Differentiating the two
The big difference is that most fungus problems can be controlled with fungicides, while viruses cannot. A fungus normally attacks the foliage, while a virus can attack every part of a plant-eventually killing it. Most fungus is transmitted through the air while most viruses are transmitted through the soil.
The most common types of fungus are leaf spots (red, brown, or black), mildew (white), and types of rust (orange and brown). Leaf spots normally affect both sides of leaves, mildew the top side, and rusts underneath. They are fairly easy to prevent and control with a fungicide.
Most viruses enter a plant through the roots. They are prevalent in poorly draining or over-watered soils that never dry out. Many viruses cause leaf wilt; you will see this in the individual leaves-or the whole plant may look sickly and drooping. The only solution when a virus is identified is to try to let the soil dry out. If you catch it early enough, some plants can recover. Unfortunately, by the time most people discover a virus problem, it’s usually too late.