What is “intermediate” disease resistance?

Clubroot

February 4, 2015 - Issue 3

A new canola variety was recently registered claiming an “intermediate” reaction to clubroot pathotype 5x, as well as resistance to established clubroot pathotypes 2, 3, 5, 6 and 8. Based on Western Canada Canola/Rapeseed Recommending Committee protocol for clubroot disease resistance, seed that is resistant (R) shows less than 30% infection based on a severity-by-incidence rating, and seed that is susceptible (S) will have more than 70% infection. An intermediate (I) reaction is somewhere in the middle.
 
A variety with intermediate reaction to 5x is not recommended on fields infected with the 5x pathotype. A minimum four-year rotation for canola is still recommended on these fields.
 
An intermediate reaction to clubroot pathotype 5x is similar to MS or MR ratings for blackleg, and to the “partial resistance” rating for sclerotinia stem rot tolerance available in some varieties. It may take the edge off full-blown infection, but economic levels of damage may still occur. An “I” rating to the 5x pathotype may reduce the size of galls and severity of infection, but in a 5x positive field, galls will still form and the probability of 5x build-up, while decreased, can still occur.  

However, for growers who do not have clubroot or have clubroot at very low levels, they benefit significantly by having an alternative source of resistance to which to rotate, since it may reduce the probability of 5x pathotype infection and build-up in their fields.

Canola Watch