Sources of Inoculum, Factors Affecting Spatio-Temporal Spread, and Yield Loss Caused by Puccinia Recondita F. Sp. Tritici on Wheat.

Abstract

Kramer-Collins 7-day spore samplers were operated continuously at two locations, Baton Rouge and Bossier City, during 1986 and 1987, and daily urediniospore counts were taken. Number and viability of spores trapped was maximum during the active epidemic period and viability was significantly inversely correlated with ambient maximum temperature. Leaf rust appeared in numerous microplots of McNair 1003 during summer. Rusts from Hordeum pusillum Nutt. and Lolium multiflorum Lam. failed to infect wheat. Surveys during mid-December in Baton Rouge and Alexandria, showed uniform leaf rust distribution over a large area and the possibility of a major exodemic source of leaf rust inoculum for Louisiana. Effects of time of inoculation on leaf rust development and factors affecting leaf rust increase and spread were determined on McNair 1004, Rosen, and Terral 812. Sequential inoculations of McNair 1003 plots were made and leaf rust development was monitored. Increase and spread of leaf rust in all plots occurred in March with the onset of warmer temperatures. Leaf rust severity was highly correlated with cumulative degree days (CDD) ($>$20 C) following inoculation. Regression analysis indicated that leaf rust development in Louisiana can be predicted using CDD and leaf rust severity two weeks before prediction. Leaf rust spread occurred at constant rate and velocity of spread was affected by cultivar resistance and time of inoculation. Inoculations at early growth stages with different levels of inoculum affected the rate of leaf rust development. Epidemics with common onset stages and different initial inoculum levels differed in AUDPC. Leaf rust epidemics initiated early with higher inoculum levels had greatest effect in reducing yield. Yield reductions were directly related to AUDPC and were mainly caused by lower grain weight. The contributions of different tiller parts to grain yield of McNair 1003 and the effect of leaf rust were determined by defoliation and inoculation. Contributions of the tiller parts estimated by the regression model Y$\sb{\rm ij}$ = $\beta\sb0$ + $\beta\sb1$F$\sb{\rm i}$ + $\beta\sb2$(F$\sb{\rm i}$-1) + $\beta\sb3$(F$\sb{\rm i}$-2) + $\beta\sb4$(F$\sb{\rm i}$-3) + $\varepsilon\sb{\rm ij}$, indicated that highest contributions to yield come from nonfoliar parts, comprising stem, leaf sheaths, ear, and awns, followed by Flag (F), F-1, and F-2 leaves. Estimated relative contributions of the different tiller parts were not altered by leaf rust and grain yield reduction was directly related to the amount of photosynthetic tissue destroyed by leaf rust and relative healthy area duration.