Disease update- determine what may be taking some of your yield away

August 16, 2017 - Issue 21

For a quick estimation of what’s going on in fields, pull at least a dozen plants in every field you scout. Then, inspect the roots for clubroot, take a look at the stems for sclerotinia and cut at the base of the stem to examine for blackleg. Repeat in suspicious-looking thin, dead or drowned out spots. Record your observations somewhere that it will be available for several years. If that seemed easy you can improve your scouting with the procedure below.

A good time to look for plant diseases is in the couple weeks leading up to swathing or shortly after. Even if disease isn’t looking like it will be a major yield robber for you this year, you never know unless you check. Plus, it is good to assess which diseases are in your field for your records and future crop and variety choices. Patches of dying or prematurely-ripening plants are obvious areas to scout, but even clean-looking fields can provide some early warning if you take time to look.

For more thorough results and an accurate assessment of your disease situation, follow these steps:

In each field randomly select <50 plants from a “W” pattern in the field, where the five points of the “W” are at least 20 paces apart.

  • For all diseases: pull up the full plants, including roots, and individually examine for presence of disease.
  • For blackleg: clip at the base of stem/top of root and look for blackened tissue inside the crown of the stem. The amount of infection present will help identify the level of risk and the best management practices for that field in following years.
  • For clubroot: examine the roots as well as the above-ground plant material. Soil samples can also be gathered from the field entrance in a “W” pattern to test for the presence of clubroot spores in the soil.
  • Walking through the field isn’t as effective and won’t give you a proper assessment of your disease situation.

When looking at the plants (including their roots), check for each of these diseases:

Blackleg

  • Look for areas with lodged or prematurely ripened plants.
  • Examine the bottom to middle areas of the stem for lesions. Black pepper-like specks (pycnidia) may appear within the lesions.
  • When blackleg is severe enough to cause yield loss, the plant will have irregular, knotty, woody cankers at the base of the stem. This infection will eventually grow through the stem, cutting off nutrient flow.
  • After clipping at the base of stem/top of root, use the 0-5 blackleg disease rating system here to identify severity of the infection along with the incidence of the disease. If more than half the area of the stem is blackened, blackleg has likely reduced the yield of that plant.
  • Read more on this disease at blackleg.ca

Clubroot

  • Above-ground symptoms, such as wilting and pre-mature ripening, should be evident in severely-infected plants.
  • Even if you don’t see above-ground symptoms, pull plants and look for galls. Light or severe infection has essentially the same risk for spreading the disease with equipment.
  • Check roots carefully for galls. If galls are present, the best management at this stage is to prevent spread within the field and certainly from field to field.
  • Hybridization nodules on canola roots, although rare, could be confused with clubroot galls. These are small, round nodules located at root nodes and, like healthy roots, are uniformly dense inside, while the interior texture of a clubroot gall is spongy or marbled.
  • Read more on this disease at clubroot.ca

Sclerotinia stem rot

  • Look for areas with lodged or prematurely ripened plants. Brown or yellow plants scattered throughout your green crop may indicate a low level infection. Sclerotinia may just infect pods or upper branches as well. When infection is on the main stem, that’s when you will see the most yield lost.
  • Examine the lower to middle areas of the stem looking for large bleached or tan lesions. As the infected stem dries, it will appear bleached or brown — like a bone — and may start to crack open or shred.
  • Infected stem tissue is often hollow and hard, black sclerotia bodies — similar in appearance to mouse droppings — may be found inside the infected stems.
  • As a general rule, yield losses due to sclerotinia stem rot will be approximately equivalent to 50% of the disease incidence. For example, if 10% of stems are infected, yield loss will be about half that — or 5%. Read more.
  • Distinguish it from blackleg by twisting the stem (and looking for sclerotia bodies inside). Canola stems with sclerotinia will shred and shatter when they are twisted, while stems with blackleg will not.

Alternaria

  • Alternaria tends to be more damaging in juncea or rapa canola. In these crops, small black spots will move up the plant, eventually reaching the pods.
  • If alternaria is to show up in napus canola, it will be later in the season when weather and time have removed the protective wax covering on the plant. UV radiation, cool/warm fluctuations and hail will break down this wax barrier.
  • Cool, wet weather can also increase alternaria infection, and rain splash can spread the disease to other plants. In cases of heavy alternaria infection (which is rare), early swathing may have an overall economic benefit to later swathing if alternaria has moved to pods. Read more.

Aster yellows

  • Common symptoms include the malformed bladder-like pods, which produce little to no yield. The disease can also result in normal looking pods that contain only a few misshapen seeds.
  • Plants might be conspicuously taller than the rest of the crop and therefore easy to notice.
  • Other symptoms include sprouting in the pod, and purplish plants and pods, although this purpling can result from many potential causes. Read more.

Verticillium stripe

  • Disease symptoms in canola include leaf chlorosis, early ripening, stunting and as the disease progresses, necrosis and shredding of the stem tissue.
  • Once the plant is fully ripe, the stem peels to reveal tiny black microsclerotia which resemble ground pepper in appearance. These microsclerotia remain on the plant stem or fall to the soil. While this may seem similar to the blackleg symptom, these specks are below the surface for verticillium stripe and always on the surface for blackleg. Read more.
  • Distinguish it from sclerotinia by checking inside the twisted stem. Twisting open a stem with verticillium stripe will have much tinier, black microsclerotia (that resemble black pepper), while sclerotinia are twisted open they will reveal small (<2cm), hard, black cylindrical bodies called sclerotia.

Foot rot and brown girdling root rot

  • Canola plants exhibiting brown superficial symptoms at ground level likely have fusarium foot rot, which produces tan brown lesions with concentric markings.
  • Another possibility affecting canola roots at adult stages is brown girdling root rot (BGRR), which is more of an issue in Polish canola.
  • Symptoms of BGRR are rusty brown lesions on the canola taproot, which may girdle and pinch off the root if severe. These diseases can be much higher in tight canola rotations. Read more.

Grey Stem

  • Grey silvery to purplish patches develop on stem. These can cover whole stems and continue to spread in stubble as plants decompose.
  • Grey stem usually infects too late to cause significant yield loss.
  • To differentiate blackleg and grey stem at the end of the season, cut the lower stem and look for dead blackened tissue in the crown — a characteristic of blackleg, not grey stem.

If you are more of a visual person, you can also check out the CCC disease scouting videos (pre-swathing version and post-swathing version) and the CCC disease scouting guide.

Further reading:

For more information on disease scouting, check out the Saskatchewan Ministry of Agriculture’s Plant Disease Scouting 101 fact sheet.

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