zucchini leaves dying
I am new to gardening so please excuse me if I ask dumb questions. :) I have 3 zucchini plants, and all were doing great for a few months. In mid- May I got my first zucchini, and since then I have got maybe 15 more. However, the last few weeks the bottom leaves on all my plants have been getting dry and yellowing, then turning brown. I have been cutting them off (should I be doing this?) but then soon the leaves above that spot do the same. Also, most of the leaves unless they are brand new growth have brown sections and are lighter green than the new leaves, but the veins are still green. At first, it did not seem to affect the zucchinis and they continued to grow big and healthy. But this week, most of the new tiny females have turned yellow and the flower dries up before it has had a chance to open. It has been very hot (I'm in California and we have been having 100+ degree weather) but I have been trying to keep the plants well watered. But could the weather be the cause? I have heard that zucchini usually does well in hot weather. One last thing: I have occasionally found little spiders and spider webs on the leaves... they seem to be eating bugs though. But could they be hurting the plant?
Thanks in advance for the help! I am trying to figure out how to post pics as I am having a hard time describing the problem exactly. The first picture shows white powdery spots that just showed up today. :(