Potato varieties for organic or reduced input production in Northern Ireland (amended Nov 2009)

Last year, we reported the results of two years’ trials of potato varieties with partial resistance to late blight, caused by Phytophthora infestans.  In 2008, we carried out a further trial to test performance against new strains of blight identified in Northern Ireland in recent years and included the Sárpo cultivar Mira, previously found to be the most resistant of those tested.  The other varieties were Santé (widely grown for organic production) and Galactica and Setanta bred at Teagasc’s Oak Park Research Centre.

The trials were planted in May and plots were either untreated or received four applications of the non-systemic fungicide ‘Shirlan’ (fluazinam) at 300 ml/ha between 2 July and 27 August.  They were exposed to a high infection pressure from adjacent unsprayed rows inoculated with a range of recent N. Ireland blight strains in early July.  The very wet weather encouraged rapid build-up of infection after the end of July.  In the unsprayed plots, Santé and Galactica were both virtually dead by 8 August, Setanta lasted about a week longer, but in Mira infection was only just over 20% by the end of August when the haulm was destroyed (Fig. 1a).  The application of ‘Shirlan’ delayed blight development; again Setanta was slower to become infected than Santé and Galactica, while Mira had only a few spots of blight (2% infection) at the end of August (Fig. 1b).

All plots were harvested and the marketable yield and tuber blight assessed after storage. The untreated plots yielded less than the treated ones; the difference was very small with Mira and was most marked in Setanta. Mira had the greatest yield of healthy tubers for both the untreated and ‘Shirlan’-treated plots (Fig. 2).

The tuber assessments were complicated by the occurrence of other fungal rots favoured by the very wet soil conditions.  Pink rot, caused by the soil-borne pathogen Phytophthora erythroseptica, was widespread and some tubers may have been infected by both P. infestans and P. erythroseptica, so it was impossible to assess these diseases separately.  The greatest percentage of rots (8%) occurred in Galactica from the untreated plots, while rots were found in approx. 6% of tubers from the treated Galactica and Mira and the untreated Mira plots (Fig. 3).  Few rots were found in either Santé or Setanta.

The relative susceptibility of the varieties evaluated in 2008 was not affected by the presence of the new blight strains.  Galactica appears too susceptible to foliage blight and tuber rots to be suitable for organic or low input production.  Santé also develops severe foliar blight under high infection pressure and levels of tuber blight have been variable between years, making this variety potentially problematic for organic production.  Mira continues to exhibit a high level of foliage blight resistance and can be grown without fungicides, but its susceptibility to pink rot could be a problem in wet years and on heavy soils.  The performance of Setanta is encouraging since although more foliage infection has been seen in this variety since the appearance of the new blight strains, its good tuber blight resistance has been maintained and the 2008 trial showed that it is also not susceptible to pink rot.

Note: Shirlan cannot be used on organic crops (Adrian Saunders).

1. Unsprayed Sárpo Mira (plot 16, foreground) with the other varieties and inoculated infector drills behind (all dead), 13 August 2008

2. Shirlan-sprayed Galactica (plot 20, foreground, nearly dead), Santé (behind, severely infected) and Mira (background, healthy), 13 August 2008

3. Unsprayed Galactica and Setanta (plots 13 and 14, nearly dead), 13 August 2008