Etiology and dynamics of the stem bleeding disease of coconut (Cocos nucifera L.) in the Philippines

The cause of the disease was determined as Thielaviopsis paradoxa (Seynes) von Hohnel. The initial symptom is the appearance of a reddish brown or rust colored liquid which oozes out from the diseased stem. Later the oozing ceases and the reddish brown areas become black. The extent of damage in naturally infected palm depended on the age of the palm. In the young palms, extensive damage of internal tissues take place in the central core of the stem. In older palms, damage is confined to the bark and seldom involves the central core of the stem. Wounding was essential to the stem bleeding infection. Injection of inoculum suspension resulted in serious decay of the central core tissues of the stem. Conidiophores and conidia of the T. paradoxa isolated from coconut were morphologically similar to those isolated from black spot of pineapple. Mycelial growth and sporulation were variously affected by different agar media, light, and pH. Continuous light, alternate light as well as darkness greatly enhanced growth and sporulation of T. paradoxa in PDA. The fungus grew well but did not sporulate under continuous darkness. T. paradoxa germinated best at 25 deg C to 30 deg C and at 97.5 percent and 100 percent RH. The fungus produced larger colonies and more spores at pH 4.0 to pH 6.0. The stem bleeding disease was found in different places in the Philippines where coconut is grown. The disease incidence was relatively higher in coconut hybrids than in the local tall cultivar. Increase in disease incidence was directly related to the occurrence of high rainfall. The spatial distribution of stem bleeding initially occurred in random foci. From the focus, the disease infected adjacent palms. The disease progress in the experimental areas in AADC, La Filipina and Zamboanga Research Center appeared to follow a sigmoid curve. Apparent infection rate of stem bleeding ranged from 0.029 to 0.218 per unit per year. The five coconut cultivars/hybrids tested were all susceptible to the disease. JVA-1 hybrid showed the most extensive damage of the central core tissues of the stem. Infection developed in banana, cacao, papaya and pineapple fruits but not in sweet potato when inoculated with T. paradoxa isolates from coconut. The fungicides Aliette, Difolatan and Benlate were effective in suppressing mycelial growth of T. paradoxa in culture and in preventing lesion development in the inoculated stem whereas Daconil and Ridomil were not effective