Sweet potato phomopsis

Initial symptoms include the appearance of small, dark, and slightly sunken spots on the epidermis of the sweet potato tuber during or after harvest.

The flesh underneath these lesions turns brown, dry, and develops a spongy, corky texture as the fungal infection progresses throughout the tuber.

Black, pinhead-sized structures known as pycnidia appear on the surface of the infected areas, serving as a diagnostic feature for positive identification.

In cases of severe infection, the foliage may show signs of chlorosis and wilting, though the disease is most frequently observed post-harvest.

As the rot advances, the tubers may become brittle, light in weight, and eventually shrivel, making them entirely unsuitable for any commercial use.

The disease is caused by the fungus Diaporthe destruens (synonym Phomopsis batatas), an ascomycete pathogen that primarily infects the storage roots.

This fungus survives in the soil and on infected crop residues, acting as a persistent source of inoculum for subsequent planting seasons.

The life cycle involves the formation of pycnidia, which release conidia that spread the disease through splashing rain, irrigation water, and contaminated tools.

The pathogen is highly opportunistic, typically invading the tuber tissues through wounds, cuts, or areas damaged by insect activity.

Once inside the host tissue, the fungus secretes enzymes that break down the plant cell walls, leading to the development of necrotic lesions and tissue decay.

Warm and moist environmental conditions significantly favor the development and spread of Diaporthe destruens in the field.

Temperatures ranging from 20°C to 28°C are optimal for the growth of the fungus and the subsequent colonization of sweet potato tissues.

Poor drainage and high soil moisture levels create an environment where the fungus can easily infect the developing tubers during the late growth stages.

Mechanical injury to the vines or roots during cultivation and harvesting operations provides ideal entry points for the fungal spores to initiate infection.

Inadequate ventilation in storage facilities, combined with high relative humidity, promotes the rapid spread of the pathogen from infected to healthy tubers.

Phomopsis root rot is a devastating disease that can cause substantial post-harvest losses, sometimes exceeding 50% of the total yield in untreated fields.

The infection significantly degrades the tuber quality, rendering the crop unmarketable and resulting in major economic losses for the farmer.

The disease limits the shelf life of the produce, as infected tubers continue to rot during transit or storage, potentially affecting surrounding healthy tubers.

The accumulation of the pathogen in the soil restricts the reuse of land for sweet potato production, necessitating costly land management interventions.

In addition to physical loss, the infection can produce mycotoxins, making the affected tubers hazardous and unfit for both human consumption and animal feed.

The primary control strategy involves rigorous crop rotation, planting sweet potatoes in the same field no more than once every three or four years.

Only disease-free, high-quality propagation material (slips) should be used for planting to ensure the absence of initial fungal inoculum.

• Proper wound healing (curing) of tubers immediately after harvest at correct temperatures.
• Strict sanitation measures to remove all infected plant debris from the fields post-harvest.
• Use of certified, fungicide-treated propagation material if the pathogen is known to be present in the region.

Storage areas must be properly disinfected and maintained at optimal temperatures with adequate airflow to minimize the risk of infection spread.

Monitoring for insect pests is essential, as managing insect populations reduces the number of entry wounds available for fungal invasion.