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Clubroot-resistant cultivars – a great tool

Photo by Janet Kanters.

Dec. 13, 2013 - Clubroot-resistant cultivars are by far the best tool available for dealing with this devastating disease in canola crops. Recent research has found that these cultivars not only produce much better yields in clubroot-infested fields, they also reduce clubroot spore populations in the soil. Just as important, this same research demonstrates the importance of crop rotation to protect that crucial clubroot resistance.

Clubroot is caused by a soil-borne pathogen called Plasmodiophora brassicae. The pathogen affects canola and other cruciferous crops such as mustard as well as cruciferous weeds such as stinkweed. The disease is called clubroot because of the large, irregular swellings (galls) that form on the plant roots. These galls hinder the plant’s ability to move water and nutrients to the upper parts of the plant, so the plant wilts, becomes stunted and may die. Yield loss has reached up to 100 per cent when susceptible canola cultivars were grown in clubroot-infested fields in Alberta.

Since clubroot was first found in a handful of canola fields in the Edmonton area in 2003, this disease has become a major concern in Alberta canola. The pathogen has been detected in Saskatchewan and Manitoba, but so far it has not caused serious disease problems in canola in those provinces.

Plasmodiophora brassicae lives in the soil as tiny resting spores, which can survive for over 15 years. The roots of cruciferous plants emit substances that stimulate the resting spores to germinate and release zoospores. These spores swim through the soil water to the plant’s root hairs and penetrate the cell walls. Once inside the root hair, they form structures called plasmodia. The plasmodia release secondary zoospores that penetrate the root cells and form secondary plasmodia.

It is this secondary phase that leads to gall formation, and it’s the galls that result in major yield loss. The galls on a single infected root can produce billions of resting spores. Those billions of resting spores are ready and waiting in the soil for the next canola crop to come along.

Fortunately, in the last few years, a number of seed companies have released clubroot-resistant cultivars. “Pioneer Hi-Bred [now known as DuPont Pioneer] was the first seed company to release a clubroot-resistant cultivar. Everybody, including me, was so excited,” says Dr. Sheau-Fang Hwang with Alberta Agriculture and Rural Development.

But as a plant pathologist, she wanted to learn more. “I wondered whether the cultivar is really 100 per cent immune to the pathogen and how growing the resistant cultivar would affect the pathogen’s spore populations in the soil.”

Hwang also wondered what would happen to the spore population if some susceptible plants were grown in field with a resistant line; for example, if there were susceptible canola volunteers, cruciferous weeds or susceptible off-types of the resistant cultivar. In addition, she wanted to know whether a resistant cultivar might act like a bait crop – by stimulating germination of resting spores but not allowing the pathogen to complete its life cycle – which would cause the resting spore population to decline more quickly over time. And she wondered if growing a resistant cultivar year after year would start to select for pathotypes (strains) of the pathogen that are able to overcome the cultivar’s resistance, a situation that would cause the resting spore population to eventually increase.

So Hwang conducted a number of studies under greenhouse, mini-plot and field conditions. Her initial work involved Pioneer’s clubroot-resistant cultivar 45H29, and she later confirmed her results using resistant cultivars from other companies.

One of her key findings was that the resistant lines produced much lower levels of resting spores than the susceptible lines. In her greenhouse studies, when she grew a susceptible line three times in a row – the equivalent of a susceptible-susceptible-susceptible (SSS) rotation – the number of resting spores and the disease impact on the plants increased after each cycle. When she grew a resistant-resistant-resistant (RRR) rotation, the resting spore population declined and disease severity on the plants remained relatively constant.

“Of course, when you grow a resistant cultivar, you get a much better yield. My study proved you also get another benefit: that the contribution of resting spores back to the field is less,” says Hwang.

Hwang also grew a susceptible line after three cycles of resistant lines (RRRS) and after three cycles of fallow (FFFS). She found disease severity on the susceptible plants in the two treatments to be the same. The fact that the effects of fallow and the resistant lines were the same suggests that the resistant line was not acting as a bait crop nor was it selecting for strains able to overcome its resistance.

As expected, all of the plants from the susceptible cultivar had galls. Surprisingly, 14 per cent of the plants grown from the resistant cultivar seeds also developed galls. So the resistant cultivar was not 100 per cent immune to clubroot. One possible reason for this is that there might be some off-types within the plant population of resistant cultivars.

“That brings up another issue: we cannot abuse these resistant lines. We still need to do rotations because there is the potential for the resistance to break down,” emphasizes Hwang.

She outlines how the resistance might break down: “For example, in Alberta canola fields, the clubroot spores are mainly pathotype 3, but there are small populations of other pathotypes. The resistant cultivars have been developed to be resistant to pathotype 3. If you keep growing these resistant cultivars, then they will suppress pathotype 3, but the other pathotypes are not being suppressed. So eventually a different pathotype could become prevalent and the cultivar’s resistance will not be effective any more.”

But she adds, “Canola is an important crop, so I expect the seed companies are developing lines that are effective against some of the other pathotypes in case these other pathotypes become common.”

In both susceptible and resistant plants, the primary zoospores infect the root hairs. However the secondary zoospores don’t cause noticeable galls in resistant plants, indicating that this is the stage of infection where the plant’s resistance kicks in. However, in Hwang’s microscopic examinations, she found very tiny galls on the root hairs of resistant plants. This is another indication that the resistance could eventually break down, especially if growers use very short canola rotations.

Hwang also examined the effects of growing various proportions of susceptible and resistant cultivars together. The results show susceptible plants likely play a significant role in the persistence of viable resting spores in a field. These findings reinforce the importance of controlling canola volunteers and cruciferous weeds in clubroot-infested fields, even if the field is not going to be sown to canola that year.

For any disease, the potential for a cultivar’s resistance to break down is higher if the resistance is based on only a single gene in the cultivar. Hwang says that the seed companies haven’t yet released information about the genetic background of the clubroot resistance in their cultivars. However, she thinks it’s possible that the various companies may be using somewhat different genetic bases for the resistance. So she suggests growers consider including different resistant cultivars in their rotation as another way to slow the breakdown of resistance in their fields.

“We are very lucky that all these resistant lines have been released,” says Hwang. “All these lines give good yields, even in heavily infested fields. And they also contribute lower amounts of resting spores to the soil. This research is in advance of any problems, so we can get ready to fight back.”

Overall, her research results point to the importance of some key practices for growers. If you have clubroot in a field, grow resistant cultivars and use crop rotation – the Canola Council of Canada suggests a four-year rotation. Consider including different resistant cultivars in your rotation. And control volunteer canola and cruciferous weeds in your fields.