The consistently high humidity this year has played havoc with my potted tomatoes - despite being under cover, they have suffered from powdery mildew. It is partly my fault for not watering more consistently when we've had those really hot days… But they are still producing good crops of tasty cherry tomatoes.
Tomatoes are actually a perennial plant, though we usually treat them as annuals. They will keep growing and fruiting until killed by the cold or disease, a factor that makes them worth some care. If you have mildew or other fungal diseases, strip off all affected leaves and cut back any badly infected stems at the base.
Established plants will throw out new shoots all along the stem, plus new stems at ground level. You can either leave these to replace old stems, or remove them and pot them up as cuttings. Regular spraying with milk spray (10% milk, 90% water) will limit the spread of mildew to new growth - you can also use dusting sulphur or tomato dust. Other helpful tactics are thinning the new growth to allow better air movement and consistent watering for pot-grown plants (note to self!).
You might notice adventitious roots emerging from the stems even if they aren't in contact with mulch or soil - this is another response to high humidity, and one you might also see on grapevines.
An interesting fact about tomato stems is that they have been found to have a second purpose. The hairs that cover them and the other parts of the plant, giving them that 'sticky' feeling, are designed to trap small insects. The dead insects fall to the ground beneath, adding nutrients. I wonder if the plant might be able to absorb insect nutrients directly through the skin of the stem. After all, if the plant produces aerial roots....
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