This section is from the book "The Gardener V1", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
Only two things bearing this designation, that I am aware of, affect these divisions of the Pelargonium; the one "spot on the leaves," the other "black-rot." Both of these attack the growth, and invariably result from decay of the foliage. The worst to be eradicated is the spot, as it seems to exist in the blood of the plant; and my experience leads me to conclude that the malady is communicated by the prevalence of an impure atmosphere or bad ventilation. This, at all events, agrees with my own experience, and I am confirmed in this conviction by invariably finding that not one of the Pelargoniums, from the strongest and most vigorous grower of the ordinary zonal section to the most tender tricolor, will exist for a few weeks in the conservatory here without spot making its appearance, and this at all seasons of the year. One of the many faults belonging to the conservatory is defective ventilation. Whether my theory be right or wrong, however, this much is certain, that no sooner are the plants placed in another plant-house, after removal from the conservatory, than they begin to mend, provided their stay in the conservatory has not been too prolonged, in which case they die of the disease.
Some authorities attribute these maladies to over-feeding, and to some extent this may be true; still I have never experienced such to be the case when the plants are grown in the open air, unless the plants had been raised from cuttings taken from those affected by disease.
The most effectual remedy to apply to affected plants is to turn them out of their pots, shake the soil from the roots, cut them back, wash those that are left after removing every diseased leaf, repot in a poor mixture made up of sand and leaf-mould, and give the plants a light, airy position.
(To be continued).