Question
I am trying to grow tomato plants for the first time.  The plant I have was purchased from a graden shop and had 1 tomato on it.  

I live in Mesa Arizona a suburb of Phoenix.  The plant was in partial sun during the hot month of September.  It is now in full sun.  The temperatures have been in the 80's and 90's.  I water the plant enough to keep the soil moist at all times.  There are two tomatos on the plant and a thrid that is now growing.

The problem is that the leaves of the plant gradually turn a very light brown.  They are not dry and brittle but feel like a normal leave.   It can occur anywhere on the plant and when it does the entire branch of the plant turns that color.

I have attached three photos to show you what I mean.  Can you tell me what is happening and how I can fix the problem.

Answer
It looks like the leaves have suffered from sun scald or from too much water. Since the plant had been out the entire month of September, it should have acclimated to the sunny conditions, so I doubt it is sun scald. You did mention that you are keeping the soil moist at all times. This is actually bad for the plant. Tomatoes should only be watered when the soil is dry 2" below the soil level. In most areas, this means watering twice per week at an average of 1.5" per week. When a plant is overwatered, the nutrients are leeched from the soil. The roots also become water-logged causing them to not be able to absorb oxygen and nutrients. The first signs are leaf discoloring and eventually no blooming.

My advice is to water less and mulch around the base of the plant with 2" of dried grass clippings, straw, or shredded leaves. This will help cool the soil and reduce the need for watering. When the soil does start to dry out, water deeply. Shallow watering causes the roots to reach for the soil instead of straight down. The results are similar to the ones described above. And finally, amend the soil every year with compost, peat, and other organic materials such as manure. These will improve the tilth of the soil, improve drainage, and provide nutrients throughout the growing season.

I hope this answered your question. Please write again if I can ever provide assistance.

Regards,

Mike