How to identify and prevent root rots in canola

If you’re not scouting for root rots, that doesn’t mean they’re not there

Survey data from Western Canada’s provincial agriculture departments don’t have really good numbers when it comes to root rots. The difficulty is not many growers are scouting for them, but that doesn’t mean they aren’t there. The pathogens that cause root rot can impact yield severely, especially if infection occurs early in the season.

The 2018 canola disease surveys for Saskatchewan and Manitoba reported root rots in six per cent of fields, says Clint Jurke, Canola Council of Canada (CCC) agronomy director. Manitoba estimated the percentage of infected plants to be around 0.3 per cent and Saskatchewan did not estimate the percentage of infected plants. Alberta has not surveyed for root rots in the last seven years.

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One of the difficulties of root rot is it’s not really understood how prevalent it is, or how much damage it causes overall. “Where we do know that root rot is causing a bigger problem is often as a secondary pathogen, particularly if you have blackleg,” says Jurke. “Root rot seems to be worse in blackleg-infected plants, especially if the plants are really heavily infected.

The pathogens that cause root rot can severely affect yields, especially if infection occurs early in the season.

“Likely what happens is the plant’s defence systems are compromised by the blackleg fungus, so now the root rot pathogen has the ability to cause a lot more damage,” he adds. “It exacerbates the whole blackleg issue. It’s one of those diseases that makes sick plants even worse.”

Agronomists also note that interactions with insects, such as cabbage flies, can further exacerbate root rot. Cabbage flies produce root maggots, and when those maggots tunnel around canola roots, they open wounds where infection can set in.

Varied symptoms

Root rot complex is caused by soil-borne fungi that affect the roots of mature canola plants. The primary pathogens involved in root rot complex include Fusarium species, Rhizoctonia solani, and Pythium species. The root rot complex itself includes foot rot, late rot, root rot and brown girdling root rot. Of the four, brown girdling root rot is the most serious. In many cases, root rot diseases are initiated by infection early in the plant’s life.

Root rot is a serious issue in Western Canada. Losses are highest when wet soil conditions occur around early flowering and followed by dry weather later in the season. In some fields, infection levels reach as high as 80–100 per cent, causing as much as 50 per cent yield loss.

Foot rot symptoms include hard, brown lesions at the base of the stem. Salmon-coloured masses will often be present in the lesions.
photo: Justine Cornelsen

Identifying root rot can be tricky since the disease complex manifests in several different ways. Seedling diseases exhibit symptoms during the germination and emergence phase of the plant’s life. Adult root rots exhibit their symptoms at the two-leaf stage and later, and are mostly evident near the end of the plant’s life.

“The infection process may have started during the seedling stage for both types of diseases, but when the symptoms manifest determines which type of disease it will be,” says Jurke.

Early symptoms of brown girdling root rot, for instance, manifest as light-brown lesions on the taproot or main lateral roots. The roots will expand and merge, and the taproot will look as if it’s wearing a belt that’s fastened too tightly. In some cases, only a short stub of the taproot remains. In the field, plants affected by brown girdling root rot will ripen prematurely, often before any seed has been set.

Brown girdling root rot is most prevalent in the Peace River region of Alberta. Jurke says this is likely due to the region’s environmental conditions, cold soils and the frequency at which canola is grown.

Foot rot symptoms include hard, brown lesions at the base of the stem. Salmon-coloured masses will often be present in the lesions. Since the lesions develop late in the season, yield loss as a result of foot rot tends to be minor. When lesions occur earlier in the growing season, they can cause premature ripening and reduced yield.

Root rot symptoms are more variable in both colour and shape. According to CCC resource material, symptoms can be grouped into four types: a light-grey, oval lesion on the upper taproot; a dark-grey discolouration of the lower taproot and internal tissue (this will later turn black); a light-brown, soft, widely spread taproot lesion; or a dark-brown, sunken, sharply defined taproot lesion.

In canola-growing regions symptoms will occur sporadically. Generally speaking, Brassica rapa varieties tend to be more susceptible to root rot. Brown girdling root rot is not commonly seen outside of the Peace River region. A test should be conducted in order to confirm the disease’s presence.

Root rot prevention

Preventing root rot starts with crop rotation, as the risk of root disease tends to increase when rotations are shorter. Jurke says growers should consider including peas or other pulse crops in the rotation to reduce disease severity. Not only will they help lengthen the rotation, but they’ll also improve nitrogen levels in the soil.

“Pulse crops are infected by different pathogens, and so the opportunities for those canola-infecting pathogen strains to increase is diminished,” says Jurke.

Interactions with insects, such as cabbage flies, can further exacerbate root rot. Cabbage flies produce root maggots. When those maggots tunnel around canola roots, they open wounds where infection can set in.
photo: Clint Jurke

Likewise, it’s also important to maintain recommended fertility levels in the soil, particularly nitrogen, phosphorus, potassium and sulphur. Nitrogen, in particular, decreases disease severity.

“When plants are stressed, then their ability to fight off root infections is diminished,” says Jurke.

Disease management starts at planting — even before farmers get out in the field. To keep disease at bay, choose certified seed, which will have a fungicide package, says Jurke.

“Risk goes up dramatically if the seed is not treated,” he says.

Note that no economical chemical controls are available for brown girdling root rot or any of the adult-plant root rot diseases.

At planting, good stand establishment — so a vigorous, uniform crop — is crucial for preventing disease. Pay attention to soil temperature, planting depth and uniformity, and make sure seeds are planted into a firmly packed seedbed, says Jurke.

“The longer seeds are sitting in an ungerminated or slow-growing state below the soil’s surface, the greater the risk,” says Jurke.

Other management tips include practicing good weed control. Any weeds that are in the Brassica family are potential hosts, says Jurke. Problematic weeds include shepherd’s purse, stinkweed, volunteer canola and flixweed.

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