Author : Ahmad Kamil bin Mohd Yunus

Summary : Phytophthora infestants (Mont.) De Bary causes late blight disease of tomato, especially in tomatoes in. The study aimed to isolate the pathogen on artificial media. The morphology and cultural characteristics were described and disease development of P. infestant isolates on tomato was also studied. In-vivo sensitivity of the isolates toward metalaxyl was tested. Disease epidemiology studies were conducted during the two tomato planting periods in Cameron Highlands. Three isolates of P. infestans from Cameron Highlands were isolated on Rye Grain Agar (RGrA), v-8 Juice Agar (V8JA), and Rye Wholemeal Agar (RWMA) all mended with antibiotics, with percentage recovery of 2, 0.1, and 0.4 percent respectively. The P. infestants isolates were designated as PIB/01, PIH/01 and PIKR/01. No growth was observed on the two other media viz. Pea Agar and Difco LBA. Successful isolation was aided by baiting using potato tuber slices. Morphologicl studies of the P. infestans isolates showed that the mean zoosporangia size was 37.25 um x 19.71 um, mean L/B ratio of 1.91 and mean short pedicel length og 1.60 um. Mean zoospores diameter measured 9.99 um. No chlamydorpores was produced in culture. The isolates required a compatible isolate to form zoospores. Direct contact method of pairing the isolates with the known A1 (P. palmivora durian isolate) and A2 (P. palmivora cocoa isolate) compatible types confirmed that all three P. infestants isolates belonged to A1 compatible type. Cultural studies showed no definite growth pattern of P. infestans isolates on all media tested (e.g., V8JA, LBA, RGrA and potato tomato Rye Dextrose Agar (PTRDA)). Mycelial growth was optimum between 18-22 degree C but no growth was observed at 30 degree C. Indirect germination of zoosporangia occured at low temperature (6-15 degree C), while direct germination by germ tube occured at 20-25 degree C. Disease development on tomato leaves was highest at relative humidity (RH) in the following order: 99, 95, 90, 85, 80, and 75 percent.