Albugo candida is a widespread plant pathogen commonly known as white rust. Despite its name, it is not a true fungus but an oomycete, a group of fungus-like organisms more closely related to algae. As an obligate parasite, Albugo candida requires a living host plant to grow and reproduce, impacting various agricultural crops and wild plants worldwide. This pathogen can cause noticeable symptoms and reduced plant vigor in gardens.
Identifying White Rust
Gardeners can identify white rust by observing distinct symptoms primarily on the above-ground parts of susceptible plants. The most characteristic sign is white or cream-colored, blister-like pustules, typically 1 to 2 millimeters in diameter. These raised pustules usually form on the underside of leaves, but also on stems and floral structures. As the disease progresses, these pustules rupture, releasing a powdery white mass of spores.
Beyond these lesions, Albugo candida can cause systemic infections leading to plant distortion. Leaves may curl or twist, and stems or flower parts can become swollen and malformed. This abnormal growth, particularly in inflorescences, is often referred to as “stagheads.” The pathogen primarily affects plants within the Brassicaceae family, which includes many common garden vegetables such as cabbage, broccoli, kale, radish, mustard greens, arugula, turnips, and horseradish. Other susceptible plants, including spinach, can also show symptoms.
How White Rust Spreads and Survives
The life cycle of Albugo candida involves asexual and sexual reproduction, enabling its spread and survival. Asexual spores, sporangia, are produced within white pustules on infected plants. Once mature, sporangia are dispersed by wind, splashing rain, and insects, carrying the pathogen to new host plants. Upon landing on a susceptible plant, each sporangium can release several motile zoospores.
These zoospores are equipped with flagella, allowing them to swim in thin films of water on plant surfaces, eventually forming germ tubes that penetrate host tissues, often through natural openings like stomata. The pathogen thrives in cool, moist conditions, with optimal temperatures for infection between 13 and 25 degrees Celsius (55-77°F). Zoospore germination is favored by temperatures between 10 and 14 degrees Celsius (50-57°F).
For long-term survival, Albugo candida produces thick-walled sexual spores called oospores, formed in deeper host tissues or within “stagheads.” These resilient oospores can remain dormant in plant debris or soil for extended periods, even several years, allowing the pathogen to overwinter. This resilience ensures the pathogen can re-emerge to initiate new infections when environmental conditions become favorable.
Managing White Rust in Your Garden
Managing white rust relies on proactive cultural practices to prevent its establishment and spread. Crop rotation is a foundational strategy; avoiding susceptible Brassica crops in the same spot for at least two to three years helps reduce oospore buildup in the soil. Longer rotations may be beneficial in drier climates. Proper plant spacing promotes better air circulation, which helps to dry leaf surfaces more quickly and creates an environment less favorable for spore germination.
Sanitation is another practical measure. Promptly removing and destroying infected plant material, including leaves, stems, and “stagheads,” reduces the inoculum available to spread the disease. Regularly cleaning garden tools and equipment prevents accidental transmission of spores between plants. Altering watering practices, such as avoiding overhead irrigation and watering at the base of plants, particularly in the evening, minimizes prolonged leaf wetness conducive to white rust development.
Selecting and planting varieties less susceptible to white rust can offer natural resistance, though truly immune cultivars may not be available for all host plants. Controlling cruciferous weeds, like Shepherd’s purse, around the garden is also important, as they can act as alternative hosts for different races of Albugo candida. Using certified disease-free seeds prevents the introduction of oospores into your garden. While cultural practices are the primary defense, fungicides targeting oomycetes, similar to those used for downy mildew, may be considered for severe outbreaks. If fungicides are used, follow label instructions regarding application timing and rotation of products with different modes of action to prevent resistant pathogen strains.