Some insects have chewing mouthparts, biting holes in leaves or other plant parts with their jaws; others have a beak which is inserted in plant tissue to suck out the sap, usually resulting in loss of color, sometimes dwarfing and other deformation.
Mites belong to the spider family with four pairs of legs; sowbugs have seven pairs; and millipedes have two pairs on each of their many seg ments.
Disease in plants is an injurious process, a deviation from normal con dition, caused by living organisms (pathogens), which may be bacteria, fungi, nematodes, and occasionally higher plants; or by viruses, on the borderline between living and non-living matter; or by unfavorable con ditions. The latter are sometimes called physiogenic or physiological dis eases.
Bacteria are one-celled organisms reproducing by simple fission. Fungi are more complex. They are made up of a weft of threads, hyphae, which form a mycelium, but lack chlorophyll and so cannot manufacture their own food, obtaining it from living plants or decaying organic matter. Fungi are divided into three main groups according to their reproductive structures: the Phycomycetes, containing damping-off fungi, downy mil dews, tomato and potato late blight pathogens; the Ascomycetes, respon sible for most of the diseases found in home gardens; and the Basidio mycetes, the mushroom group, which includes rusts as well as fungi causing wood rots.
Ascomycetes have summer or asexual spores, called conidia, which are borne in chains, as in the mildews, or in special fruiting bodies called pycnidia or acervuli. A new cycle is initiated by ascospores, sex spores re sulting from a union of two cells, which are formed in club-shaped sacs called asci. These line open apothecia, which look something like minute goblets on long or short stems, or are in a roundish closed body called a perithecium, which appears to the naked eye like a black dot on stem or leaf.
Particularly important to gardeners are hard-walled resting bodies, sclerotia, which enable fungi to survive for months in the soil or on old plant parts. Some are round, reddish tan, and resemble mustard seeds; some are black and rather closely appressed to plant tissue.
Symptoms of plant disease are of three types. The majority are necrotic with part or all of the tissue dead or dying. Limited dead lesions on leaves or fruits are spots, but on canes and other woody structures they are called cancers. Blight indicates sudden death and blackening or col lapse of leaves, shoots, sometimes flowers; wilt means interference with the water conducting system so the whole plant droops; rot is the term for disintegration of tissue into a soft, wet or a hard, dry mass. Damping-off refers to seedlings rotting at the base.