Superelongation disease, caused by Elsinoe brasiliensis, confirmed on cassava in Trinidad and Tobago

Cassava (Manihot esculenta) is an important crop in the Caribbean. In 2007, a new disease was observed on cassava in fields in the north, south and central areas of Trinidad. Disease incidence ranged from 5-100%. Field symptoms included raised, pale, tan-coloured corky lesions on the petioles and midribs of leaves (Fig. 1) and stems (Fig. 2). Infrequently, chlorotic spots with necrotic centres were seen on the leaf lamina, which dried and fell out giving a ‘shot hole’ appearance. Leaves were often twisted and curled. Samples were sent to the Global Plant Clinic, UK for diagnosis.

Sphaceloma manihoticola (teleomorph Elsinoe brasiliensis) was consistently isolated from the leaf material. Colonies on potato carrot agar were slow-growing, pulvinate, and fissured. Most colonies were bright red and tomentose. Conidiophores were philidic, forming a continuous layer or grouped into fascicles. Conidia were hyaline, nonseptate, and ellipsoid, 3-6 (4.65) x 2-2.5 (2.21) μm. Species identity was confirmed morphologically at CABI, UK and cultures placed in the Genetic Resource Collection (IMI 395825 and 395826).

In Trinidad, pathogenicity tests were conducted in polythene tents on one month-old cassava cultivar ‘MMex’. Cuttings were sprayed with a conidial suspension (2×10⁶ conidia per ml) of S. manihoticola originally isolated from diseased cassava. Control plants were sprayed with sterile distilled water (SDW). All plants were incubated in a polythene tent for 21 days at 22-30°C and misted with SDW at least three times daily. Disease symptoms identical to those observed in the field appeared 7-10 days after inoculation. S. manihoticola was reisolated from these symptoms. Control plants did not develop symptoms.

This is the first record of E. brasiliensis on cassava in Trinidad and Tobago, although it is widespread in Barbados, the Dominican Republic and Panama. E. brasiliensis is considered a major pathogen of cassava that can drastically reduce yields (Alvarez & Molina, 2000). Besides the typical leaf and petiole lesions, severe infection of young plants can result in the elongation of internodes resulting in thin and weak stems (due to the hormone gibberellin), which accounts for the common name superelongation disease. This symptom is frequently seen in severely infected young plants, but was not observed in Trinidad. However, it is not unusual for plants suffering from this disease to exhibit no elongation of the internodes, especially during the dry season (Krausz, 1976).