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Diagnosis, forecasting, risk assessment and control of stem-base diseases of wheat using new molecular technologies

HGCA PROJECT REPORT 216 

Diagnosis, forecasting, risk assessment and control of stem-base diseases of wheat using new molecular technologies 


by

D W Parry

HRI East Malling

February 2000

Abstract

Diagnosis, forecasting, risk assessment and control of stem-base diseases of wheat using new molecular technologies

PCR was used to identify and quantify all fungal pathogens of wheat stem bases in nine field experiments at three locations in England. The main aims were to establish relationships between amounts of pathogen DNA determined by PCR, stem-base disease severity and yield loss, to apply quantitative PCR to provide robust data on the efficacy of new fungicides against stem-base diseases and to investigate its use in developing a risk assessment system based on threshold amounts of pathogen. Additionally, an appropriate field-sampling procedure was to be developed.

 

Quantifiable amounts of fungal DNA and disease were not always present before stem extension, when decisions to apply fungicides are taken. PCR confirmed that symptoms were often identified incorrectly at this time. The early development of pathogens did not often relate to disease severity at grain-filling or to yield losses.

 

Cyprodinil most effectively controlled eyespot by decreasing both pathogens, Tapesia yallundae and T. acuformis (the most widespread species), and sometimes contributed to increased yields. Prochloraz controlled eyespot erratically, its effectiveness dependent mainly on the presence of T. yallundae and, partly, on rainfall events soon after application. Azoxystrobin contributed to yield increases most consistently. Although it decreased sharp eyespot and its pathogen, Rhizoctonia cerealis, these effects were insufficient to account for much of the yield increases. The effects of fungicides on eyespot were sometimes greatest on the most susceptible cultivars. Amounts of Tapesia DNA were usually consistent with cultivar susceptibilities.

 

The only pathogens of brown foot rot present in significant amounts were Microdochium nivale vars nivaleand majus. They appeared not to affect yield or to respond greatly to fungicides. The susceptibility of cultivars to these pathogens was often similar to their susceptibility to eyespot, suggesting that they respond to the same host resistance genes or are often secondary colonisers of eyespot-infected plants. The significance of M. nivale on shoot bases needs further investigation.

 

It is suggested that quantitative PCR, more than other methods, can provide accurate evidence of early, extensive disease development that indicates risk. It can be used on a field scale, using appropriate sampling patterns and bulking of samples, as a routine laboratory-based procedure. However, none of the methods currently available can provide precise threshold information on which to base a decision to apply fungicide.

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