Diseases of the Sweet Potato (Ipomoea batatas) in Guyana

1. FUSARIUM WILT (FUSARIUM OXYSPORUM) SYMPTOM
The disease is caused by fungi that persist in the soil for long periods of time.
Symptom: Include a darkly tanned rot that extends into the root and usually has a sharp line separating the rot from the healthy portion of the root. Stem infections appear as swollen areas at the base of the stem that have internal vascular discoloration.
Cultural Control:
• The disease can be avoided by practising two-year rotations out of sweet potatoes and by using clean planting material.
Chemical control: • Use Carbendazime, Cuprasan and Rizolex for effective control.

2. SOIL ROT (STREPTOMYCES IPOMOEA)
Symptom: Roots develop black necrotic lesions which traverse its entire cross section (Figure 6). Usually, the necrotic portion of these roots breaks off when the roots are dug, leaving a small portion of the lesion at the broken end of the root. Vine growth may be severely stunted with lower leaves bronzed or yellow and the plants wilted. Lesions on roots are necrotic, black, roughly circular in outline, and often have cracks radiating from the centre.
Cultural Control:
• Phytosanitary measures are important to prevent the introduction of Streptomyces ipomoea into non-infested soils.
• Cultivating equipment should not be moved from infested fields to non-infested fields and /or should be disinfested before movement into a new area.
• Storage roots from affected fields should not be used for producing transplants.

Chemical Control: • The use of Banrot, Rizolex, Captan or Mankocide is recommended for effective control. These can also be used as pre-plant drenches.

3. BLACK ROT (CERATOCYSTIS FIMBRIATUM)
Symptom : Tubers with this disease show irregular sunken spots which are dark brown to black in colour. Very often black dots are scattered across the spots (Figure 7). These black dots contain the spores of the fungus causing the disease. Pink masses of the spores are sometimes seen oozing from the black dots.

Cultural Control:
• Use disease free sprouts or tubers for planting;
• Practice crop rotation; • Use extra care in the storage of planting materials- ‘slips’ and tubers; and • Practice good farm sanitation.

Chemical Control: • The use of fungicides such as Captan, Manzeb and Rizolex is recommended.

4. ROOT-KNOT NEMATODE BLACK ROT (MELOIDOGYNE SPP.)

Symptom : Underground symptoms are primarily galls on roots, tubers, abnormal formation and function of the root system. Non-specific above-ground symptoms include patchy stunted growth, discolouration and leaf chlorosis, excessive wilting during dry, hot conditions, stunting of whole plants, reduced yield and quality, and sometimes premature death. Root-knot is generally more severe in sandy soils and under adverse environmental conditions.

Cultural Control:
• Practice good farm sanitation; • Practice crop rotation; and • Flood fallow Biological Control: • Plant marigold flowers, tulsi, neem in and around cultivated areas

Chemical Control: • Use any approved nematicide

5. VIRAL DISEASES
Symptom: Viral disease on sweet potato is caused by a synergistic combination of sweet potato feathery mottle virus (SPFMV) and sweet potato chlorotic stunt virus (SPCSV).
Chlorotic dwarf virus is caused by the synergistic interaction of three viruses: two potyviruses, SPFMV and sweet potato mild speckling virus (SPMSV), and a crinivirus (SPCSV).
SPVD is transmitted by grafting. The individual viruses are transmitted by their insect vectors. Symptoms vary with plant genotype but typically include severe stunting of plants, the production of small leaves which are often distorted, narrow and crinkled and with a chlorotic mosaic and/or vein clearing, giving affected plants an overall pale appearance.

Cultural Control: • Use of disease-free planting material and crop rotation are the most reliable means of controlling these diseases and its insect pest’s vectors. • Use of resistant varieties.

Chemical Control: • Spraying with insecticides such as Fastac, Decis or Karate on the undersides of the leaves to kill aphids. • Spraying insecticides such as Vydate L, Newmectin and Admire is an effective control measure for whiteflies.