Sweet potato scab

The first signs of scab appear on young leaves, stems, and petioles as small, circular, brown or gray spots. Over time, these spots enlarge and coalesce.

Severely affected leaves curl and deform, leading to stunted growth of the entire vine. Deep ulcers may form on petioles, hindering nutrient transport to the leaves.

In the affected areas, epidermal tissue proliferates, forming characteristic corky outgrowths or wart-like bumps, similar to classic scab symptoms on other crops.

If the terminal buds are infected, they may die off, stimulating undesirable lateral branching and bushy plant growth, while the new shoots remain infected.

• Darkening and curling of leaf blades
• Small brown spots on stems
• Corky outgrowths on petioles
• Reduced overall rate of vegetative growth
• Death of growing points at early stages

The causal agent of the disease is the ascomycete fungus Elsinoë batatas. This is a highly specialized pathogen that affects exclusively sweet potatoes (Ipomoea batatas).

The infection persists in plant debris, in the soil, and on infected planting material (vines/cuttings). Spore dispersal occurs via rain splashes, irrigation water, and wind currents.

The fungus attacks the aerial parts of the plant at various growth stages. Secondary infection continues throughout the growing season under favorable weather conditions.

The fungal mycelium develops within plant tissues, causing characteristic deformation of the epidermis and the formation of fruiting bodies. Infected cuttings are the primary source of spreading the pathogen to new fields.

The biology of the pathogen is closely tied to high humidity, making it extremely aggressive in tropical and subtropical regions where the crop is cultivated.

The primary factor for active disease development is high relative humidity. Prolonged rainfall creates an ideal environment for spore germination.

The temperature range most favorable for infection is between 20–28 degrees Celsius. Under these conditions, the fungal incubation period is significantly reduced.

Dense planting, which restricts proper air circulation and prevents foliage from drying out quickly after dew, significantly increases the risk of an outbreak.

Poor agricultural practices, such as excessive overhead irrigation and lack of field drainage, contribute to the accumulation and spread of the pathogen.

The presence of weeds from the Convolvulaceae family may facilitate the survival of the infection, although the fungus is strictly specialized for sweet potatoes.

Sweet potato scab primarily damages the photosynthetic surface area. Due to leaf deformation, the plant loses its ability to accumulate nutrients effectively.

This results in lower tuber yields, as tuber development depends directly on the health of the aerial parts. Heavily damaged plants produce small, unmarketable tubers.

The damage also manifests in reduced quality of planting material. Using cuttings from infected plants leads to early disease emergence in the next growing cycle.

In cases of severe infection, the crop can lose its market value entirely, rendering it unsuitable for sale or long-term storage.

In regions with favorable climates, the disease can cause yield losses of up to 30-50 percent if timely protective measures are not implemented.

The main method of control is the use of healthy, pathogen-free planting material. Cuttings should only be sourced from certified, scab-free plantations.

Crop rotation is an essential requirement. Planting sweet potatoes in the same field should be avoided for at least 3–4 years after the last cultivation cycle.

It is important to remove and destroy plant debris promptly after harvest. The fungus cannot survive in the soil for long without access to fresh plant tissues.

At the first signs of the disease, fungicide applications, particularly copper-based or recommended systemic products, should be used to manage fungal spots.

Choosing scab-resistant sweet potato varieties is highly recommended, provided they are adapted to the specific region, to minimize the need for chemical intervention.