Relationship of the severity of Septoria tritici blotch of wheat to sowing time, rainfall at heading and average susceptibility of wheat cultivars in the area

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Abstract
The severity of epidemics of Septoria tritici blotch (STB) in wheat, caused by Mycosphaerella graminicola, was recorded for a 38-year period at Temora in southern New South Wales. The disease was rated as severe in 11 years, moderate in 11 and nil to light in 15, while very wet conditions prevented sowing in one year. The correlation of disease severity (S, where 0 =nil, 7 =very severe) with environmental and management factors was examined: the correlation was positive with days from sowing to heading and with rainfall ( R-4W , R+4W, mm) and the number of rainy days in the 4-week periods before and after heading; negative with the time of sowing (DS, day of year) and with mean daily maximum temperature in the 4-week periods before and after heading. Days from sowing to heading were negatively correlated with sowing day, and rainy days and mean daily maximum temperature were correlated with total rainfall in the same time period. Addition of these terms did not significantly improve the prediction of severity. The cumulative sum of the recursive residuals from this regression showed a trend with time that was associated with the average susceptibility (SAV, where 1 =highly resistant, 7 =extremely susceptible) of wheat cultivars to STB grown in the district in the previous year. The second model showed that the reduction of the average susceptibility of cultivars grown in an area will reduce the severity of STB. It provided justification for minimum disease standards for cultivars to be grown where STB is potentially severe. Further, it explained the distribution of severity of STB in New South Wales.