Clubroot lifecycle

The life cycle of Plasmodiophora brassicae, the pathogen that causes clubroot

Source: Ohio State University

Resting spores germinate in the spring, producing zoospores that swim very short distances in soil water to root hairs. These resting spores are exceptionally long-lived, with a half-life of about four years, but they can survive in the soil for up to 20 years. For example, Swedish research in clubrootinfested spring rapeseed fields found that 17 years were needed to reduce the infestation to nondetectable limits.

The longevity of the resting spores is a crucial factor contributing to the seriousness of the disease. A substance secreted from the roots of host plants stimulates resting spore germination. After the initial infection through root hairs or wounds, the pathogen forms an amoeba-like cell. This unusual cell multiplies and then joins with others to create a plasmodium, a naked mass of protoplasm with many nuclei. The plasmodium eventually divides to form many secondary zoospores, which are then released into the soil.

These second-generation zoospores re-infect the roots of the initial host or nearby plants. They then invade the cortex (interior) of the root. Once in the cortex, the amoeba-like cells multiply or join with others to form a secondary plasmodium. As this plasmodium develops, plant hormones are altered, which causes the infected cortical cells to swell. Clusters of these enlarged cells form “clubs” or galls.

Clubroot galls are a nutrient sink. Severely infected roots of canola cannot transport sufficient water and nutrients for aboveground plant parts. Symptoms will vary depending on the growth stage of the crop when infection occurs. Early infection at the seedling stage can result in wilting, stunting and yellowing of canola plants in the late rosette to the early podding stage. Clubroot infection that occurs at later stages may not show plant wilting, stunting or yellowing. However, infected plants may ripen prematurely, and seeds will shrivel. Infected plants will reduce the yield and quality (oil content) of the crop.

Clubroot

Photo by Saskcanola

Recipe for Clubroot Management (excerpts from clubroot.ca)

1. Vigilantly scout all canola fields (early) for symptoms, even if growing a CR (Clubroot Resistant) variety. Clubroot disease symptoms are most noticeable late in the season and remain visible during and after harvest. Producers are strongly encouraged to familiarize themselves with clubroot symptoms (https://www.canolacouncil.org/canolaencyclopedia/diseases/clubroot/identifyclubroot/) and start scouting now before the galls degrade. Infected plants, removed from the soil late in the season, may have galls that appear brown and peaty. These galls may even detach from the root if they are too degraded. Scout heavy traffic areas (entrances, exploration, gas wells, etc.), low areas/water runs, and areas where dust/snow tends to drift and settle. It is good practice to pull plants and examine roots whenever you inspect later-stage canola.

2. Keep a minimum two-year break between canola crops. This crop rotation is crucial in the stewardship of genetic resistance. With a 2-year gap between clubroot hosts, we see a rapid decline in living resting spores. Longer breaks may be required when spore loads are high.

3. Seed CR varieties and understand if/when to deploy different sources of CR. Planting CR varieties before identifying clubroot or it becomes established will keep resting spore loads manageable. Rotation of resistance genes could also be essential to maintain resistance efficacy. With repeated use of varieties with the same resistant traits under high spore loads, virulent races can multiply. The effectiveness of resistance may be seriously compromised.

4. Limit activities that can introduce foreign soil or cause erosion. Minimum tillage and equipment sanitation (as simple as knocking off visible dirt before leaving a field) will significantly reduce the risk of moving infested soil around. Note that wet soil conditions increase the amount of soil that clings to equipment.

5. Control host weeds. Common host weeds include stinkweed, shepherd’s purse, flixweed, all mustards and volunteer canola. They need to be controlled within three weeks of emergence to prevent a new batch of spores.

Clubroot galls on stinkweed and shepherd’s purse

6. Isolate field entrances and hot spots. Use patch management strategies to reduce spore loads, such as grassing the affected area and limit soil movement. Diseased patches that are visibly worse than the remainder of your field may have billions of more spores per gram of soil than elsewhere. These patches are often the first place where clubroot resistance breaks down. Removing these hot spots from cultivation for a few extra years significantly reduces clubroot spread and decreases resistance breakdown. Having separate field entrances and exits could reduce the amount of infested soil leaving the field on machinery.