooler temperatures at night may help keep fungus under control (as long as the plants aren’t frozen - I put mine in the garage)
  • Begin hardening off the young seedlings early so they are well-conditioned and can be moved outdoors earlier

Here are some more detailed thoughts to expand on the bullet points above:

Temperature

Consider reducing the daily temperature to 16°-18°C (60°-65°F). Germinate the seeds indoors at room temperature 21°C (70°F), but move them to a slightly cooler temperature after they sprout. The optimal growing temperature for most brassicas is in the range of 16°-18°C (60°-65°F), whereas seedlings of warm-season crops like squash, tomato, and cucumber do better when growing at a warmer temperature in the range of 18°-24°C (65°-75°F). 

You may also consider reducing the night temperature. This may mean keeping the plants at a daytime temperature of around 16°-18°C (60°-65°F), and then reducing the temperature down to about 10°-13°C (50°-55°F) in late evening/early morning. The growth of certain problematic wilt fungi (such as Fusarium oxysporum f. sp. conglutinans) is slowed at temperatures at or below about 16°C (60°F), and these plants are quite cold-hardy and can still actively grow below 16°C (60°F).

Sanitation

Fungi are commonly transferred mechanically from infected soil (pots, tools), contact with infected plant tissue, flying pests like fungus gnats and flies, windblown soil, and even on brassica seeds. Clean and disinfect all your tools and work surfaces prior to seed starting. Use a sterilized seed-starting soil-free mix guaranteed to be pathogen-free. Buy fresh seeds from a reputable company with proper sanitation practices. 

Light

The tall spindly growth of the seedlings could possibly be due to a lack of light. Your seedlings love the natural outdoor light, and likely need a stronger light when grown indoors to keep them from reaching. Use a very bright light for seed starting and position it a few inches above the pots/seedlings (follow the instructions for your specific plant light). 

Moisture

Consider using small seed-starting trays with a smaller volume of soil for each plant. Larger pots contain a lot of soil, which tends to stay quite saturated and hold more water than the individual cells of a seedling tray that can be bottom watered.

Cultivars

Choose cultivars that are resistant to common pathogens like Fusarium. Consider an early-maturing hybrid variety that will mature quickly. Cornell lists quite a few disease-resistant varieties on their website (https://www.vegetables.cornell.edu/pest-management/disease-factsheets/disease-resistant-vegetable-varieties/).

Seeding

Plant about 3 seeds in each cell. When the seedlings have developed their first true leaves, thin your seedlings by trimming the weak ones off at the soil line with scissors, leaving only the strongest in each pot.

Transplanting

Transplant the seedlings when they are still quite young (with the stem smaller than a pencil). This will include the pre-transplant process of hardening off the seedlings for 1-2 weeks prior to planting them out. Early exposure to mild environmental stresses should help strengthen the seedlings and perhaps keep the pathogens at bay. You can start exposing the seedlings to small amounts of cooler temperatures, outdoor sunlight, fresh air, and wind as soon as they have sprouted their first true leaves and have been thinned.

Hopefully, implementing these changes will lead to healthier brassica seedlings. Thank you very much for submitting your gardening question, and we look forward to your brassica conundrum becoming a brassica victory!

Answer by:

Volunteer Master Gardener Advisor

Okanagan Master Gardeners

A chapter of the Master Gardeners Association of British Columbia

References

Dying brassica seedlings

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