Question
Have squash plants in bins -were very large and healthy until started to bloom. leaves started to get what looked like burnt edges and now turning yellow like they have been too dry but soil has always been moist.  Too much moisture?.  Also the weather has turned very hot -up to 90+degrees some days but aren't zuchinni supposed to be hot weather plants?

Answer
Hi Barbara,

Based on your description, I suspect your plants are experiencing abiotic scorch.  Since the temperature has recently gotten hot and the water need has increased, the plants may not have been prepared for the drastic change.   The root system may not be adequate to support such vigorous plant growth.  

You may want to fertilize the plants will a root stimulating fertilizer.  This will be one that contains a high amount of phosphorus.  If you read the label on a fertilizer bag, you will see a number such as 10-20-10.  These numbers refer to the amount of nitrogen-phosphorus-potassium in the fertilizer.  You would want to stay away from products high in nitrogen (ie. 29-2-5) and use something with a large middle number.  

Continue to water your plants as needed.  It is best to water deeply early in the day and if at all possible, overhead irrigation should be avoided.  I think it is unlikely based on your description that the problem is due to a leaf spot pathogen, but it is possible.  You may want to review some of the images of leaf spots at the following website: http://vegetablemdonline.ppath.cornell.edu/diagnostickeys/CucurLeaf/CucLeafKey.h...
Overhead irrigation causes lots of splashing which can spread pathogen spores to healthy leaves.  This is way drip irrigation or pointing your waterer directly at the soil is recommended.

Good luck!