Cercospora appears as small, reddish-ringed spots on the leaves that make Swiss chard unmarketable. (When they appear on beet, these rings are darker red. They do not make the beet unmarketable, however.) Rainy spells in warm weather are almost certain to bring on this disease.
Most of the literature on leaf spot advises the farmer to deep-plough crop residue, implement three-year rotations, ensure the seed is free of the pathogen, and so on. In my experience, however, these measures are of little help; if weather conditions are favourable, leaf spot will appear regardless.
In moist climates and with heavy dew and high humidity, make sure the leaves are dry in the morning before using overhead irrigation. This limits the time that free water is on the leaves, thereby reducing the chances of the fungus spores germinating.
Older leaves are likely to be infected initially and should be removed from the plant if they are unmarketable. With fungicides, it is better to be proactive and start spraying as soon as conditions favour the development of the disease. Use copper-based and other fungicides alternately.
When spraying preventatively, always use non-systemic products and make sure the leaves are completely wet to avoid gaps; these will allow spores to germinate. Systemic products should preferably be reserved for using at the onset of first symptoms.
When it first appears, this disease is often mistaken for powdery mildew. This is because the symptoms in Swiss chard differ from those seen when downy mildew attacks other crops. The leaves curl, especially in the centre of the plant where new ones are forming, and eventually become completely covered by the fungus. Downy mildew is very difficult to control in Swiss chard. In fact, I have never come across a farmer able to control it by chemical means.
It usually occurs at the end of winter and spring, and only in some years. Some plants may be very badly affected, while those alongside them may show no symptoms at all. Downy mildew can disappear on its own when the weather changes, leaving the new growth untouched.
Cucumber mosaic virus
This affects either individual plants or groups of plants and slows the growth rate. If you suspect that any leaves are infected, hold them up to the light. Those with mosaic virus are likely to have fainter mottling than leaves from a healthy plant. This is not a foolproof method, however, so prevention remains better than cure. And this means keeping aphids out of the crop.(In any case, aphids don’t go down well with customers!)
There is another fungus disease that occurs in warm and cooler, wet weather and attacks the leaves (the midrib and petiole), manifesting as blotches rather than circular spots. It usually develops in patches in the land and spreads from there, unlike leaf spot, which generally appears across the whole land, with symptoms prevalent in patches.
So far, my research has turned up nothing that looks like it. Remedies that normally control leaf spot seem to have little or no effect on this disease. From what I can establish so far, only carbendazim seems to be effective.