VIEW ARTICLE

Achlya klebsiana and Pythium species as Primary Causes of Seed Rot and Seedling Disease of Rice in California. R. K. Webster, Department of Plant Pathology and Agricultural Extension Service, University of California, Davis 95616; D. H. Hall(2), Jacob Heeres(3), C. M. Wick(4), and D. M. Brandon(5). (2)(3)(4)(5)Department of Plant Pathology and Agricultural Extension Service, University of California, Davis 95616. Phytopathology 60:964-968. Accepted for publication 15 January 1970. DOI: 10.1094/Phyto-60-964.

Within a few days after seeding, water-sown rice often is severely infected by seed-rot and seedling disease resulting in reduced stands. The disease, although prevalent throughout the rice-producing areas of California, is generally more severe when temp are cool and unfavorable for the growth of rice. Achlya klebsiana or Pythium species were usually isolated from decayed seeds and infected seedlings. Outgrowths of whitish hyphae characteristically radiated from cracks in the glumes or from the collar of the plumule. The standard California varieties (Caloro, Colusa, and Calrose) were equally susceptible. Isolates of A. klebsiana did not differ significantly in pathogenicity. Optimum temp for growth of A. klebsiana isolates in culture was 27-30 C, but pathogenicity did not differ significantly between 20-30 C (a mean of 52.8% of the plants became diseased over this range). Pythium isolates grew well in culture at 30 C, but were significantly less pathogenic (17.2% plants infected) than at 25 C (48.6% plants infected) or 20 C (95.5% plants infected). Although these results are consistent with field observations, cool temp apparently increase disease severity primarily through an adverse effect on growth of rice seedlings rather than by favoring growth of the pathogen.