I've had this basil plant at home terrace since 2 months ago , 4 days before I've noticed one black small bug under leaves but the day after it disappeared , I am used to cover it with a net shield (not sure if it pass probable amount of sun light) , now after my first and second flower grew i noticed that leaves are getting so weak and may be about to fall , my question is what is the reason of leaves weakness and how to stop it ??

plant's environment : I live in north Africa (Egypt) day time temperature varies from 26C to 28C with fine sun light that last for about 5 hours , at night temperature varies from 18C to 21C , today humidity is 49% and wind speed 14 km/hr .
plant's current care routine : I don't use any care routine I only irrigate is with water . I use my finger tips to indicate the time for irrigation .

Weak stems usually mean not enough sunlight. Basil plants require a lot of light. Full sun means over 6 hours a day - between your netting and only 5 hours of sun a day, it could be that your plant just isn't getting enough.

sherouk

I don't know how much nutrition your soil holds, but I think starting to add a little liquid fertilizer to the water would be a good idea if you haven't already done so. A weak solution will do. You could also try to cut a few of the basil plants to give the remaining a little more light and room to grow.

sherouk,
Yours is a good question and the photos are excellent. Thanks to you I have done some quick research and have learned that most of the world's basil is grown in Egypt!

I agree with @william that you might want to add a diluted liquid fertilizer.

Are you growing to save the seeds for replanting? or to use the leaves in cooking? If you are not attempting to save seeds you could pinch off the flowers to encourage new leaf growth.