Phakopsora pachyrhizi is a fungus that causes the disease known as Asian soybean rust. In Brazil, it is called "rust" due to the rusty appearance it causes in infected leaves. Since its introduction into the country in 2001, Asian soybean rust has caused significant losses, with productivity reductions that can exceed 80% in epidemic conditions without adequate management.
Taxonomic classification of Phakopsora pachyrhizi:
- Kingdom: Fungi
- Phylum: Basidiomycota
- Class: Pucciniomycetes
- Order: Pucciniales (formerly Uredinales)
- Family: Phakopsoraceae
- Genre: Phakopsora
- Species: Phakopsora pachyrhizi
Crops attacked
Asian rust mainly affects soybeans. However, the fungus infects more than 150 species of the Fabaceae family, including Phaseolus (common bean), Pueraria lobata and Neotonia wightii. There are reports of infection in non-legume species.
Symptoms
The first symptoms appear as small dark spots on the lower third of the plant, ranging from grayish-green to light brown.
These lesions evolve, forming protrusions on the underside of the leaf, known as uredia, from which uredospores are released to spread the disease.
Infected leaves turn yellow, dry and fall prematurely, compromising grain filling and drastically reducing productivity.
The lesion density can reach up to 100 pustules/cm², significantly reducing the photosynthetic area and grain weight.
Etiology and characteristics
Asian rust is caused by the biotrophic fungus Phakopsora pachyrhizi, which requires living plants to complete its cycle.
Its rapid spread and ability to adapt make it one of the greatest challenges for modern agriculture.
Uredospores, the reproductive structures of the fungus, are responsible for initiating new infections and are widely spread by air currents. The dispersion speed of uredospores can reach 10–20 km/h in moderate winds, with studies indicating transcontinental transport in jet streams.
Phakopsora pachyrhizi It is not transmitted by seeds and depends on the wind for the dispersal of its uredospores.
Favorable conditions for infection include temperatures between 15°C and 25°C and at least six hours of leaf wetness.
Frequent rainfall and high humidity contribute to severe epidemics of the disease.
The fungus survives in the off-season on living plants, making a fallow period essential to reduce the initial inoculum.
Control
Effective management of Asian rust requires a combination of strategies:
- Toilet void: period of absence of live soybean plants in the off-season to reduce the fungus population.
- Resilient and early cultivars: use of resistant varieties or sown at the beginning of the recommended season to escape the period of greatest disease pressure.
- Monitoring: regular inspections to detect early signs of disease. Tools such as spore collectors help in making decisions about the use of fungicides.
- fungicides: preventive application or at the onset of symptoms, using products with different modes of action to avoid fungal resistance. Rotation and mixing of fungicides are essential to maintain the effectiveness of treatments.
- Genetic resistance: Cultivars with resistance genes offer partial protection, but the virulence of the fungus limits their effectiveness. Stacked genes are under development.
Note on resistance: the resistance of P.pachyrhizi to site-specific fungicides, such as triazoles (FRAC 3) and strobilurins (FRAC 11), is a growing challenge. Susceptibility tests indicate EC50 (effective concentration for 50% inhibition) up to 10 times higher in resistant populations.
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