Thesis (M.Sc.)--Chulalongkorn University, 2007

Fifteen phytopathogenic fungi isolates were isolated from pineapple leaves showing symptoms of heart rot, collected at Cha-Am district, Phetchaburi province. All isolates were subjected to pathogenicity test on healthy suckers of pineapple. Three isolates as RM1, RK1 and Phytophthora caused heart rot disease. Based on fungal morphology and analysis of DNA sequencing, those fungi were identified as Fusarium oxysporum f. sp. vasinfectum, Lasiodiplodia theobromae and Phytophthora parasitica. The screening of antifungal activity of twelve compounds of benzoic acid and cinnamic acid derivatives at 100 ppm against three isolates was conducted. n-Butyl 4-hydroxybenzoate revealed the best activity of mycelial growth on agar plate assay against F. oxysporum, L. theobromae and P. parasitica with %inhibition of 73, 87 and 80%, respectively. n-Butyl 4-hydroxybenzoate at 250 ppm completely inhibited mycelial growth of all fungi in broth and completely inhibited zoospore germination at 100 and 250 ppm for P. parasitica and F. oxysporum, respectively. n-Butyl 4-hydroxybenzoate at 3,000 ppm completely inhibited pineapple heart rot disease caused by P. parasitica when tested by detached leaf method and as preventative of the disease on suckers in laboratory. Synergistic at EC[subscript 50] 1.4 ppm of metalaxyl and a mixture of metalaxyl and n-butyl 4-hydroxybenzoate at the ratio of 4:1 showed inhibition of mycelial growth not different from that observed from solely metalaxyl.