Sugarcane downy mildew

Sclerospora sacchari is an obligate pathogen belonging to the kingdom Chromista, phylum Oomycota. It is the causative agent of downy mildew, a significant systemic disease affecting sugarcane production.

The pathogen reproduces by forming sporangia on conidiophores, which emerge from the stomata on the lower surface of leaves under conditions of high humidity.

Oospores are formed within the infected host tissue, serving as a survival mechanism that allows the pathogen to persist in the soil for extended periods.

The pathogen utilizes motile zoospores for dispersal in water films, whereas conidia are effectively disseminated by wind to neighboring plants.

As a systemic pathogen, the mycelium spreads throughout the vascular system of the sugarcane plant, interfering with nutrient transport and overall metabolic efficiency.

The primary host is sugarcane (Saccharum officinarum), although other grasses can be susceptible depending on the geographical location and strain of the pathogen.

The disease causes substantial reductions in sugar content and juice quality, which are critical metrics for the commercial sugar industry.

Infected plants show severe stunting, reduced stem diameter, and shortened internodes, leading to a massive loss in overall biomass production.

Severe infections frequently result in the death of the meristematic tissue, leading to the collapse and death of the entire plant.

Due to its systemic nature, the pathogen is easily transported through vegetative propagation materials, making disease management and quarantine vital for trade.

Development of the disease is highly favored by warm and humid conditions, with an optimal temperature range typically between 20°C and 28°C.

Sporulation is most intense during nighttime and early morning hours when humidity levels are high and the foliage is covered in dew.

Rainfall is a major driver of epidemics, as free water is necessary for the production and swimming movement of zoospores toward host tissues.

During dry spells, the disease progress slows down, yet the pathogen remains present in the field as oospores buried in crop residues.

Airborne conidia allow for rapid spread across fields during the main growing season, provided that dew or rain is present for infection to occur.

Initial symptoms present as narrow chlorotic streaks that develop along the leaf veins, gradually expanding as the infection matures.

A distinctive white, downy fungal growth appears on the abaxial leaf surface, which corresponds to the sporulation stage of the oomycete.

As the disease progresses, leaves may become curled, brittle, and exhibit necrotic lesions, eventually leading to premature desiccation.

Systemically infected plants often display excessive tillering, resulting in a stunted, bushy appearance that distinguishes them from healthy stalks.

• Chlorotic leaf striping.
• White, fuzzy sporulation on leaf undersides.
• Stunted growth and shorter internodes.
• Significant decrease in juice quality.
• Premature leaf necrosis and wilting.

The implementation of resistant varieties is the most effective and sustainable strategy for controlling sugarcane downy mildew.

Strict phytosanitary and quarantine measures prevent the introduction of infected planting material into regions currently free of the disease.

Hot water treatment of seed canes (typically 50-52°C for 2 hours) is a standard practice to eradicate internal mycelium before planting.

Good agricultural practices, including the removal of infected stools and debris, are crucial to reducing the primary inoculum source in fields.

Fungicide applications are sometimes used in nurseries to protect young crops, but reliance on genetic resistance remains the primary management tool.