Lasiodiplodia theobromae causes a damaging diebackof cocoa in India

Cocoa (Theobroma cacao) is mainly grown as a mixed crop in coconut/arecanut gardens in India. In September 2000 a rapid dieback of cocoa was first observed in Hunsur Taluq of Mysore district, Karnataka, India. Affected trees show sudden yellowing and browning of leaves (Fig. 1) followed by rapid dieback of branches from the tips. Infected twigs have brownish, short vertical streaks in the vascular tissues. Affected trees continue to decline for several days and they soon die, although new flushes still develop at the collar regions during the monsoon (Fig. 2). Pods do not develop in diseased trees but if formed earlier become mummified. Trees are affected at all growth stages and throughout the year. Mortality is most severe from March to May.

Investigations of the disease began in 2003 but it was not until 2007 that the cause was consistently associated with a fungus isolated from affected twigs. Colonies were initially milky white on potato dextrose agar plates, later turning to grey-white with black pycnidia on the surface. Young conidia were hyaline, unicellular, ovoid in shape and on maturity one-septate and ellipsoidal (Fig. 3) and confirmed as Lasiodiplodia theobromae by CABI, UK (IMI 394314). Sterile cotton swabs containing L. theobromae mycelium were rubbed on pre-wounded stems of eight-month-old seedlings. Typical vascular streaking (Fig. 4) and dieback symptoms appeared within 20 days, exhibiting a similar aggressive response to L. theobromae on cocoa in Cameroon (Mbenoun et al.,2007). L. theobromae was reisolated from the inoculated seedlings. Control swabs with sterile water did not produce symptoms.

L. theobromae is a non-prominent pathogen of sixty plant hosts in India (Varma & Bilgrami, 1977) and is the cause of charcoal pod rot of cocoa (Nambiar & Nair, 1972). This is, however, the first report of a damaging dieback and death of cocoa plants, as witnessed in Mysore district. Currently confined to an area of 260 ha, the incidence of the disease in cocoa gardens varies from 10 to 56%. With the rapid expansion of cocoa in India underway, this disease is a potential threat to future production.