Hello Sarina ,
could be a nitrogen deficiency in the zucchinis,as they are shallow rooted and the only available N would be from the lucerne ,so maybe put some dynamic liffter in water ,disove as much as posssible and pour on th e top side of the plant ,slowly ,we dont want to have the wrong osmatic affect ,so keep about 20cm away from plant centre and drip feed.
the problem on the leaves is called pawdery mildew,very common in zuccs and qucerbets ,I have always believed it is seed born ,although the so called xperts dont agree.although when i was farming I believed the seed and chemical companies ,had there heads together .
to find an organic cure would be amazing as the chemical cure is very xpensive and sometimes doesnt work .
good luck Tiny