White rust of salsify

Taxonomy and nature of the pathogen: Cystopus tragopogonis (syn. Albugo tragopogonis) is a member of the family Albuginaceae, belonging to the class Oomycota. Unlike true fungi, it is a water mold that acts as an obligate parasite. It is a highly specialized pathogen that survives solely by infecting the living tissues of specific host plants.

Diseases and susceptible crops: This pathogen causes a disease commonly known as white rust. It primarily attacks members of the Asteraceae family. Key crops affected include salsify (oyster plant), sunflower, lettuce, and various ornamental flowers. The disease is also widely distributed among wild species, which often serve as an overwintering reservoir for the pathogen.

Biology and life cycle: The pathogen's life cycle is characterized by the formation of white, chalky pustules on the host's surface, which break the epidermis to release sporangia. These are dispersed by wind and water splash. Under favorable conditions, the zoospores infect the host directly. Oospores are also produced, providing a resilient survival mechanism in soil and infected plant debris during periods of environmental stress.

Developmental conditions and impact: White rust thrives in cool, moist environments. High humidity levels combined with moderate temperatures accelerate the release of zoospores and the rate of infection. The pathogen causes significant morphological changes, including swelling of plant parts, chlorosis, and necrosis. The resulting systemic stress reduces the host plant's vigor, yield potential, and aesthetic quality in ornamental cases.

Control and management strategies: Effective management of Cystopus tragopogonis requires a combination of sanitation and chemical strategies:

• Strict crop rotation schedules, avoiding Asteraceae crops in the same field for several years.
• Removal and destruction of infected plant material to reduce inoculum levels.
• Weed management, specifically targeting wild hosts that harbor the pathogen in the field margins.
• Use of appropriate fungicides, such as metalaxyl or copper-based compounds, applied preventively when environmental conditions favor disease development.