In the disease known as Beet-rot, the roots turn black at the tip, where the tissues shrivel and become grooved and wrinkled extensively. Inside the flesh also blackens and finally rots. In earlier stages, only the vascular bundles are brown and blocked with gum-like substances. In advanced stages there is much gummy material in the lumina, and even large cavities filled with this gum may be found.
The rot of Cherries, Pears, Apples, Plums, etc., in store may be due to several fungi, of which Botrytis, Monilia, Mucor, Penicillium, and Aspergillus are the chief. The fruit may be attacked while still on the tree, but very often fungi and bacteria gain access to the tissues, through bruises, cracks, etc., formed in the fruit lying in the storage baskets or on the shelves.
Rot in Onions, Hyacinth bulbs, etc., is frequently due to the access of Botrytis or Sclerotinia, followed by moulds, yeasts, and bacteria in the stores.
Sour-rot in Grapes, and other fleshy fruits which need much sun to ripen them, is probably a usual result of continued cold, wet weather at the cropping season, setting in when the fruits are beginning to swell.
Flux.—It is a common event to see fluids of various kinds issuing from wounds in trees, or congealing in more or less solid masses about them; and owing to the prevailing tendency to compare plant diseases with those of animals, we find such expressions as Gangrene, Ulcer, and so forth, applied to these "open sores." In so far as such outflowings frequently indicate diseased states of injured tissues which are incapable of healing up, the analogy is perhaps a true one; but it must be remembered that very different structures and processes in detail are concerned. Moreover, liquid excretions more or less indicative of diseased states are by no means confined to wounds or definitely injured tissues, in which case such terms are wholly misapplied.
Honey-dew.—The leaves, or other organs, of many plants are sticky in hot weather, owing to the excretion of a sweet liquid containing sugar, the consistency and colour of which vary according to circumstances. This honey-dew must not be confounded with the normal viscidity of certain insectivorous plants—e.g. Sundew—or with the sticky secretion on the internodes of species of Lychnis, etc., where it plays the part of a protection against minute creeping things.
Honey-dew is often met with on Lime trees, Roses, Hops, etc. In many of these cases the honey-dew is excreted by Aphides, which suck the juices of the leaves and pour out the saccharine liquid from their bodies. The sweet fluid is in its turn sought after by ants, and also serves as nutritive material for various epiphytic fungi—e.g. sooty mould, Capnodium, Fumago, and Antennaria—which give the leaves and honey-dew a brown or black colour. Certain Coccideae also excrete honey-dew, especially in the tropics.
At least one case is known where honey-dew is formed as the result of the parasitic action of a fungus, namely Claviceps purpurea in its conidial stage on the stigmas of cereals, and this may be compared with the sweet odorous fluid excreted by the spermogonia of certain Aecidia. In both cases the sweet fluid attracts insects which disperse the spores.
Honey-dew may also be formed without the agency of fungi or insects, when hot and dry days are followed by cool nights, with a saturated atmosphere, e.g. Caesalpinia, Calliandra and other trees in the tropics, which are called rain trees owing to the numerous drops of fluid which drip from the leaves under the abnormally turgescent conditions referred to.
Cuckoo-spit.—The leaves of Willows, Meadow grasses and herbs, etc., are often seen with froth on them, in which is a green insect, Aphrophora, which sucks the juices from the tissues and excretes the frothy watery cuckoo-spit from its body.