Sclerotinia, commonly known as Sclerotinia stem rot, is a crop disease caused by fungi. This soil-borne fungus is called Sclerotinia sclerotiorum and creates white mold on a number of different broad-leaf host families. It begins as a resting spore protected by hard walls in the soil; these resting spores are called sclerotia. Sclerotia are robust and can handle unfavorable weather conditions; it can survive in this state for roughly 3 years. The sclerotia germinate in the spring and can do this in 2 different ways. Germination commonly occurs through carpogenic germination, which can take approximately 2 to 3 weeks and is often the result of conditions of prolonged moisture on the soil’s surface. This form of germination creates apothecia, which are small structures that are similar to mushrooms. They can operate for up to 10 days and prefer a temperature range of between 50 and 60 degrees Fahrenheit. While in operation, these mushroom-like fungi can release millions of spores into the air. The spores are likely to travel up to 150 meters from the apothecia, unless high winds are present; in this case they can travel much farther.
When carpogenic germination occurs, above-ground infection is common and is likely to cause severe damage to canola or pulse crops. The second form of germination for sclerotia is called myceliogenic germination. This germination process involves tiny hyphae forming in the soil and will infect a plant through their roots, often leading to an infection in the basal stem. Myceliogenic germination is more likely to affect sunflower plants. Once infection occurs through either germination process, injury will be caused mainly by fungus enzymes that are produced, which deteriorate the plants cell-wall. These enzymes are toxic to the plant and can eventually lead to death. Spores will develop on infected areas which can travel to soil or plant debris, possibly leading to a second round of infection.
Sclerotinia Identification and Habitat
When infection first occurs, the plant develops grey or white lesions that seem water-soaked and soft. These lesions can appear on both stems and leaves. Lesions later turn fragile and will look bleached in color. The leaves eventually shred and can lead to stem collapse. The lesions may also look as if the tissue is rotting in tight rings around the initial point of infection. Parts of the plant above this area will most likely change to light green or yellow, leading to wilting and then death. Lesions can also cause the plant to ripen prematurely. Sclerotinia may infect an entire field or sometimes just individual plants in different areas, most likely congregating near spots where the soil has high moisture levels. Areas that are heavily infected often emit a ‘rotten egg’ smell.
When temperatures range between 68 to 78 degrees Fahrenheit and there are high humidity levels, lesions are capable of quick development. Weather conditions that are extremely dry will slow or even stop the growth of lesions on infected plants. Dense crop canopies can also encourage fungal development. Overall, weather and moisture levels are the main determinants for Sclerotinia severity. Bad infection is common when not only moisture levels are high, but also when this condition is present when canola plants are in bloom.
Sclerotinia Management and Control Methods
Practice crop rotation, especially with non-susceptible plant types such as grasses or cereals. With that being said, it is hard to maintain a Sclerotinia-free field as air-borne spores may travel from other fields to infect new areas and certain broad leaf weeds can maintain Sclerotinia in the soil during these years of rotation. Always use clean seed and do not exceed suggested seeding rate as this can create higher crop density and thus promote infection. Increasing plant ventilation can help deter major Sclerotinia infections. Having satisfactory control is often dependent on using an integrated management system that includes the use of both cultural and chemical methods, especially in years experiencing high moisture.
The use of fungicides on this disease can improve yield and seed quality during grading. Some effective fungicides are Lance, Quadris, Astound, Rovral Flo, or Vertisan. Always be sure to carefully read the label for cautions and proper application before use. These sprays should be applied when the majority of canola plants have turned yellow to make sure petals will be covered. When fungicides are able to reach the optimum amount of petals, this can ensure the chemical will penetrate the entire canopy and protect vulnerable infection sites. These fungicides can only stop infection on the petals that were present during application; any after this point will still be vulnerable to the fungus. Additionally, the spray cannot help stem infections. Therefore, application before petal drop is important.
Alternative Sclerotinia Names
- • Sclerotinia stem rot
- • Sclerotinia sclerotiorum
- • Cottony rot
- • Watery soft rot
- • Stem rot
- • Blossom blight