Effect of temperature on the incidence of nodal foot rot symptoms in winter wheat crops in England and Wales caused by Fusarium culmorum and Microdochium nivale

The predominant Fusarium or Fusarium-like species infecting winter wheat stem bases and causing foot rot symptoms at growth stage 75 was determined, by isolations in England and Wales during 1992. Of the four principal species causing foot rot in the UK, Fusarium culmorum, Fusarium avenaceum, Fusarium graminearum and Microdochium nivale, F. culmorum or M. nivale were predominant in 31 countries at wheat growth stage 75. The mean percentage incidence of nodal Fusarium foot rot symptoms for each county was considered in relation to thermal time calculated from local monthly mean air temperatures from February to July and assuming a base temperature ( T b) of 0°C. This data was plotted together with similar values calculated for a limited number of counties in 1987, 1990 and 1991, for which published information on the predominant stem base Fusarium species present was available. Multiple regression analysis was applied to this relation, taking the predominant foot rot pathogen species present into consideration by the use of dummy variables. This showed that the slopes, estimated from the regressions of nodal foot rot symptom incidence (caused by F. culmorum or by M. nivale) on thermal time, were not significantly different but had significantly different y-intercepts. This indicated a difference in temperature requirement to cause symptoms between the two pathogens and was in agreement with the difference in relative growth rates of F. culmorum and M. nivale with temperature in vitro. The sub-optimal portion of each pathogen's in vitro growth rate/temperature curve was used to calculate the specific T b for each species. These T b values were used to calculate thermal times for the development of nodal foot rot symptoms in each county. Simple regression analysis of the mean percentage incidence of nodal Fusarium foot rot for each county in 1992 against thermal time showed that thermal time could account for just over 50% of the variability in symptom incidence in areas where either F. culmorum or M. nivale were predominant ( y = 0.021 x + 1.257, P = 0.01, r 2 = 0.53). However, the use of thermal time alone was insufficient for the prediction of symptoms and it is concluded that the inclusion of measurements of another factor, most probably soil moisture, is required to make more reliable predictions of Fusarium foot rot symptoms.