Sorghum downy mildew

The pathogen's mycelium grows intercellulary, spreading through the plant's vascular tissues. Under high humidity, it produces conidiophores and conidia on the leaf surface, facilitating further spread.

A critical biological feature of this pathogen is the production of oospores, which allow it to survive in soil and seeds for several years, serving as the primary inoculum source.

Peronosclerospora sorghi exhibits high genetic variability, leading to the emergence of new physiological races capable of overcoming resistance in previously tolerant cultivars.

Development depends heavily on soil temperature during germination. Optimal conditions for infection involve moderate temperatures and moisture during the seedling emergence phase.

The primary hosts for this pathogen include various sorghum species such as grain sorghum, sweet sorghum, and sudangrass. Maize can also be affected by the disease.

The disease significantly reduces plant population density. Severely infected plants often die at the seedling stage or experience extreme stunting in growth.

Systemically infected adult plants exhibit pronounced dwarfing. These plants often fail to produce fully developed panicles, or they produce barren, malformed floral structures.

The economic impact is substantial, leading to severe yield losses in both grain and biomass. In epidemic conditions, yield losses can easily exceed 50-80% in infected fields.

Pathogen damage to tissues makes the plant susceptible to secondary infections by bacteria and other fungi, which further accelerates the overall crop decline.

Primary infections typically occur during the seedling stage, when spores penetrate the mesocotyl or root system of the plant from contaminated soil.

Mass dissemination of conidia happens during the active growing season, favored by temperatures between 20–25°C and high relative humidity (above 90%), usually at night.

Wind is the primary vector for conidia distribution, allowing the pathogen to spread across large areas from initial hotspots within a field.

The most critical window for infection is from germination to the 5-6 leaf stage, as the plant is most vulnerable to systemic colonization during this period.

Toward the end of the season, oospores form within the tissues of infected plants. These spores are returned to the soil with crop residues, ensuring the pathogen's survival through winter.

The first noticeable sign is the development of light-green or chlorotic spots and streaks on young leaves, often described as a "striped" pattern along the veins.

Under high humidity, a dense, whitish, or grayish downy growth appears on the underside of the leaves, consisting of the pathogen's reproductive structures.

As the disease progresses, chlorotic areas become necrotic and turn brown. Leaf tissue may shred, giving the plant a distinctive "shredded leaf" appearance.

A typical sign of systemic infection is abnormal plant development: plants appear bushy or stunted due to excessive lateral tillering and lack of proper apical growth.

• Chlorotic streaks along leaf veins
• Downy white growth on the leaf underside
• Stunting and panicle deformation
• Premature leaf tissue necrosis
• Excessive tillering of affected plants

The most effective strategy is the use of resistant sorghum hybrids and cultivars that have been bred for tolerance to regional strains of the pathogen.

Crop rotation is essential; sorghum should not be planted in the same field for at least 3-4 years to allow for the natural degradation of oospores in the soil.

Seed treatment with systemic fungicides, such as metalaxyl, is the standard practice for protecting seedlings from soil-borne infection during the critical early stages.

Deep plowing of crop residues is recommended to bury infected debris, which accelerates the decomposition process and reduces the inoculum load for the next season.

Foliar fungicide applications during the growing season are generally limited in efficacy and are usually only considered when economic thresholds are met and infection is detected early.