The leaves that are dying are juvenile foliage that naturally dies as the plant grows and matures. There in nothing wrong with the plant and this process is normal. Plants have the ability to abscise foliage that is no longer needed. Most plants will stop sending water and nutrients to the bottom leaves as new leaves higher up on the plant begging to grow and require more of the plants energy than the older leaves. These leaves will slowly yellow and dry until they fall from the plant (or remain and be unsightly on some species). When a plant first produces it first set of leaves this foliage is called "juvenile". This type of leaf is actually the oldest growth and the first to die as a plant grows. This is what is happening with your plant.
There are several fungal diseases that cause leaf spot. "Leaf spot" is a common descriptive term applied to a number of diseases affecting the foliage of ornamentals and shade trees. The majority of leaf spots are caused by fungi, but some are caused by bacteria. Some insects also cause damage that appears like a leaf spot disease.Leaf spot may result in some defoliation of a plant. An established plant can tolerate almost complete defoliation if it happens late in the season or not every year. Small or newly planted that become defoliated are more at risk of suffering damage until they become established. Keep the leaves picked up that fall and destroy, do not compost them. Spray with a fungicide now and when new leaves emerge.
Your problem is likely powdery mildew, an opportunistic but difficult disease. Powdery mildew first appears as pale yellow spots on stems, petioles, and leaves. These spots enlarge as the white, fluffy mycelium (fungus) grows over plant surfaces and produces spores, which give the lesions a powdery appearance. If not controlled plants may die. There are remedies ranging from selection of resistant varieties to sprays of neem oil in the early stages. At this stage, suggest you take off the affected leaves, begin spraying the plants, and try to arrest the disease to save the crop. When you do spray, spray in the late afternoon or early evening to prevent sun scald on the foliage. It may be too late, but we hope you can save your zucchini plants.
This tomato is under near lethal stress. The rolled leaves and prominent veins would seem to indicate that its resources are stretched to the limit. Possible stressors: buying a transplant already in flower and fruit and transplanting, too small a pot, overwatering, cold nights (below 50), Improper fertilization etc. The purpling I am seeing is usually related to lack of Phosporous uptake which can be due to cold soils or wrong pH. If you want to continue to grow this plant, you need to remove all flowers and fruit, use a liquid fertilizer on the leaves and soil (fish emulsion is good), check the pot for good drainage (never just use ordinary soil, it compacts). We presume that the pot has good drainage holes. Tomatoes should never sit in water.
We see a few things going on. In the top photo, if those leaves were anywhere near the soil level, we would think snails and/or slugs. Here is a great link for those pests:
We can't make out any definite issue in the photo in the lower left.
In the photo in the lower right, there might be a couple of aphids. Here is a link for those pesky critters:
Some white coloration on the leaves is normal. Excessive whitish "trails" in the leaf is a sign of leafminers. You guessed it, another link...2 actually:
The last one is a great overview.
Since you mention over-watering, its possible that the root system is weak (not enough oxygen in a wet soil) and the plant is not thriving because of this. We don't see the yellow "spots" you refer to, but yellowing can be from nutrient deficiency (which also occurs with unhealthy roots even if you keep adding fertilizer), or if you have specific spots of yellow tissue it could be caused by Thrips insects or any number of things. Make sure your plant is getting full sun for 6-8 hours per day and if in a container, you only water when the soil is dry at the depth of two or three inches. Apply enough water to get drainage from the pot. As the plant grows you'll need to water more often. See this link from the Univ of CA on "all things Tomato"
These are one of the yellow long peppers - there are several types and from a photo alone it's impossible to tell which one you have. There are long Italian frying peppers, banana peppers, and several others. The good news is that once they turn yellow they are ripe so there is no confusion there - peppers that turn red when ripe go from green to red without a stop at yellow, but those that are ripe when yellow go directly from green to yellow.
Most yellow peppers are mild, so you shouldn't have to worry that they will be spicy. But since "heat" is different from person to person, you might cut into a yellow pepper and touch your pinky finger to that cut and then put it to your lips to taste in order to tell if you think it's spicy or not. Enjoy!
There are two things that can cause such symptoms on pepper plants. The first is tobacco mosaic virus, and the second is aphid damage. If it's the virus the problem will get worse and you'll see spots and yellowing of leaves too. There is no cure. This virus can be spread from plant by insects. Read more about it here: http://www.extension.umn.edu/garden/yard-garden/vegetables/tomato-tmv-disease/
The other possibility is aphid damage. If aphids suck out plant juices under the leaves the cells of the plants are then distorted and the plant grows in a crinkled condition. Look carefully under the leaves of the plants to see if there is sign of aphids or aphid litter. Aphids can be green, white, red or black - the litter can be tiny black or white specs...so small that the underside of the leaf can just look dusty or dirty. Spray the undersides of plants with insecticidal soap if you suspect aphids.
If it's aphids the plants may grow out of the damage and be fine - viruses on plants usually have no cure.
This could be a viral disease, and/or a combination of issues. Virus diseases infect tomatoes and can cause leaves to curl, discolor, and/or develop mosaic patterns (which appears in your photo to be affecting the stem and leaves). Its hard to say from the photo, but it appears there may be white, powdery mildew on the plant? This is a fungal disease that affects tomatoes (and many other plants), but powdery mildew would not cause the stem mottling and leaf curl seen here. Additionally, environmental issues, like too much water in the soil around the roots can cause plant distortion from excess release of ethylene gas as a result of "drowning" roots.
We've included some links that may help you to narrow down the cause, but we also recommend that you get a local opinion from a Master Gardner group, home/farm advisor's office, or a local garden center.
This could be a viral disease, and/or a combination of issues. Virus diseases infect tomatoes and can cause leaves to curl, discolor, and/or develop mosaic patterns (which affect the stem and leaves). Its hard to say from the photo, but it appears there may be white, powdery mildew on the plant? This is a fungal disease that affects tomatoes (and many other plants), but powdery mildew would not cause the stem mottling and leaf curl seen here. Additionally, environmental issues, like too much water in the soil around the roots can cause plant distortion from excess release of ethylene gas as a result of "drowning" roots.
We've included some links that may help you to narrow down the cause, but we also recommend that you get a local opinion from a home/farm advisor's office, or a local garden center.