Light leaf spot symptoms in oilseed rape

Leaves, flowers, stems and pods may all show symptoms of light leaf spot infection. With management centred on timely detection and understanding pathogen pressure, recognising the tell-tale signs of this disease is essential.

Caused by the pathogen Pyrenopeziza brassicae (asexual stage, Cylindrosporium concentricum), light leaf spot produces more than one infection cycle per season (a polycyclic disease).

Small structures (less than 1 mm) called apothecia develop – mainly on infected crop debris – and release airborne spores (ascospores). These spores can travel for several miles and land on oilseed rape crops. Detection of spores (in spore traps) as early as May, and throughout the summer, suggest that infection of autumn-sown crops may occur as they emerge.

Moisture encourages spore germination and initial infection. A long incubation period follows, with asymptomatic growth. During winter, the production of rain-splashed spores (conidia) spread the disease up the plant or to adjacent plants. Consequently, the disease develops in patches, with warmth and moisture favouring spread. As the pathogen remains active at temperatures lower than required for crop growth, the disease can multiply on plants during the winter months. Infection cycles typically occur every four to eight weeks.

The production of apothecia on senescent leaves results in further flushes of airborne spores during stem extension.

The first symptoms do not typically show until late autumn or early winter. Initially, light-green circular lesions form on the leaf surface (often in association with trapped water droplets). On the tissue bordering the lesion, very small white spots (spore masses) may be visible. These develop into discrete lesions, with pink-tinged centres. In late winter and spring, leaves may curl, distort, become brittle and crack. Whole leaves may even die. Severely infected plants are more vulnerable to winter kill.

Symptom expression is highly variable, depending on environmental conditions and cultivar. Some resistant varieties may display black flecking, as a resistance response to the disease. Symptoms can sometimes be confused with damage from fertiliser scorch or frost. However, the latter tends to have a sharply defined margin. Phoma stem canker lesions are also sharply defined, compared with light leaf spot lesions.

When the disease spreads to stems and lateral branches, elongated fawn lesions, surrounded by black speckling, appear. The stems may form horizontal cracks as they extend. Under humid conditions, white spore masses also form. Under favourable conditions, the disease can spread to and distort pods, which turn brown and shatter prematurely.

Initial and subsequent foliar symptoms of light leaf spot.

Oilseed rape with light leaf spot infection resulting in stem distortion.