By Mackenzie Hladun, MSc AAg, Crops Extension Specialist
f your crops are starting to take on a black to olive-green tinge, it is likely because there is a sooty mould in the crop. Sooty moulds are caused by saprophytes: fungi that live off the nutrients from dead tissues and help with the decomposition process. There are more than 100,000 saprophytes in the world, and there are even more when you consider facultative saprophytes. Facultative saprophytes are fungi that can survive off the nutrient of dead tissues if they must. Many plant-disease causing fungi are facultative saprophytes and can over winter in field residue.
Sooty mould, or black mould, is often caused by Alternaria spp., often found in combination with Cladosporium spp. It can also be caused by Stemphylium spp. and Epicoccom spp. Sooty moulds can be found on cereals, oilseeds and pulses. It will have a fuzzy dark green or black appearance on the outside of the seed pods or glumes. If there is a prolonged, wet/humid harvest, then the sooty moulds have the perfect environment for growing and producing spores. When combined, the spores are released and disturbed, and will likely land on the combine. In a year with lots of sooty moulds, the standing/swathed crop and the combines can look black.
Powdery mildew, caused by an obligate parasite fungus named Erysiphe cruciferarum, is also often seen in canola during harvest. Instead of black mould and dust, powdery mildew looks like white powder on canola stems and pods, and causes puffs of white dust during swathing and harvest, and leaves a white powder behind. Combines and swathers can be covered in white dust if there are perfect conditions for powdery mildew growth.
Saprophytes do not survive on living tissues, only dead. Because of this, there is no effect on yield. However, if harvest is delayed and the sooty moulds have a chance to grow and infect the grain, quality may be compromised. Black point and kernel smudge are also kernel diseases in wheat caused by Alternaria alternata, Cochliobolus sativus, Pyrenophora tritici-repentis, and Fusarium, Aspergillus, Cladosporium and Penicillium species. The presence of these diseases impacts the grade of the grain. Check out the Official Grain Grading Guide to learn more.
Controlling sooty moulds, powdery mildew and other saprophytes is best done by using crop rotations and managing inputs. Using a crop rotation with a minimum of three years between the same crop will help with breaking down the disease residue in the field and will reduce infection. Excessive nitrogen and irrigation will increase canopy growth, creating a better environment for disease growth and increase the incidence of disease. Use the right rates, place, and time of input application to ensure nitrogen and irrigation levels are the best they can be for your crop. Fungicides applied at the time of harvest are not economical and pre-harvest intervals should be kept in mind when making applications.
Sooty moulds can be alarming when present in high enough populations to cause puffs of black (or white in the case of canola powdery mildew) dust and cover the combine. Although not affecting yield, these late season diseases can affect quality of the harvest grain. For more information about sooty mould or powdery mildew, contact your local crops extension specialist or call the Agriculture Knowledge Centre at 1‑866‑457‑2377.