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Resistance

Screening Tobacco Seedlings for Resistance to Phytophthora parasitica var. nicotianae. John L. McIntyre, Department of Plant Pathology and Botany, The Connecticut Agricultural Experiment Station, New Haven, 06504; G. S. Taylor, Department of Plant Pathology and Botany, The Connecticut Agricultural Experiment Station, Windsor, 06095. Phytopathology 66:70-73. Accepted for publication 10 July 1975. DOI: 10.1094/Phyto-66-70.

A technique was developed for screening 2- to 3-week-old tobacco seedlings for resistance to black shank, caused by Phytophthora parasitica var. nicotianae. Tobacco seedlings of different cultivars at the 4-leaf stage were placed on moistened filter paper disks in square petri plates and sprayed with approximately 3 ml of a zoospore suspension of race 0, 1, or a Connecticut (C) isolate of P. parasitica var. nicotianae. Black shank symptoms developed within 3 days after the seedlings were sprayed with a suspension of 10⁴ zoospores/ml and within 5 days disease resistance ratings of the cultivars and breeding lines tested paralleled results obtained by others from field and greenhouse observations. The seedling data indicate that the C isolate was different from races 0 and 1. Stem inoculations of 8-week-old differential tobacco cultivars and lines L8, 1071, and mature Nicotiana nesophila with races 0 or 1 of P. parasitica var. nicotianae gave the expected black shank symptom responses. Stem inoculation of 1071 with the C isolate gave results comparable to race 0, while results obtained with N. nesophila were comparable to race 1. Inoculation of L8 with this isolate caused some necrosis of stem tissue. These results suggest that the C isolate of P. parasitica var. nicotianae may be more virulent than or different from races 0 and 1 of the pathogen.