Soyabean root and stem rot caused by Phytophthora sojae in Lorestan province.
A widespread root and stem rot disease has occurred in soyabean fields in the Lorestan province of Iran in recent years. To determine the causal agent and the extent of damage caused by the disease, several soyabean fields in the province were surveyed during June-July 1995. None of the observed fields were free of contamination and the disease incidence ranged from 15 to 20% in the various regions surveyed. Of 27 diseased samples collected, eight Phytophthora isolates were recovered using PARPH medium. Abundant sex organs were formed in single strain culture on LBA. Oogonial dimension ranged from 32.5 to 45 µm with an average of 37.4-39.2 µm. Non-papillate sporangia with some extended and or nesting proliferation, characteristics of the species, were formed on 6-day old mycelial mats exposed to continuous light at 25±1°C. Mature sporangia were globose to ellipsoid and measured 28.3-87.4×17.2-52.3 µm (average 54.6×36.5 µm). The cardinal temperatures for growth were 5°C (min.), 24-26° (opt.) and 35°C (max.). Based on morphological and limited physiological characteristics, all isolates were identified as Phytophthora sojae. This constitutes the first report of the disease and its causal agent in Iran. Pathogenicity tests were conducted with all isolates of P. sojae using either whole seedling immersion (WSI) technique (104 zoospores/ml) in the laboratory or inoculating 3-day-old soyabean cv. Williams seedlings with mycelial discs (5 mm diam.) in the glasshouse. The average lesion sizes on taproots were measured 48 h after inoculation in the lab and % mortality of treated plants was determined 15 days after inoculation in the glasshouse. The results showed that all isolates were pathogenic to this soyabean cultivar, exhibiting different degrees of virulence.