I’m posting this article why….? You guessed it. The leaves on two of my tomato plants are turning yellow and I know ‘zip’ about vegetable gardening . Oh, it gets even worse . I don’t know anyone who knows anything about vegetable gardening. So what does one (me) do when one (I) finds oneself (myself) lost in a world devoid of all knowledge pertaining to vegetable gardening….hummmm….I KNOW!!! One (me) turns to the the all knowing Cyber Gods and That’s exactly what I did. I ran (not walked) directly to my computer and did an internet search asking The Powers that Be: ‘Why are the leaves on my tomato plant turning yellow’? *BAM* INSTANTANIOULY all of these answers just came to me. I’m telling you: ‘it’s a miracle’.
The article below gave me the best info.
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By Vanessa Richins
If only there was a simple answer to this. There are many different factors that can cause tomato plants to develop yellow leaves.
Here are some of the most common reasons:
- Under-watering: When plants don’t get enough water, they start to wilt and lose color. Under-watered plants are also more prone to attack from diseases and pests.
- Over-watering: On the flip side, overwatering can also cause yellow leaves. Over-watering can also cause root rots.
- Aphids: If you see sticky honeydew and small green, black, or red insects, you have aphids. A strong spray of water can knock these off. Ladybugs love to eat aphids.
- Nitrogen: Another cause of yellow leaves could be a lack of nitrogen. Test your soil with a basic kit available at home improvement stores or nurseries, or you can send in your soil for more detailed testing through your local cooperative extension. The cost varies by location but for example, it costs $15 here in Utah. If this is the case, add compost or a nitrogen fertilizer.
- Tomato Hornworm: If you see chewed yellow leaves, you may have tomato hornworms. Check the plant for these large, fat green worms. The easiest remedy is to just pick them off.
- Iron: Another nutrient deficiency signaled by yellow leaves is lack of iron. Add an iron chelate to the soil or the leaves, depending on the form.
These are only some of the causes. Other possibilities include: Early blight, psyllids, sunburn, curly top virus, whiteflies, flea beetles, and Septoria Leaf Spot.
If you are unsure of the cause, you can call a fellow gardener, ask here on Tomato Casual, or consult your local extension office.
Zucchini is a warm weather crop and will not tolerate frosts. It requires full sun to partial shade and will do well in almost any soil with good drainage. Zucchini is a heavy feeder and the addition of 
Try to keep the water off the leaves and foliage. Insufficient water will cause the fruit to fall off before it matures. Leaves will wilt during very hot weather, but will recover when watered. 
