Characterization of Resistance to Corky Ringspot Disease in Potato: A Case for Resistance to Infection by Tobacco Rattle Virus

Corky ringspot disease (spraing) is incited by the tobacco rattle virus (TRV) transmitted by stubby root nematode (Paratrichodorus spp.). The resulting damage consists of internal and skin surface necrotic blemishes which render the tubers unmarketable. Resistance to the disease has been extensively documented in breeding programs. High levels of resistance are known for a number of varieties and breeding lines. Because tubers that are symptomless but virus infected have been documented, the nature of resistance is a point of debate. In two field locations where a total of 1,259 tubers of resistant breeding lines were RT-PCR tested, 5.6 and 2.0% of symptomless tubers gave positive PCR tests in the different fields. Nearly all of these were from three clones that would have been classified as susceptible by symptoms in later testing. Taken as a whole 58% of the symptomatic tubers gave positive PCR tests. Standard commercial susceptible cultivars showed 34% symptomless tubers with positive PCR results. We have shown here that tubers of most resistant lines are not positive for the virus when asymptomatic tubers are tested by RT-PCR, while symptomatic tubers are mostly positive. Within selected resistant materials, clones with the highest level of asymptomatic positives also showed susceptible responses as assessed by visual ranking in later tests. Although RT-PCR fails to amplify from symptomatic tubers in some cases, the results of this study suggest that most resistant breeding lines do not test positive as latent carriers of the virus. Therefore, lack of symptoms appears to be an expression of resistance to infection by TRV.