Wartdisease is caused by the fungus: Synchytrium endobioticum. This fungus causes considerable yield loss and forms tumors on stems, stolons and tubers. Until last year two races of this fungus were recognised in The Netherlands: race 1 and race 2. Nowadays also race 6, which is very similar to race 2, is present. Fortunately cultivars resistant to race 2 are most of the times also resistant to race 6. As can be noticed in the variety list, most of the Averis cultivars are resistant or less susceptible to the races 1 and 2.

Recently another race is found. Race 18.