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Field Crop Diseases to Scout for this Season - Spring 2021

Submitted by: David Kaminski, Field Crop Pathologist with Manitoba Agriculture and Resource Development

Article originally published in MCA's spring 2021 edition of The Fence Post newsletter.

So much of the focus in 2020 was on the effects of the global pandemic. Did you realize that 2020 was designated by the United Nations to be the International Year of Plant Health? Covid-19 pushed plant health to the back burner. Yet farmers deal with plant health concerns every year; these include diseases, insects, weeds and soil fertility. Here are some fundamental actions that can reduce the impact of these yield robbers:

  • Consider crop rotation carefully,
  • Choose resistant cultivars, and
  • Apply prophylactic pesticides judiciously.

Are you a wheat grower in Manitoba? Depending on the market class(es) you produce, you may have to choose from cultivars with limited genetic tolerance to Fusarium head blight (FHB). For example, durum wheat has all but disappeared from Manitoba with, at best, a moderately susceptible (MS) rating to FHB. The fungicides that suppress FHB are not enough, especially in the eastern prairies, which typically receive more rainfall. On the other hand, many CWRS varieties have moderate resistance (MR) to FHB. Even so, in disease conducive conditions, a carefully timed fungicide will likely be warranted.

Barley is also highly susceptible to FHB. With even tighter standards for levels of the DON toxin in malt barley, it takes a higher level of management to ensure acceptable quality. Furthermore, your buyer might specify that fungicides cannot be used on barley intended for malt even though they are registered on the crop. Consider the Keep-It-Clean directive; we all bear responsibility to ensure that end products are free from pesticide residues, to prevent market implications.

Winter cereals often escape FHB infection by flowering earlier, at a time when it is less likely to be hot and humid. Again, available resistance is limited so there are years when winter wheat can be hit hard. Hybrid fall rye is much more prone to infection with ergot, than FHB. The glumes remain open longer during rainy periods, allowing access to the insects that introduce ergot spores from native grasses.

Despite drier than average conditions across Manitoba in 2020, some bacterial diseases were problematic. Bacterial leaf streak (BLS) was documented in some CWRS fields, especially in areas that suffered from intense rainfall events and/or hail injury. Fungicides cannot control bacterial diseases; hence, the impact of a disease like BLS is “unmasked” when fungicides have been applied. Oat growers often see bacterial disease, too. Those who apply fungicides to manage foliar diseases may be disappointed with the lack of results.

Corn is another grain crop that can be adversely affected by Fusarium fungi. They cause stalk rot and/or ear rot, with the potential for toxins to render the grain unsuitable as animal feed. In a tight corn rotation, for example in the production of silage corn, another bacterial disease, Goss’s wilt can be an issue. In Manitoba, fortunately, we have yet to see the bacterium cause actual “wilt.” Rather, the disease appears mostly on foliage.

Let us turn to some of the diseases that infect broadleaf crops.

Flax acres are likely to increase in 2021 in response to a record commodity price in 2020. New growers should be aware of a number of agronomic practices that can affect yield. One is the crop’s dependence on mycorrhizal fungi, which are not supported by canola. If canola was grown in the year before flax, phosphorus uptake will be limited. The inoculum for pasmo, a fungal disease, is widespread and crops should be scouted mid-season to determine whether foliar fungicides are needed. If one has been pushing the rotation with flax, you may need to watch out for Fusarium wilt. Fusarium oxysporum lini is unique to flax, but this wilt can be very serious, at least in patches in the field. Get an agronomist or diagnostic lab to determine whether it is Fusarium wilt or one of several abiotic causes of stunting and death.

Sunflowers have a variety of disease issues, but the one that is most difficult to manage and most likely to result in quality and yield reductions. Sclerotinia is a fungus that attacks most broadleaf crops and can cause basal stalk rot, mid-stalk rot and head rot. Sunflowers are vulnerable to infection from the vegetative stage right though to harvest. Minor diseases on sunflowers include downy mildew, other stalk rots and Botrytis head rot.