Comparative efficacy of fungicides for control of late blight in potatoes

In each of the three years from 2003 to 2005, a range of fungicide spray programmes were evaluated in field experiments at two sites for the control of foliar and tuber blight. The sites were located in Herefordshire and Ayrshire. The same treatment protocols were followed at each site where comparisons were made between:

• fungicides applied early in the development of the crop from 100% crop emergence (Protocol 1)
• established and recently-introduced fungicides as main season (canopy stable stage to crop desiccation) fungicide treatments together with commercial spray programmes from agrochemical manufacturers (Protocol 2).

The fungicides were applied to the blight susceptible variety King Edward. Unsprayed guard areas surrounding the trials at both sites were inoculated with isolates of the late blight pathogen to stimulate the establishment of an early epidemic.

Evaluation of early season fungicide applications (Protocol 1)

There was a clear benefit in terms of foliar blight control from early season fungicide use under both high and low disease pressure conditions.  The benefit remained evident for several weeks after the early treatments had stopped suggesting that fungicides were suppressing blight inoculum before visible symptoms were evident. This could indicate that fungicides are most effective when inoculum levels are low. There were statistically significant differences in the relative efficacy of some of the fungicides tested although this tended to vary in different seasons reflecting the weather conditions and the severity of the foliar epidemic.

Evaluation of fungicides and commercial fungicide programmes (Protocol 2)

There was a significant disease challenge (foliar and/or tuber blight) at the two sites in all three years of the project although this varied in intensity depending on prevailing weather conditions.  In all of the experiments, the fungicides tested gave good control of foliar blight compared with the untreated controls. Fungicides were less effective when disease pressure was extreme and fungicide applications were delayed due to adverse weather conditions. It was clear that the intrinsic curative properties (or kick-back activity) of certain fungicides could only be expressed when they were used shortly after high risk periods.

Resistance to fungicide weathering is an important attribute of fungicides. In 2004, the effectiveness of the core treatments in terms of foliar blight control reflected the relative rainfastness of the fungicides. There were differences between fungicides in their relative performance for the control of foliar blight. However, this was not always the case. Where the severity of foliar blight was low in an experiment the results should be treated with caution.

In 2005, the curative property of certain fungicides was evaluated. However at both sites, there was limited opportunity to test the benefit of using a fungicide with curative properties because very few Smith Periods occurred.

During the period of this investigation high levels of tuber blight were recorded. In 2004, in Ayrshire, the epidemic was particularly severe and control of tuber blight was associated with the control of foliar infection (indirect control). In 2004 in Herdefordshire, the severity of foliar blight was low and differences between fungicide performance were considered to be an effect on the tuber infection process (direct control).