I am currently growing a variety of lettuce cultivars aeroponically (indoors, climate controlled) and have noticed some yellowish-brown growth on the topside of the plant and some black dots (around the size of a ball point and smaller) on the underside of the leaf. I have a few ideas on what may be causing this. My first instinct was to blame nutrient deficiency. However, all of the plants growing around the affected area are bright green and seemingly very healthy. They all receive the same amount of nutrients. My second thought was maybe that it was fungal. I applied a diluted baking soda and dish soap solution hoping that it would kill off any fungal growth. I am still waiting on the results of that. I also noticed that some of the roots were rotting and choking out the healthy, white roots. I cleared out the black roots to make way for the healthier ones. I am worried that this may be bacterial in nature. I have quarantined the affected plants just in case.
Let me know what you think of this and thank you,
Thanks for getting in touch with Toronto Master Gardeners. Based on your written description it sounds like the lettuce is infected with Botrytis Crown Rot, a type of fungus. If you want a more definitive diagnosis you might consider sending a few pictures of the infected lettuce. Botrytis, which can present in a number of ways, affects many types of fruits and vegetables. Lettuce plants near maturity may appear healthy but develop crown rot close to harvest. Diseased plants will start to wilt, lower leaves turn yellow then brown, and the entire plant will collapse. The black spots you describe may be black sclerotia (hard fungal resting structures measuring from 1/8 to 1/4 inch in diameter). You mention that the growing environment is temperature controlled but it might be too damp for the lettuce. Get rid of any of the plants that look infected and the best way to dispose of it is to burn it. Botrytis is easier to prevent than to treat so when you grow lettuce in the future you might consider the following:
- Buy high-quality seed of recommended varieties.
- Plant in a light, well-drained, well-prepared, fertile seedbed with good air circulation.
- Sterilize the seedbed soil before planting with heat.
- Avoid over fertilizing (especially with nitrogen), and wet mulches.
- Fertilize plants on the basis of a soil test.
- Keep the greenhouse and seedbed soil somewhat dry after planting. Circulate the air with fans when heating greenhouses to keep warm, dry air in motion,
- Keep water off of the foliage.
For more information about botrytis, please review the flowing sites: