Virulence of an Endemic Isolate of Peronospora tabacina from Texas to Nicotiana tabacum and N. repanda. Moshe Reuveni, Department of Plant Pathology, University of Kentucky, Lexington 40546. William C. Nesmith, Malcolm R. Siegel, and Toney M. Keeny. Department of Plant Pathology, University of Kentucky, Lexington 40546, and Southwest Texas Junior College, Uvalde 78801. Plant Dis. 72:1024-1027. Accepted for publication 28 June 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-1024.

Isolates of Peronospora tabacina, causal agent of blue mold of tobacco, were obtained in 1983 and 1984 from naturally infected wild Nicotiana repanda plants near Uvalde, TX, and were increased on N. repanda in a growth chamber. Disease severity was similar on both N. tabacum ‘Ky 14’ and N. repanda plants following serial inoculations with sporangiospores and was positively correlated to inoculum density. The host plant (Ky 14 or N. repanda) used to maintain the isolates did not affect disease. N. repanda plants became systemically infected and supported sporulation for 25–60 days (30,000–240,000 sporangiospores/cm² of leaf tissue). These findings differ from those we obtained in earlier experiments with a Kentucky isolate (Ky 79). Virulence of the Ky 79 and Texas 84 isolates of P. tabacina was compared directly on leaf disks of Ky 14 plants of different ages under controlled conditions. The Texas isolate caused greater disease severity than the Kentucky isolate. This disease severity was temperature-dependent during the infection period. The data indicate that the Texas isolate is more virulent than the Kentucky isolate, and that virulence may be a useful marker for comparison of the two isolates. The epidemiological significance of these findings is discussed.