Elsinoë potatoes (Saw.) Viega & Jensen (teleomorph)  

Sphaceloma potatoes (anamorphe)

Sweet potato scab

General

• Worldwide widespread fungal disease specific to sweet potato.
• Sometimes attributed to Sphaceloma batatas which is actually the asexual form of the fungus (anamorph).
• Symptoms affecting young leaves and stems of the plant.

• Susceptible botanical family(s)

The sweet potato is the only known vegetable

• Production areas affected :

• Organs attacked

Symptoms

• Symptoms :
  • Appearance of small black necrotic lesions along leaf veins and on young stems. These alterations elongate and take on a corky appearance and an often rusty color along the leaf veins. The latter show more or less a blocked development and a tense aspect.
  • Comparable lesions on petioles causing their deformation locally.
  • Corky areas on stems of a rather beige color as on the petioles.
  • In strong attacks, the leaves are curled up, the stems are erect.

• Signs : Can't we see sporulation of the fungus?
• Possible confusion : Little possible confusion

Biology

• Storage : the fungus can survive on plant crop debris. Its other modes of preservation are still poorly understood.
• Infection : After germination, the fungus penetrates plant tissues and produces corky patches.
• Sporulation  : 
• Dissemination : Naturally, spores produced by corky lesions are dispersed by wind and splashing rainwater. In addition, dissemination can be done by using infected cuttings
• Favorable conditions : presence of free water on the plants (rain or dew) favoring the germination of the spores.

Protection

• Use tolerant or resistant varieties.
• Respect the principle of crop rotation.
• Avoid planting near a field that has been heavily impacted.
• Use healthy cuttings showing no symptoms of scab. The production of seed-bed cuttings improves the sanitary quality of the plant material produced.
• Remove the leaves from the cuttings before planting in a preparation area away from the field, to limit the risk of disease development at the start of the crop.
• Encourage aeration and sunshine of the crop (planting density and choice of plot according to its orientation).