What Rots will be ‘Hot’ in 2020?

The 2019 growing season started of dry across most of Western Canada and turned wet starting about mid-summer all the way through to winter snow fall, including an October blizzard in parts of Manitoba, Saskatchewan and Alberta. Now, over 4.2 million acres (2.5M in Saskatchewan; 1.6M in Alberta and over 260,000 in Manitoba) are still in the swath or standing in the field to-date in spring 2020.

The excess moisture can increase the incidence and severity of root rot diseases in many crops including cereals, canola and other Brassicaceae vegetables or pulses. Fungal species such as Pythium, Phytophthora, Rhizoctonia, Aphanomyces and/or Fusarium are known to cause root rots in many crops, and the prevalence of one species over the others is dictated by soil moisture and temperature.

For instance, as long as the soil temperature is cold, Rhizoctonia will thrive regardless of soil moisture levels. Fusarium is more predominant in high moisture soils, no matter the temperature, and the fungi can also express well in warm and dry soils. Pythium, on the other hand, is favored when the conditions are wet leading to root rots and damping-off symptoms.

Fusarium species are the most predominant cause of root rot in cereals, pulses and many other field crops and vegetables. Fusarium causes severe damages to the root system early in the season, which interferes with the normal uptake of water and nutrients. When the disease pressure is high, this can lead to premature death of the plants or yield losses as high as 60–80%; depending on the susceptibility of the crop. With the wide range of host crops, and lack of effective control measures, Fusarium root rot diseases are challenging to manage. The most common species is Fusarium avenaceum that can infect almost all cultivated crops with a preference for peas and lentils in which most damage is inflicted.

Figure 1. Fusarium root rot in field peas.

How root rots damage a plant

Root rots cause the plant’s roots to decay and rot while suffering a lack of oxygen due to submersion under water or following the effect of toxins produced by the pathogens. Plants suffering from root rots tend to exhibit stunted growth with wilting symptoms, yellowing and necrosis of the foliage. This causes significant reduction in uptake of water and minerals and a large reduction in photosynthetic activity. In addition, once the senescence process starts ethylene production increases, which spreads in the field triggering a premature ripening. Production of ethylene shuts down the production of auxins, which leads to further growth stunting, lower development of roots and a poor seed set. Ethylene also interferes with the plant ability to produce cytokinins and remain green; fixing carbon and photosynthesizing.

Factors contributing to (or lessening) root rot severity

Several factors contribute to the severity of root rots in the field. These include:

  • Crop rotation – the tighter the crop rotation using susceptible crops, the higher the levels of root rots that may be detected in the field;
  • Poor drainage – root rots are often first detected in the low parts of the field where water stagnates. Improving drainage helps reduce the excess water from occupying all of the pore spaces of the soil and improve oxygenation;
  • Excess moisture – most root rots thrive in waterlogged and saturated soils after heavy rains. Excess water helps the mobility of the spores of the pathogens and the spread of the disease (i.e., Aphanomyces, Pythium, Fusarium);
  • Weed control – rotating herbicides allows for control of some of the weed species that are susceptible to root rots that could serve as a refuge or alternate host for the pathogen in the absence of the host crop. Controlling these weeds can lessen the potential for severe root rots from developing in the field.

What rots can we expect in 2020?

Saturated soil conditions from last fall have spread pathogen spores and propagules throughout the soil and increased the expected risk of root rots during spring 2020 as species such as Pythium, Phytophthora, Aphanomyces and Fusarium thrive under wet conditions. Regardless of the temperatures expected in the spring, saturated soils planted to susceptible crops will have a high level of disease incidence and severity.

In cereals, one should expect Fusarium root rots, (take-all in cereals-on-cereals rotation); Alternaria, the causal agent of the black point disease on grains that can also cause some damage to the root system. In pulses, the biggest risks are from Fusarium root rot and Aphanomyces. In canola, clubroot, although technically not considered a root rot, thrives under wet conditions. Blackleg is also on the rise in this crop as the pathogen populations had evolved and developed resistance to the deployed resistance varieties.

Strategies to lower the impact of root rots

Soilborne pathogens and diseases are hard to manage as inoculum can come either from the seed, soil or be brought in by equipment, but there are several strategies that can be used to lower the disease below the economical threshold.

First, a soil and seed test need to be carried out for each field planted to susceptible crops or crops which are at elevated risk. These tests should provide an indication on the prevalence of the fungal species that could cause root rots. These tests should also guide the choice of the appropriate seed treatment.

The second line of defense is the use of seed treatments that provide anywhere between 2 and 4 weeks of fungicidal protection to the seedlings. This window of protection starts from seeding time not at crop emergence, which represents a challenge should the crop become slow emerging when conditions are cooler/colder. Combining these seed treatment fungicides with seed dressings containing nutrients such as phosphorus, potassium, calcium, zinc and manganese (i.e., Pulse Primer; Pulse Pak; Primer Mn) help mitigate the effect of cool/cold conditions, hence prolonging the protection against root rot diseases.

In terms of cultural practices, deployment of genetic resistances, crop rotation, drainage, residues management and delayed seeding are often practiced whenever possible to reduce the chance of spread of inoculum or to avoid the optimum conditions for the start of the disease. For instance, in Fusarium infested soil, the practice of seeding pulses mid-May rather than early May revealed to be good; as it promotes rapid seedling establishment due to the average temperatures being warm enough for plant growth and development but not enough for the proliferation of Fusarium species.

Most of the Canadian Prairies fields are cultivated under minimum- to no-till systems. Residues carrying the inoculum are often not buried, which increased the chances of infection. In addition, longer rotations are not practiced nor economically feasible because of the lack of resistant species to some of these diseases as well as the fact that some of the common weed species are able to be a refuge for these root rots in the absence of the host crop. For instance, lambsquarters, redroot pigweed, shepherd’s purse, chickweed, Canada thistle, field violet, alfalfa, volunteer canola, cereals and flax can all be an alternate host for different species of Fusarium.

How can OMEX help?

OMEX offers a wide range of Primers, Starters, Foliars, PGRs, Biologicals and Biostimulants that contain micronutrients, molecules, co-factors, activators and technologies able to stimulate a plant’s natural defenses against pathogens causing root rots and prolong the efficacy of some of the effective fungicide seed treatment that are commonly used.

For instance, Primers are formulated with key nutrients such as P, K, Zn, Mn and Ca that have the ability to fulfill the nutritional needs of the seedlings up to the 3-5 leaf stage, preventing deficiencies and stimulating the innate defense responses of the crop. To these Primers a series of PGRs, Biologicals and Biostimulants such as salicylic (SA) and jasmonic acids (JA) or Agriflora Soil can be added to stimulate the plant’s natural defenses. These additives can also be included in Starters that are formulated with TPA and Carboxylate to maximize availability and reduce to phosphorus tie-up and leaching. When placed in-furrow with the seed products such as Agriflora Soil, these additives create a biofilm around the root system, shielding it from infection by pathogens.

Foliars, on the other hand, are formulated with highly available nutrients for maximum uptake and translocation to prevent and correct deficiencies and compensate for a plant’s inability to absorb nutrients through its roots. Foliars also represent an opportunity to restore the plant’s hormonal balance, especially increasing cytokinins (i.e., Kinetin) and auxin (i.e., IBA) to counter the excess production of ethylene due to the disease. The inclusion of SA or JA can be used to stimulate defense responses and enhance the efficacy of some of the foliar fungicides.

Talk to your local Ag Retailer or get in touch with your OMEX representative to learn more about OMEX products and technologies, and how they fit in a strategy to reduce the impact of root rots on your farm.