Light leaf spot is normally associated with symptoms seen on the leaves, but it can have detrimental effects on stem health throughout the season.
This is because it is a polycyclic disease, meaning it can cycle more than once per season, leading to multiple infections.
Dr Georgia Mitrousia, oilseed rape breeder at Limagrain, explains: “The [movement of the disease] is called cylindrosporiose and it causes secondary infections in the stem to occur, unlike phoma where what you see at the end is predetermined by what you see at the beginning.”
While the leaf curling and distortion caused by light leaf spot seen in late winter and spring makes the plant more vulnerable to winter kill, cylindrosporiose can cause the stem to distort and horizontal cracks can appear as the stem extends.
Infection of the pods can also lead to premature pod shatter.
“Cylindrosporiose in the stem is the part of the disease that is not so well known but is being explored a lot now on the continent where it is more common,” Dr Mitrousia says.
“Yield losses associated with this disease can be up to one tonne per hectare, making it the most economically damaging OSR disease.”
Resistance ratings The severity of light leaf spot has also increased in recent years, despite the availability of varieties with good resistance.
Trials by Limagrain across the UK and Europe found that despite their high resistance ratings, some varieties were more affected by cylindrosporiose than others.
Dr Mitrousia says: “We have seen great differences in the severity of the light leaf spot disease symptoms on stems between different varieties and across our germplasm. Looking at the symptom variation between our germplasm, there is strong evidence to suggest the phenotype on the more resistant individuals is genetically linked.”
Limagrain’s emphasis on breeding for disease tolerance and resistance includes a focus on stem health.
“A healthy stem makes for a healthy crop, with better defence mechanisms against insects and abiotic stresses, plus more efficient nutrient and water uptake.”
Improved varietal resistance has seen the incidence of other stem health-compromising diseases such as phoma cause less of a problem in recent years, but for verticillium stem stripe, the pathogen is more complicated.
“It’s a soil-borne disease and it can cause great losses through canopy collapse and seed shedding. It’s more of a tricky one because you don’t see it until the end of the season and there is no chemical solution,” says Dr Mitrousia.
“The disease severity is dependent on weather patterns and yield losses can really vary at 3-34%. Increased yield losses are associated with high temperatures and drought stress prior harvest.”