Answer: The tomato problem you are describing is most likely not a result of mulching or not mulching, but rather in all probability a serious disease. You might wish to take a sample of the leaves (showing both yellowing and spots) to your local County Extension for a positive identification of the problem; their telephone number is 424-9485. That way you will know exactly what problem you are facing.
Unfortunately, in most cases diseased plants should be dug up and removed from the garden and destroyed, along with any plant debris, in order to slow the spread of the disease and reduce the chances of reinfection next year. In addition, you should then follow a rotation of several years between growing tomatoes (and other members of the same family) in that location. From now on you should also look for disease resistant varieties as indicated by capital letters (such as VFFNT) on either the seed packet or plant label.
With regard to slug control, this can be accomplished over time by hand picking and dropping them into soapy water, using traps baited with beer, or commercial slug baits placed according to the label instructions. Some gardeners also report good results from lightly cultivating the soil in early spring to disturb them.
An organic mulch such as straw is almost always a good idea in the vegetable garden as it serves many purposes including weed control, soil temperature moderation, water retention and feeding the soil as it decays over time. It also keeps soil (and certain associated soil-borne nasties) from splashing onto your plants and produce.
In the case of the tomato disease, however, mulch would not have prevented it. I'm sorry about your tomatoes.
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