Fusarium wilt is most prevalent in indoor gardens and greenhouses where it does happen outdoors. Reservoirs with nutrient solutions above 75° F (24° C) create a perfect condition for Fusarium to flourish to keep the reservoir cooled.
If water is an issue, then you will need to drain the reservoir and refill it with fresh water as well as nutrients. Fusarium wilt starts showing small spots on older lower leaves.
Interveinal chlorosis develops on the leaves very quickly and leaf tips may curl before wilting along with dying off. With fusarium wilt, either part of the plant will wilt and even possibly the whole cannabis plant.
The disease starts in the plant’s xylem, which is the base of the fluid transport system. The reason the cannabis plant wilts is that the fungi clog up the plant’s fluid flow in the tissue.
You can check your plants for this, by cutting one of the main stems in half and looking for a #0excludeGlossary color inside. Cleanliness is the best prevention, along with using fresh, sterile growing mediums.
Not reusing old growing mediums will help prevent this and avoid too much nitrogen fertilizer. Add hydrogen peroxide to the reservoir and keep the water temperatures down.
If there are any parts of the plants that are infected, cut them off and throw them away. You can use Streptomyces griseoviridis or Burkholderia cepacia and Trichoderma for biological treatment.
If you want to spray your plants, you can treat seeds with a chemical fungicide to kill the infection. Chemical fungicides are not very effective on plant foliage.