Clubroot – Estimating yield loss

How much canola yield can clubroot steal? Clubroot is a major disease of canola in Canada and the pathogen, Plasmodiophora brassicae, continues to spread across the Prairies. No region in Alberta, Saskatchewan and Manitoba is safe from the pathogen. And no plant is safe from infection and yield loss. But how much yield can be lost to clubroot?

When clubroot was first establishing itself around the Edmonton region 20 years ago, fields were being completely wiped-out by this disease. But since resistant cultivars were first introduced in 2009, can this disease still cause complete yield loss in the field?

Andrea Botero-Ramírez, a PhD student in Stephen Strelkov’s lab at the University of Alberta, tried to answer these yield loss questions by measuring yield loss in the greenhouse and in the field. She succeeded. Botero-Ramírez found that in the greenhouse, yield declined by 0.9 to one per cent for each one per cent increase in the clubroot disease severity index (DSI). So when DSI was at 100 per cent, greenhouse results show an almost total (90 to 100 per cent) yield loss (see Fig. 1). In the field, declined between 0.54 and 0.62 per cent for each one per cent increase in DSI, so there was a yield loss of between 54 to 62 per cent when the DSI was 100 per cent. This variation in yield loss depended on the cultivar being tested.

Large differences between greenhouse and field results in this study can be explained. Greenhouse conditions are ideal – and these results indicate the potential of clubroot yield loss in ideal conditions. Field studies are more variable because soil spore levels are more variable. In the field study, P. brassicae inoculum loads in the soil varied from low to moderately high (zero to 100,000,000 spores/gram of soil), which is typical for most infested fields dealing with clubroot. Together, these studies can represent the range in which clubroot can steal yield.

A proper yield loss estimate begins with scouting to measure the disease severity index.

For the study, Botero-Ramírez evaluated roots eight weeks after seeding for clubroot severity on a 0 to 3 scale where 0 = no galling, 1 = few small galls (small galls on less than one-third of the roots), 2 = moderate galling (small to medium galls in one-third to two-thirds of the roots) and 3 = severe galling (medium to large galls in more than two-thirds of the roots). These ratings were plugged into the following formula, developed by Horiuchi and Hori (1980) and modified by Strelkov et al. (2006).

“n” is the number of plants in each severity level; N is the total number of plants evaluated; and 0, 1, 2 and 3 are the symptom severity classes. This formula provides a disease severity index (DSI) for clubroot.

Botero-Ramírez’s field results suggest 0.6 per cent yield loss for every one per cent DSI. This is a good starting point, but under severe conditions (high disease loads and ideal growing conditions) the yield loss may be even greater, as the greenhouse results suggest.

Measuring yield loss is an essential tool for farmers and agronomists to make good management decisions, and now we can now with confidence estimate canola yield losses due to clubroot in any field.

Figure 1 shows results from Botero-Ramírez’s greenhouse (orange line) and field (yellow and grey lines) results. In the field, yield loss from clubroot is about half of the disease severity index (DSI).