Abstract: The most destructive disease of Para rubber seedling is a leaf fall caused by Phytophthora botryosa. The bioactivity of chitosan against P. botryosa infection was studied in vitro. The growth rates of P. botryosa in potato dextrose agar were determined with different concentrations of chitosan. A chitosan concentration of 0.5 mg mL-1 was sufficient to cause clear growth inhibition (about 87% reduction in diameter). Mycelia and oogonia wilt were observed in scanning electron micrographs of P. botryosa. The biodegradable chitosan therefore inhibits the in vitro growth of P. botryosa and may have potential for practical disease management of leaf fall.