Graduate Thesis Or Dissertation
 

An evaluation of field methods of disease assessment in wheat (Triticum aestivum) for stripe rust (Puccinia striiformis)

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  • The need to identify a simple but accurate method of stripe rust assessment that best predicts host response prompted this investigation. The study was divided into two plot types, one protected with the fungicide Bayleton and the other unprotected. Ten winter wheat cultivars were used and evaluated. Grain yield, test weight, kernels per spike and kernel weight were measured for each plot. Stripe rust severity was visually assessed at four different times according to the level of infection in the spreader row (a collection of cultivars susceptible to stripe rust). The four selected levels of infection were chosen based on the logarithmic increase in disease progression. The levels of infection (1-10%, 10-25%, 25-502 and 50-100%) were the four critical points chosen because of their placement along the disease progress curve. Several methods of disease assessment were compared and correlated with redacted kernel weight, which was used as a measure of yield loss. A coefficient of infection (CI) was used as the measure of disease assessment. The CI is calculated from the field readings by multiplying the percent severity by the value for infection type. A coefficient of infection was obtained for each plot at four specific dates. Disease levels in the unprotected plots and kernel weights were compared for each time of observation and correlations were determined. The data showed that the single visual assessment showing the highest correlation of all assessments compared in the study as they related to reduced kernel weight (r - =.791) was the third observation recorded as a coefficient of infection on May 30 when the level of infection in the spreader row was 25 to 50 percent or when the stripe rust epidemic was increasing logarithmically or in the logistic phase. Two other methods of assessment found less effective were analyzed using a transformed value for percent severity. Disease severity is usually measured on a logarithmic scale but for this computation, an arithmetic value for percent severity was used. When this value was included in determination of a coefficient of infection, there were lower r values associated with kernel weight than when the untransformed values for percent severity were used. The second additional assessment method utilized only the transformed percent severity reading to measure the amount of disease present with no regard to infection type. The correlations with these observations were also extremely low. Finally the area under the disease progress curves (AUDPC) were measured for the unprotected plots and correlated with kernel weight. When all ten cultivars were averaged and AUDPCs and kernel weights correlated, an r value of -.505 was obtained. Evidence provided in this study supports the use of a critical point model in that a single visual assessment of stripe rust was identified to estimate loss. AUDPCs were found to most accurately reflect host response to the presence of stripe rust infection. The strategy of an assessment program must be adapted to the characteristics of the pathogen under investigation and to the breeding program in which it will be implemented.
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