Stem canker on sunflower is caused by the fungal pathogen "Phomopsis helianthi" (teleomorph = "Diaporthe helianthi"). There are many "Phomopsis" species that have a broad host range, but "Phomopsis helianthi" is the primary pathogen that infects sunflower, although it has also been seen to infect other, secondary hosts. Some "Phomopsis" species have been found to be able to cause disease on several host genera. Sunflower hosts that found in high densities are particularly susceptible to attack by the pathogen due to the microclimate formed in crowded areas. This microclimate has a high humidity and allows for infection to proceed rapidly. There has also been recent molecular evidence to suggest there are several genetically distinct species of "Phomopsis" that can cause disease on the sunflower hosts.