I planted tomato plants about three weeks ago - most of them have been doing really well, but this one is having some problems (pics attached). The watering system waters daily. I weeded the area + applied fertilizer- is it a nutrient deficiency or disease?
Unfortunately, you're not going to get a straight answer on this question but I'll try to help you. Yellowing leaves are typically caused by a nutrient deficiency.... However, yellowing leaves can be caused by under watering or over watering, under fertilizing or over fertilizing. It can be a fungus issue like blight or a virus caused by white flies etc. To make matters worst, when a gardener has an issue they tend to over correct and end up exaggerating the problem. Unfortunately without knowing exactly how much and what fertilizer you used along with other conditions with your soil it's tough to say with 100% certainty what your issue is.
What I can say to help is: try to grow varieties that are disease resistant or that are better adapted to your climate and area. For example, in my area (zone 7a NYC) cherry tomatoes varieties grow amazingly. When I try to grow other varieties I get mixed results.
The other thing you can focus on is to work on your soil health (add as much rich compost as you can afford to your soil). When I started making my own compost + vermicompost and adding that to my soil it was a game changer for my garden.
Hopefully this help. But I don't think anyone can answer this question by hitting the nail on the head.
Your tomato plant is likely suffering from more than one problem. Over fertilizing and over watering.
Unfortunately, fertilizer will NOT cure a unhealthy plant. It just causes more harm. Fertilizer should only be used on healthy plants that are growing vigorously. Even then, I always recommend that you use only 1/3 of the strength in the directions on the label. I prefer liquid fertilizers that I dilute with distilled water. It is 1 part liquid fertilizer to 2 parts distilled water. If using a granulated fertilizer, make the fertilizer according to the directions on the package and then dilute it just like a liquid fertilizer (1 part fertilizer/ 2 parts water). Using a diluted fertilizer can mean that you will have to fertilize more often but it is better to under fertilize a plant than over fertilize.
Unless the soil is getting dry everyday, you should not be watering it everyday. You should allow the top inch of the soil to feel dry to the touch and then water the plant thoroughly.
Prune back the yellow leaves and stems. Also check carefully to see if there is any sign of insects on the plant. An unhealthy plant is much more susceptible to insects than a healthy plant is. A population of insects, like aphids, can get really big, really fast and cause enough damage that the plant can't recover from. If you see any signs that uninvited guests are eating your plants, a safe way to keep the population of insects to a minimum is to spray the plants with soapy water. Mix a few drops of dish washing liquid (I prefer Dawn) with distilled water in a spray bottle. Spray the plant thoroughly. I usually spray my outdoor plants at least once a week
There are more than one cause for this symptom, but from your description, it seems you may be water >>too<< frequently. For almost all plants, grass, etc. best watering approach is to soak them well >>infrequently<< and thereby help encourage root growth/expansion, increasing overall hardiness of the plant. Repeat thoutrough soaking when top inch or so (not just the top) of dirt is dry to the touch.