The use of PCR diagnostics to monitor development of eyespot in winter wheat

Abstract

Background
This project aimed to evaluate the potential for using PCR (polymerase chain reaction) diagnostics to monitor the development of eyespot in winter wheat.

Field trials were carried out at a site in East Lothian to re-examine the timing of fungicide applications in winter wheat crops to control common eyespot (Pseudocercosporella herpotrichoides). There was a strong association between eyespot levels and yield and weaker correlations between degree of lodging and yield and eyespot levels and lodging.

The fungicides cyprodinil and prochloraz showed the most significant levels of control of eyespot when fungus when assessed visually, and these fungicides were then used the evaluate the use of PCR diagnostics to monitor the development of the rye (R) and the wheat (W) strains of the eyespot fungus as it developed in two season. The results of the PCR quantification were variable, but several trends emerged over the two seasons.

Results

Eyespot control and yield benefits can be achieved with cyprodinil and procholaz sprays. Prochoraz had to be used early in the season, during tillering, for maximum effect on eyespot levels and cyprodinil after the start of stem extension. Spraying outside the optimum window allowed the eyespot populations to recover following treatment even if initial reductions were achieved.

Prochloraz applied too late did not reduce the eyespot population sufficiently to influence the levels at the end of the season. In contrast cyprodinil applied too early achieved an initial reduction that could not be maintained until the end of the season. The findings of the work suggest there is potential for using sequences of fungicides to achieve season long control of the eyespot pathogen.

Both fungicides were more effective at controlling the W strain than the R strain of eyespot. Cyprodinil gave a more persistent reduction in R strain eyespot than prochloraz. Control of the R strain with prochloraz was initially good but the population often recovered. The optimum timing for W strain control with prochloraz was from mid-tillering to GS 31. The W strain population could recover within two months of application after control with cyprodinil so that cyprodinil was better applied as late as possible.

PCR analysis showed that the R strain predominated in both seasons but both strains declined naturally at stem extension as old leaves died and were shed from the stem base, along with their associated eyespot lesions. There was no indication of a competitive effect between the two strains, but W levels were very low in comparison to the R strain.

PCR and visual assessments up to stem extension were not useful in determining eyespot levels at the end of the season so thresholds for treatment were not successful. In one season there was a significant correlation between W strain levels at stem extension and the final levels at the end of the season, indicating how thresholds may have been more effective when the W strain was the dominant strain of eyespot in the UK.

Conclusions

This project highlights the use of PCR diagnostics as a tool for understanding whether treatments are effective, and if so, why they were successful. Using the technique it was possible to chart the initial efficacy of the fungicides following application, and the duration of control. The work demonstrated that where reductions in eyespot levels, measured as the amount of fungal DNA present, were large enough and persistent enough, eyespot levels at the end of the season were reduced. The technique demonstrated how the eyespot population was able to recover following treatment, so that the most successful treatments at reducing eyespot DNA reduced eyespot DNA levels initially, were also able to maintain this reduction for two or three months. Although in every case levels of eyespot DNA increased following the initial reduction after fungicide application, levels of eyespot DNA at the end of the season were sometimes still lower than in the untreated controls. Where this was not the case and DNA levels were not lower at the end of the season, a reduction in the level of visual symptoms and yield benefits could be demonstrated for those fungicide treatments where the initial reduction in DNA levels following treatment had been sufficiently large and persistent.