Gary is right that the bottom picture is a clear case of blossom end rot (BER) in squash. Once an individual squash fruit shows the dark mushy area on the end, it should be removed; it will not develop right. But by itself the BER is not a disease and does not kill the plant. Whether a particular squash fruit is going to show BER or not is determined at the time the fruit is set. BER is a physiological disorder caused by the plant being unable to uptake calcium from the soil. Usually the problem is not a deficiency of calcium in the soil, it is something causing the plant to be unable to absorb it. This can be a number of things including low night time temperatures, unusually cool or hot weather, drought, or wet soil conditions, or especially a lot of wet-dry cycles, when the plant gets dried out and then wet again. If you can fix the problem, the plant should go on to produce healthy fruit.
HOWEVER, that is not the cause of those diseased leaves. The leaves are probably showing a bad case of powdery mildew. Remove all the affected leaves and then treat all the remaining leaves with an anti-fungal, including baking soda solution OR hydrogen peroxide OR diluted milk. But all those things work better preventatively or if you catch it at the very beginning. Yours looks very bad and they may not be able to be saved.
For future reference here is what powdery mildew looks like at the beginning:
Little greyish or white dots that look kind of dusty/powdery (where the disease gets its name)