Cocoa Research Unit, The University of the West Indies, St Augustine, Trinidad Effect of Crinipellis perniciosa Infection on Abscission of Cacao Cotyledons, Reserve Mobilization and Dry Matter Partitioning L. A. L. A. Motilal Motilal 1 , G. G. Sirju-Charran Sirju-Charran 2 and and T. N. T. N. Sreenivasan Sreenivasan 1 AuthorsÕ addresses: 1 Cocoa Research Unit, University of the West Indies, St Augustine, Trinidad, Republic of Trinidad and Tobago, West Indies; 2 Department of Plant Science, University of the West Indies, St Augustine, Trinidad, Republic of Trinidad and Tobago, West Indies (correspondence to L. A. Motilal. E-mail: lamotilal@yahoo.com) With 6 figures Received December 6, 2002; accepted July 28, 2003 Keywords: Crinipellis perniciosa, cotyledon, abscission, partitioning, cacao Abstract Infection by Crinipellis perniciosa on cacao seedlings prevented the abscission of cotyledons which normally abscise within 32–61 days from seed sowing. The early growth and development of control and artificially inoculated seedlings was investigated for root dry mass (RDM), shoot dry mass (SDM), shoot-to-root ratio (SRR), total dry mass (TDM), cotyledonary dry mass (CDM) and cotyledonary water content (CWC). Coty- ledonary reserve mobilization was estimated at 69 and 56% at 5 weeks and 79 and 64% at 8 weeks in control and infected seedlings, respectively. Infection induced the enlargement of tissues but was not accompanied by an increase in dry mass. Infection both delayed and reduced the utilization of cotyledonary food reserves resulting in slower accumulation of RDM, SDM and TDM culminating in a reduction of growth by a factor of 1.6 at 8 weeks. However, the SRR of inoculated plants was similar to that of control plants, suggesting that the balance between the root and shoot systems exists which is unchanged by the effect of the patho- gen. CWC was decreased upon senescence to an aver- age of 65% at abscission. The prevention of cotyledonary abscission in infected seedlings is sugges- ted to be due in part to the CDM remaining above a critical level of 20% of the initial CDM. The implica- tion of this response in relation to the infection bio- logy of the pathogen is discussed. Introduction Theobroma cacao L. (cacao) is widely cultivated in the humid tropics and is an economically important tree crop for many countries of the Pacific, South East Asia, West and Central Africa, Central and South America and the Caribbean (Wood and Lass, 1985). In the Americas and the Caribbean, witchesÕ broom disease (WBD) may be considered as the most hazardous disease of cacao as a result of its widespread distribution and destructiveness (Wheeler and Sua´rez, 1993) and the lack of suitable chemical control (Purdy and Schmidt, 1996). The pathogen, Crinipellis perniciosa (Stahel) Singer, is a hemibiotroph with four known biotypes, of which the C-biotype is specific to cocoa (Bastos and Evans, 1985; Bastos and Andebrhan, 1986; Griffith and Hedger, 1994). Two pathotypes of the C-biotype are thought to exist: A, which is more virulent and is present in Bolivia, Ecuador and Colombia; and the less virulent form B, which is present in Brazil, Trinidad, Tobago and Venezuela (Wheeler and Mepsted, 1988). The pathogen attacks actively growing tissues of flowers, flower cushions, cherelles and vegetative buds resulting in hypertrophy and proliferation of axillary buds and shoots (Baker and McKee, 1943). Detailed descriptions of symptoms have been published by Baker and Holli- day (1957) and Rudgard (1989). Holliday (1955) first mentioned the non-abscission of cotyledons in artifici- ally inoculated seedlings. This is in direct contrast to normal seedlings whose cotyledons abscise approxi- mately 8 weeks after germination (Goodall, 1950). Cacao cotyledons are predominantly food storage organs (Olofinboba, 1975). Over the first 6–8 weeks, seedling growth is determined mainly by the size and composition of the cotyledons and the capacity to trans- fer their reserves (Atanda, 1971). Abscission is the shedding of plant organs when they become non-functional, damaged or infected (Sex- ton and Roberts, 1982; Clements and Atkins, 2001). Non-abscission has been reported in mutants of Lupi- nus angustifolius (Clements and Atkins, 2001) and in several host–pathogen complexes (Cohen, 1967; Rivera and Wright, 1999) including the bacteria-mediated wit- chesÕ broom disease of pea (Oduro and Munnecke, 1975). There is however, a dearth of quantitative infor- mation on the underlying physiological responses of U. S. Copyright Clearance Centre Code Statement: 0931–1785/2003/5110–0546 $ 15.00/0 www.blackwell.de/synergy J. Phytopathology 151, 546–552 (2003) Ó 2003 Blackwell Verlag, Berlin ISSN 0931-1785