Rose Black spot
Rose black spot is caused by the fungi Diplocarpon rosae, a pathogen that before the 1960's had very little attention because of high pollution levels in cities restricting it. However, due to drops in Sulphur dioxide levels in the atmosphere and a better air quality from then onwards it has had a massive economic impact on the rose industry, estimated in the 10’s of billions and is presently the most damaging widespread disease of cultivated roses. Initially a rose black spot is poorly defined purple-brownish sblodge which expands rapidly on the upper leaf surface with visible strands of hyphae growing outwards. This may be followed by yellowing leaf tissue and leaves falling off. Sometimes all the leaves may fall off the plant up to twice in one season resulting in decreased plant vigour, weakening the plant and making it susceptible to further problems. Fallen leaves will then carry the disease saprophytically over winter until spring, when the fungi becomes active again and acervuli disperse conidia (spores) as a result of rain splash making it a localised dispersal. These conidia (spores) then may infect further foliage if it remains wet for 7 hours.
Avoidance tactics include refraining from watering above the plant to minimize splash as well as removing and destroying diseased foliage. A range of chemical fungicides may be used throughout the growing season. Some advances in genetic research have shown reduced severity of blackspot in some cultivars. So, pick your roses carefully, or you might get pricked.
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