Root rot :  Rhizoctonia solani

Symptoms

• The infection result in stunted growth.
• Ultimately the entire plant dry. 
• Rhizoctonia solani causes more losses and can attack older plants. 

Management

• Soil sterilization controls the diseases.
• Drenching with Bacillus subtilis 2g/litre of water

Image source:

https://negreenhouseupdate.info/photos/gerbera-%E2%80%93-web-blight-rhizoctonia