VIEW ARTICLE

Ecology and Epidemiology

Characterization of Wheat Leaf Rust Epidemics in Louisiana. K. V. Subba Rao, Former graduate student, Department of Plant Pathology and Crop Physiology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge 70803, Present address: Postdoctoral researcher, Department of Plant Pathology and Crop Physiology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge 70803; G. T. Berggren, and J. P. Snow. Professors, Department of Plant Pathology and Crop Physiology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge 70803. Phytopathology 80:402-410. Accepted for publication 7 November 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-402.

Two experiments were conducted to study the effect of time of inoculation on leaf rust progress and to study the spatiotemporal spread of leaf rust in Baton Rouge, LA, during the 1986?1987 and 1987?1988 wheat-growing seasons. Epidemics were generated by sequential inoculation of different plots of a leaf rust-susceptible cultivar, McNair 1003, at 15-day intervals from 1 February to 15 March 1987, and from 1 December 1987 to 1 March 1988. Depending on the time of inoculation, the incubation period after inoculation varied from 8 to 18 days and appeared to be a function of the prevalent temperature. Early inoculations resulted in long, apparently stationary periods of development and greater areas under the disease progress curve (AUDPC). Leaf rust increase in all the plots occurred at the same time in March. The average apparent infection rates (r) of the epidemics were significantly different for different dates of inoculation. The r-values varied from 0.076 to 0.153/day during 1986?1987 and from 0.047 to 0.157/day during 1987?1988. Leaf rust severity was highly correlated with cumulative degree days (>20 C) following inoculation. The spatial and temporal spread of leaf rust was studied on the cultivars Rosen, McNair 1003, and Terral 812, which varied in degree of leaf rust resistance. An inoculum point source was established by transferring two uniformly rusted McNair 1003 seedlings to the center of each plot. Leaf rust spread in time and space was measured by estimating severity on five tillers at 2, 4, and 6 m from the inoculum source. Spatial gradients (g), r, and a velocity parameter (v) were calculated for each cultivar. The logistic model adequately explained the development of leaf rust in time and space. Cultivar resistance affected leaf rust severity and extent of spread. The r- and v-values for Rosen were significantly higher than for Terral 812, whereas g was significantly higher in Terral 812, indicating that Terral 812 was the most resistant cultivar. Yield reductions in all the cultivars were significantly related to AUDPC and distance from focus. The significance of distance of spread in estimating yield reduction is discussed.