‘Living with the Enemy’; The crucial role of varietal tolerance in potato cyst nematode management

Potatoes are globally important food crops (ranked 4th in the world), providing an important source of carbohydrate, dietary fiber and vitamin C. In 2022, 126,584 ha of potatoes was grown in the UK (Defra, 2022), fulfilling 73% of the country’s requirement. Crop yields are threatened, however, by a prevalent and highly persistent pest; the potato cyst nematode (PCN) (Globodera pallida and G. rostochiensis). Nematicides have provided growers with an important tool for protecting yields where modest populations of PCN are found (2-15 eggs g-1 soil), but only one ‘robust’ nematicide remains in the armoury, fosthiazate, and this could be revoked under current legislation. Varieties with resistance to G. pallida (the dominant species – found in 89% of potato growing land in England and Wales) are becoming more widely available in some potato markets e.g. processing. While varietal resistance reduces nematode population growth, under heavy infections, it does not protect the yield of the plants. Tolerance, on the other hand, is the ability of the potato to tolerate parasitism by PCN and continue to produce an acceptable yield. Used without resistance, there is a risk that a tolerant variety would lead to further multiplication of PCN, causing problems in future potato crops. However, there are varieties, such as Camel, that possess both resistance and tolerance to PCN. Such varieties, could provide a sustainable (pesticide free) and a long-term solution to controlling PCN population growth while also providing yields to match increased demands.

There is currently no simple way of measuring whether a variety is PCN-tolerant or not which significantly impacts our ability to develop and deploy varieties with increased tolerance. Tolerance is a complex trait that has been attributed to a combination of root responses, root growth characteristics and general plant stress response (Blok et al., 2018). Moreover, it can be challenging to assess under field conditions due to variations in environmental conditions (Gartner et al., 2021). On the other hand, glasshouse studies with pre-determined population ranges enable tolerance to be assessed using the Seinhorst Yield Loss Model (Seinhorst, 1965 and 1998). This approach is used in work on other nematode genera such as root knot nematodes (Meloidogyne spp.) (Teklu et al., 2022) but less commonly for PCN. However, the above method is tedious, time consuming and requires large quantities of inoculum. Potato breeders need, a rapid, simple and robust method of measuring tolerance. This could be achieved through measuring biochemical factors that might be associated with tolerance, such as variations in phytohormones, e.g. abscisic acid, which accumulates in response to plant stress. Cytokinin concentration may also vary between tolerant and intolerant plants due to production in meristematic tissue, coinciding with the invasion points of PCN (Grove, 1999). Evans and Haydock (1990) indicate that tolerance may partly be influenced by differences in the hatching factors produced by different varieties, but this has not received a great deal of attention. Finally, with changes in global climate, it is important to understand how varietal tolerance is impacted by external abiotic factors such as water stress and different soil types and whether the stability of tolerance is altered by different conditions. In 2022, extreme weather conditions affected potato yields in European countries such as Belgium, Holland, France, Germany; The North-Western European Potato Group indicated a yield decline of 6% as compared to the previous year (NEPG, 2022).