What is the reason of yellowing of garlic and onion leaves

Constantly changing weather is causing the garlic and onion crop to face a lot of problems.
The problem of yellowing is quite visible in the garlic and onion crop due to which there is a huge impact on their growth and development.
The yellowness of garlic and onion can also be caused by fungal and insect borne and nutritional problems.
If it is causes by fungus, use Kasugamycin 5% + Copper Oxychloride 45% WP @ 300 gram/acre or Thiophanate Methyl 70% WP @ 300 gram/acre
If it is because of nutritional problem, use Seaweed@ 400 ml/acre or Humic acid@ 100 gram/acre
If it is due to insect, use Profenofos 40 % + Cypermethrin 4% EC@ 400 ml/acre or Fipronil 40 % + Imidacloprid 40 % WG@ 80 gram/acre.