Use of morphometric analysis for characterization of tobacco root growth in relation to infection byPhytophthora parasitica var.nicotianae

Abstract

Development of tobacco root systems was characterized under controlled environmental conditions by use of morphometric root analysis. According to the classification scheme of this system, roots terminating in apical meristems are defined as first-order roots. Elements of second-order roots begin where two first-order roots merge, and so forth. Growth of root systems was similar for susceptible and resistant tobacco cultivars in nonautoclaved and autoclaved soils. During 15 days of growth subsequent to transplanting of 2-week-old plants, relative multiplication and extension rates of first-order and second-order roots were constant. Apparent unit extension rates of first-order and second-order root elements increased through 15 days of root system growth. Classification of tobacco root systems by the morphometric scheme provided a useful means of partitioning susceptibility of tissues to infection by Phytophthora parasitica var. nicotianae.Zoospores applied at the tips of first-order roots were most successful in causing infections; 73.3% of the roots inoculated with 16 zoospores per root tip became infected. Percentages of infections after inoculation of first-order root tissues 2 cm behind root tips or after inoculation of second-order roots were 10 and 4.3%, respectively.

Early season root production and zoospore infection of cultivars of flue-cured tobacco that differ in level of partial resistance to Phytophthora parasitica var. nicotianae

“…Ferrin and Mitchell (1986) found that initial inoculum level was important in epidemic development in moderately resistant G-28 but not in susceptible Hicks. The rapid proliferation of first order roots also may increase the chances of infection by P. p. nicotianae since first-order roots are more susceptible to infection by P. p. nicotianae than second-or thirdorder roots (English and Mitchell, 1989). The larger root system of susceptible cultivars was primarily the result of increased root branching which greatly increased the number of first order roots present (Fitter, 1982).…”