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Resistance

A Comparison of Resistance to Phytophthora parasitica in Tomato. N. S. Blaker, Postgraduate researcher, Department of Vegetable Crops, University of California, Davis 95616; J. D. Hewitt, Assistant professor, Department of Vegetable Crops, University of California, Davis 95616, Present address: Northrup King Company, P.O. Box 1827, Gilroy, CA 95020. Phytopathology 77:1113-1116. Accepted for publication 16 February 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1113.

The basis of resistance in tomato to Phytophthora parasitica was examined using three resistant genotypes, CX8303, 27-1A, and LA1312 and two susceptible cultivars, 6203 and Peto 343. Seedlings were grown hydroponically and inoculated with 2 × 10⁶ zoospores. Microscopic examination of root tips 5 hr after inoculation showed no genotypic differences in the number of zoospores encysted on 2 cm of root tip. Linear colonization of taproots of young seedlings was examined by growing plants in root boxes and inoculating root tips with approximately 200 zoospores. After 3 days, roots were sectioned into 1-cm lengths, which were plated sequentially on P₁₀VP medium. In 27-1A and LA 1312, P. parasitica was only isolated < 2.5 cm from the root tip, whereas the fungus was detected 5.5–6.1. cm from the point of inoculation in the other three genotypes. The number of propagules of P. parasitica per gram of root tissue (ppg) in inoculated root systems was estimated by using root maceration and dilution plating methods. LA 1312 and 27-1A had the lowest level of infection (2.1 and 3.2 × 10³ ppg, respectively). CX8303 had significantly less infection (6.8 × 10³ ppg) than Peto 343 and 6203 (14.1 and 9.7 × 10³ ppg, respectively). Root growth after inoculation, or mechanical pruning in the absence of P. parasitica, was measured in three genotypes by using a grid intersect method. Four weeks after inoculation, total root lengths in 27-1A, CX8303, and Peto 343 were 1,714, 1,314, and 427 cm, respectively. There were no genotypic differences in total root length 2 wk after root pruning. All three genotypes had root lengths of 830–910 cm. Differences in root growth after inoculation may be due to differences in severity of root infection.