Graduation date: 1981

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Three isolates of Curvularia belonging to Curvularia tuberculata (isolates 93-020 and 93-022) and Curvularia oryzae (isolate 93-061) were obtained from diseased Cyperus difformis, Cyperus iria, and Fimbristylis miliacea, respectively, in the Philippines in 1993. Under greenhouse conditions, these fungal isolates caused high mortality and significant plant dry weight reduction in C. difformis, C. iria, and F. miliacea when sprayed at the rate of 1 x 108 spores/m3. Cross-pathogenicity of the isolates was demonstrated in three other sedge weed species. C. difformis, C. iria, and F. miliacea were killed but C. rotundus was resistant. Most of the thirteen rice varieties tested were resistant to the fungal isolates. The order of decreasing pathogenicity to rice was C. oryzae (93-061), C. tuberculata (93-020), and C. tuberculata (93-022). The infection process of C. tuberculata and C. oryzae was similar. Spore germination was polar for C. tuberculata and bipolar for C. oryzae. Germ tube growth was random and branching. Appressoria were formed preferentially over epidermal cell wall junctions on sedge hosts and over stomatal apertures in rice. Complex infection cushions were observed only on sedge hosts. Infection hyphae developed inter- and intracellularly, causing epidermal cell walls to separate and mesophyll cells to shrink and collapse. The vascular bundles were not invaded. Colonization of susceptible weeds was rapid and conidiophores emerged from the stomatal aperture between 96 to 120 hours post inoculation (HPI). Resistance to C. tuberculata and C. oryzae in C. rotundus and rice was expressed as a delay in appressorial formation, inhibition of fungal growth after penetration, and lack of sporulation.

Six pathogenic fungal species were isolated from naturally-infected Echinochloa species and evaluated as biological control agents for E. crus-galli, E. colona, and E. glabrescens in rice. Bipolaris sacchari, Curvularia geniculata, and Exserohilum monoceras were non-pathogenic to rice and caused high mortality of Echinochloa species. E. monoceras was selected for further study. Under regulated greenhouse conditions, an inoculum dose of 2.5 $ times$ 10$ sp7$ conidia/m$ sp2$ killed E. crus-galli and E. glabrescens seedlings while 5.0 $ times$ 10$ sp7$ conidia/m$ sp2$ caused 100% mortality of E. colona seedlings. The 1.5-leaf stage was the most susceptible growth stage for all three Echinochloa species. E. glabrescens was most susceptible to E. monoceras infection, E. crus-galli had an intermediate susceptibility, and E. colona was least susceptible. The optimum temperature for 100% mortality was between 20 and 30 C for all Echinochloa species, whereas the minimum dew period for 100% mortality was 16 h for E. colona, 12 h for E. crus-galli, and 8 h for E. glabrescens. Under screenhouse conditions and in the absence of an artificial dew period, over 90% of Echinochloa seedlings were killed when inoculum was sprayed in an oil emulsion or when applied as a dry powder to the water surface of a simulated paddy field. Maximum conidia production occurred on V-8 juice agar or centrifuged V-8 juice agar, at 28 C in the dark. No conidia were produced in liquid media. Of various agricultural products tested as solid substrates, the highest sporulation (1.81 $ times$ 10$ sp6$ conidia/g dry weight) occurred on corn leaves. Host range tests on 54 plant species in 43 genera and 19 families showed that Rottboellia cochinchinensis, was also highly susceptible to this fungus. Of the crops tested, only corn seedlings were lightly infected under optimum greenhouse conditions but no disease occurred on corn under field conditions. Bipolaris sacchari, Exserohilum monoceras, and E. oryzae

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