This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
The havoc and devastation caused by parasitical Fungi exceed that caused by all other injurious agents, and where they have once established themselves, there is greater difficulty in dispossessing them than is the case with insects. There are many species or varieties, or what are now known in some cases to be different stages or conditions of the same species, constituting what is popularly called Mildew. It is still doubtful whether these Fungi attack perfectly healthy plants, or whether they are the effect of bad health, the cause being attributed to unfavourable conditions of temperature and soil, which produce decay of the epidermis and thereby expose the plant to the attacks of these parasites. However that may be, there are always accompanying unhealthy symptoms, and mildew is most flourishing in a cold cloudy season. There is, moreover, a great difference in the predisposition of different varieties of the same species (e.g. Roses) to the attacks of these insidious organisms; some are subject to mildew almost every season, whilst others as rarely betray a trace of its presence, even though growing in the midst of infected plants. This much is certain, that plants in a healthy, vigorous condition will outgrow the disease much better than stunted, weakly ones will. But of course this does not materially aid in the elucidation of the first cause of the appearance of these parasites on different plants.
Mildew in all its forms, if taken at an early stage, before it has spread too widely, may be extirpated by the application of flowers of sulphur. Sulphur is, perhaps, more effective in its action if applied dry; but this being a tedious process, it is usually mixed with other ingredients in water, and the plants syringed with the mixture. Under glass, the remedy against red spider (a minute parasitical insect), namely, moisture, is favourable to the development of mildew; but if flowers of sulphur be mixed with the water, or placed on the hot-water pipes, there is little to fear from either of these pests. The red spider is never so troublesome in the open air, though in dry, hot seasons it sometimes does great damage. It is usually found on the under surface of the leaves, and increases with astonishing rapidity, soon covering the whole leaf, causing it to turn yellow and fall off. Some of the Junipers are very subject to its attacks, unless planted in humid places which are natural to them.