Ground Cover Supplement : GC Supplement - Foliar fungal diseases of pulses
8 Canola CURIOUS CANKERS CONFIRMED AS BLACKLEG Sowing time and variety resistance rating can be used to minimise new blackleg symptoms By Dr Steve Marcroft UNUSUAL BLACKLEG SYMPTOMS on canola stems and branches observed across southern Australia during 2014 were likely the result of early-sown crops developing faster than normal and hitting the blackleg disease peak during early flowering. Symptoms included lesions and cankers on stems and branches, and inside the stems, causing the pith to become blackened (Figure 1). Lesions varied in size from one centimetre through to the entire length of the plant and in many cases branches died prematurely. Some lesions contained pycnidial fruiting bodies (small black dots); however, many lesions had no fruiting bodies. In some instances the stem and branch lesions/ cankers appeared to cause more yield loss than typical crown cankers. Normal blackleg infection typically occurs as lesions on the cotyledons and the first true leaves but infection can develop on any plant part. The fungus grows from the lesion to the crown (the junction between the stem and roots) where it causes a canker, blocking the vascular tissue and causing the plant to fall over and die (Figure 2). EARLY-SOWN CROPS Molecular marker technology confirmed that the pathogen associated with the infected canola stems and branches was indeed the blackleg fungus. Members of the GRDC-funded National Canola Pathology Program believe the unusual blackleg symptoms were caused by environmental conditions. Normally blackleg infection occurs from May through to July, which usually coincides with the vegetative stage of the canola crop (when no stem/branches have yet formed). In 2014, crops were sown early and warm conditions through early winter meant that plants were more developed than usual by July. Blackleg infection was at its most intense when canola crops had elongated, produced branches and had their first flowers. The 2014 blackleg infection was exacerbated by frost damage to green (and therefore soft) canola branches, which provided an entry point for the blackleg fungus. DISTINGUISHING BLACKLEG AND SCLEROTINIA STEM ROT Blackleg stem and branch infection can be misdiagnosed as sclerotinia stem rot. Sclerotinia does not produce the pycnidial fruiting bodies (small black dots – see Figure 2 below) characteristic of blackleg but instead produces sclerotia – small, dark and hard survival structures (Figure 2, page 4). □ GRDC Research Code UM00051 More information: Dr Steve Marcroft, Marcroft Grains Pathology, 03 5381 2294, email@example.com SOURCE: STEVE MARCROFT SOURCE: STEVE MARCROFT FIGURE 1 Stem and branch blackleg symptoms observed across southern Australia in 2014. Blackleg canker on canola branch. Blackleg lesions progressing up canola stems. FIGURE 2 Typical blackleg symptoms on canola leaves and crown. Leaf lesions from blackleg normally occur during autumn and winter on cotyledons and leaves. Typical canola crown canker caused by blackleg. Blackleg lesion on canola stem. Blackened pith inside infected canola stem. FIGURE 3 Typical sclerotinia symptoms. Sclerotinia causes white, bleached lesions that normally completely girdle the stems and branches. Sclerotinia does not produce the pycnidual fruiting bodies (small black dots) characteristic of blackleg but instead produces sclerotia – small, dark and hard survival structures (pictured). SOURCE: STEVE MARCROFT Early-sown canola is more likely to be flowering and elongating during peak blackleg severity and suffer stem and branch cankers as a result. MANAGEMENT OF STEM/ BRANCH INFECTION ¢ Time sowing date so that stem elongation occurs in the normal flowering window rather than in winter, when blackleg infection is at its most intense. ¢ Sow varieties with effective major gene resistance for blackleg. In 2014 varieties from blackleg resistance Groups D, E and F did not develop the stem/branch cankers, even at sites where varieties from Groups A, B, C and S did develop branch cankers. ¢ It is important to note that seed-dressing fungicides and Prosaro® applied at the four-to-six-leaf growth stage will not protect stem and branches from blackleg infection.
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