Pachymetra root rot of sugarcane

The primary symptom is severe decay and reduction of the root system, where roots become soft, necrotic, and brownish in appearance.

Affected plants show poor vigor, stunted growth, and visible wilting of leaves during periods of water stress, as they can no longer efficiently absorb water.

Tiller numbers are significantly reduced, leading to thinning of the crop stand and uneven development across the field.

The root system loses its mechanical integrity, making the cane prone to lodging even under moderate wind conditions.

• Loss of primary and secondary root mass.
• Yellowing of older leaves due to nutrient deficiency.
• Root tissues exhibit a soft, water-soaked rot that turns brown to black.

The disease is caused by the soil-borne oomycete pathogen Pachymetra chaunorhiza. It is a highly specialized parasite that exclusively infects sugarcane.

The pathogen survives in the soil for several years in the form of thick-walled oospores, which are resistant to harsh environmental conditions.

When environmental conditions are favorable, oospores germinate and produce zoospores, which are attracted to the root exudates of the sugarcane plant.

Once the zoospores reach the host roots, they encyst and penetrate the root cortex, establishing an infection that spreads through the root system.

The biological cycle of the fungus is strictly tied to the presence of living sugarcane roots, making it a persistent challenge in long-term cane production.

The development of the disease is highly favored by moist soil conditions, as free water is necessary for the movement of zoospores in the soil profile.

Soil temperatures between 20°C and 30°C provide the optimal environment for the pathogen to thrive and for the rapid progression of the infection.

Continuous sugarcane cultivation promotes the buildup of high inoculum levels in the soil, exacerbating the disease severity year after year.

Poorly drained soils or heavy clay textures are particularly conducive to the spread of the disease due to prolonged water retention.

Frequent mechanical soil disturbance can damage roots, creating open sites for the pathogen to enter and colonize the root tissue more aggressively.

Pachymetra root rot causes significant reductions in cane yield due to the plant's inability to maintain proper growth rates and nutrient uptake.

The disease negatively impacts the commercial sugar content (CCS) of the cane, as stressed plants produce lower levels of sucrose.

Increased susceptibility to lodging complicates mechanical harvesting operations and increases labor costs for gathering fallen stalks.

Severe infestations reduce the productive lifespan of a ratoon crop, often necessitating early replanting of affected fields.

The cumulative effect of root rot damages the overall economic efficiency of sugarcane plantations, impacting the long-term profitability of the industry.

The most reliable control method is the deployment of resistant or tolerant sugarcane varieties that have been bred to withstand the pathogen.

Implementing a crop rotation system with non-host crops helps reduce the soil inoculum density and breaks the infection cycle.

Improving soil drainage through land leveling and sub-surface drainage systems minimizes the standing water required for zoospore transmission.

Strict sanitation protocols for farm machinery prevent the spread of infested soil from affected fields to clean agricultural areas.

Regular diagnostic testing of soil and root samples is recommended in high-risk zones to assess the pathogen population before planting.