Black spot of rose. It is also known as leaf blotch of roses.
Diplocarpon rosae; this is the sexual name of the fungus. It is also known by its asexual state as Marssonia rosae.
Worldwide. Asia, Africa, North, South, and Central America, the Caribbean, Europe, Oceania. It is recorded from Australia, Fiji, New Caledonia, New Zealand, and Papua New Guinea.
A common fungal disease throughout the world, worst in warm (20-25°C) wet weather. Irregular black spots up to 10 mm diameter with feathery borders develop on the leaves especially the top surfaces, sometimes surrounded by yellow halos. The spots merge forming large black patches, and the leaves age early, turning yellow and fall. Spots also appear as dark purple or black spots on the young stems (called 'canes').
Spread of the fungus occurs as spores in wind-blown rain, and rain-splash. Usually, it is the lower leaves that are first infected and then the disease progresses upwards. The spores need to be in water for at least 7 hours to germinate.
The fungus survives in a number of ways depending on weather conditions. In temperate areas it survives on fallen leaves or in infected canes left on the soil when roses are pruned to remove dead or weak growth. In the tropics, it survives dry times on leaves, stems and also bud scales remaining on the plants.
A serious disease of roses, with many susceptible varieties. The loss of leaves leads to a general weakening of the plant over time and progressively fewer flowers. Susceptible varieties may be defoliated entirely.
Look for the characteristic black spots, especially prominent on the upper surfaces of the leaves. Look for the yellow haloes around the spots, and leaves that fall prematurely. Look for spore-producing bodies in the spots using a hand-lens.
There are many rose species and hundreds of varieties, so obtain advice from nurseries, garden centres, or from growers' catalogues about tolerance or resistance to black spot before making a purchase. Many shrub and floribunda rose varieties have resistance.
AUTHOR Grahame Jackson
Information from Ned Tisserat, Colorado State University. (https://wiki.bugwood.org/HPIPM:Diplocarpon_rosae); and Wikipedia. (https://en.wikipedia.org/wiki/Diplocarpon_rosae); and from CABI (2018) Diplocarpon rosae (black spot of roses). Crop Protection Compendium. (www.cabi.org/cpc). Photo 1&2 Florida Division of Plant Industry, Florida Department of Agricvulture and Consumer Services, Bugwood.com.
Produced with support from the Australian Centre for International Agricultural Research under project HORT/2016/18: Responding to emerging pest and disease threats to horticulture in the Pacific islands, implemented by the University of Queensland and the Pacific Community.
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