White rust of sweet potato

The disease is caused by the oomycete Albugo ipomoeae-panduratae. It is an obligate parasite that requires living host tissue to complete its life cycle and maintain its population.

This pathogen belongs to the group of white rust fungi (Peronosporales). It reproduces by forming sporangia, which are released and disseminated to infect healthy leaves and stems under favorable conditions.

The fungus can also survive unfavorable seasons by forming thick-walled resting spores known as oospores, which remain in the soil or on crop debris, ready to initiate new infections.

The mycelium grows within the intercellular spaces of the host, utilizing specialized structures called haustoria to absorb nutrients directly from the plant cells without immediately killing them.

As the pathogen grows, it ruptures the host epidermis to produce white pustules, which characterize this specific type of plant disease across various host species in the Convolvulaceae family.

Symptoms appear as small, white to creamy-colored pustules, primarily on the underside of leaves, which correspond to chlorotic or pale spots on the upper leaf surface.

As the infection progresses, these pustules can coalesce, leading to severe yellowing and eventual necrosis of the foliage, which significantly weakens the plant's photosynthetic capacity.

Affected stems and petioles often exhibit abnormal swelling, distortion, and stunted growth, as the parasite induces hyperplastic changes in the host tissue during its development.

In humid conditions, a white, powdery mass of spores becomes visible on the pustules, which can be easily detached and spread by wind, rain splashes, or contaminated tools.

Severe infections lead to premature senescence and leaf drop, reducing the canopy cover and negatively impacting the overall biomass and storage root development of the sweet potato plant.

Albugo ipomoeae-panduratae thrives in high-humidity environments. The presence of free moisture on the leaf surface is a prerequisite for the germination of sporangia and subsequent infection.

Cool to moderate temperatures, typically between 15°C and 22°C, combined with prolonged wet periods such as heavy dew or continuous rainfall, create the perfect conditions for outbreaks.

Poor aeration in dense plantings contributes to the localized build-up of humidity, making these areas significantly more prone to rapid spread of the white rust pathogen.

The disease cycle is accelerated by periods of fluctuating weather that provide the necessary moisture for sporulation followed by drying periods that facilitate the dispersal of spores.

Inappropriate field management, such as overhead irrigation that keeps foliage wet for extended periods, creates an environment that heavily favors the proliferation of the oomycete.

The primary economic impact is a substantial reduction in yield due to the loss of functional leaf area, which limits the carbohydrate synthesis necessary for root tuber development.

Plants affected by white rust are weakened and become more susceptible to secondary infections, such as soft rot, which further reduces the storage quality of the harvested tubers.

Severe defoliation limits the plant's ability to store sufficient energy, resulting in smaller, lower-quality tubers that may be rejected by commercial markets or have poor nutritional value.

In nursery settings, the pathogen can cause significant losses in vine production, rendering the infected material unsuitable for propagation and endangering future plantings.

• Reduced overall tuber yield.
• Increased risk of secondary infections.
• Loss of propagation material viability.

Implementing a strict crop rotation schedule, avoiding Convolvulaceae species for at least three years, is essential to reduce the inoculum levels remaining in the soil.

Sanitation practices, including the removal and destruction of all infected crop residues after harvest, are critical to eliminating primary sources of wintering spores.

Using only healthy, certified propagation material is the most effective way to prevent the introduction of the pathogen into new, clean growing areas.

Cultural control includes ensuring adequate plant spacing to promote airflow, and using drip irrigation instead of overhead spray to keep leaf surfaces dry.

If the infection reaches economic threshold levels, preventive application of copper-based fungicides or systemic oomyceticides can help manage the spread during the growing season.