Physiological disease is a term under which is included all those diseases the cause of which cannot be attributed to some parasitic organism. Their origin is variously attributed to abnormal enzymic activity, disturbed nutrition, and the like. The best-known of these are peach-yellows, chlorosis of the vine, tip-burn (Fig. 1291), mosaic disease of tobacco and leaf-roll disease of potatoes.

The various parasitic organisms cause disease in one of two ways, either by the secretion of toxines and enzymes which at once kill the plant tissues and change them into forms readily available as food for the invader; or the toxins and enzymes secreted merely stimulate or irritate the plant tissues in such a way as to result in abnormal tissue growth or diversion of the food substances of the host to the advantage of the parasite making its home between or in the cells of the host. Both types of disease-production have the same ultimate result, the serious injury or destruction of the infested plant, although the former is usually the more rapid and destructive. Of the first type, rots, blights and leaf-spots are the best examples, and are characterized by the rapid death and destruction of the affected tissues; of the second type, galls, leaf-curls, rusts and smuts are good examples and are characterized by a rather long period of association of the parasite with the living tissues of its host before marked injury or death of the plant results.

The causal agent is usually associated with the tissues of the host, either the dead or living, during its entire cycle of development. The apple-scab pathogen, Venturia inaequalis, will serve admirably to illustrate. It passes the summer on the surface of leaf and fruit. In the autumn when the infested leaves fall to the ground, the fungus, which as a parasite has invaded only the cuticle of the leaf or fruit, now penetrates the dead tissues and develops there during the autumn the winter form of fruit bodies, the minute globose black perithecia, in which during the warm days of early spring the ascospores are rapidly developed. These ascospores (Fig. 1292), eight in a sac, ripen and are discharged by the spring rains that come during the blossoming period. The old leaves on the ground are filled with millions of these minute perithecia with many sacs of ascospores in each perithecium. The spores are shot into the air during the rain and being exceedingly light are carried to the opening leaves and forming fruits, where they germinate, sending out mycelial threads into the cuticle of leaf or fruit forming the characteristic dense dark green or black mats or crusts, the scab-spots. The leaves become crumpled and injured, the young fruits grow one-sided, or if the stem be attacked, soon drop from the tree, thus giving no set of fruit.

On the scab-spots the conidia or summer spores cut off from the tips of upright branches in great numbers, are carried by the wind to other leaves and fruits where, with the next rain, they germinate and give rise to new scab-spots and more conidia.

The life-cycle as given for the apple-scab fungus is typical of many of the fungous pathogens of our crops. It must be remembered, however, that each pathogen has habits peculiar to itself; hence the necessity for the most careful study of each that we may know its habits and peculiarities and thus be able successfully to combat it. The following illustrations will serve to explain and impress this point.

Tip growth of yellows.

Fig. 1282. "Tip growth" of yellows.

Left-hand specimen shows two small-leaved tips appearing in October, two or three of the normal leaves still remaining near the top. The middle specimen shows numerous tips appearing in August. Right-hand specimen is a healthy twig, for comparison.

Plowrightia morbosa, the fungus causing black-knot of plums and cherries, requires two seasons to complete its life-cycle. The first season there appears on the knots only conidia, followed the second season by a crop of ascospores, produced in perithecia, which form a black crust on the surface where the conidia were earlier produced. Other pathogens like Exoascus cerasi, the "witches broom" pathogen of the cherry, lives from year to year as mycelium in the branch and twigs of the broom-like growths it excites, producing each season a crop of spores on the under sides of the leaves. The blister-rust fungus of the white pine, Cronartium ribicolum, also lives from season to season in the tissues of the pine, producing each spring a new crop of spores. This pathogen exhibits another habit peculiar chiefly to certain of the rust fungi, namely that it has another stage or spore form on an entirely different host plant, in this case, the currant, especially the European black currant. The apple-rust pathogen, Gymnosporangium macropus, exhibits the same habit, passing the winter in galls formed on the twigs of the red cedar. In the spring spores appear on these galls, which germinating in situ give rise to other minute spore bodies, the sporidia.

These sporidia are carried by the wind to the young apple leaves and fruit, giving rise there to the rust disease so destructive to certain varieties like the Mcintosh and York Imperial. The spores formed on the rusted leaves and fruit of the apple are carried to the cedar, originating a new crop of galls and thus completing the life-cycle.

The tufted shoots of peach yellows.

Fig. 1283. The tufted shoots of peach yellows.

While some pathogens may develop in both living and dead tissues of their host, as we have seen in the case of the apple-scab fungus, other pathogens like the rust organism just described or the potato-blight pathogen, Phytophthora infestans, require to be constantly associated with the living tissues of their host The last-mentioned fungus passes the winter as mycelium in the tissues of diseased tubers, grows from thence up through the new shoots, slowly killing them and forming thereon the first crop of conidia, which, carried by the wind to nearby healthy plants, produce the primary infections of the season. The successive crops of conidia produced during the season on the blighted tops are washed into the soil by the rains, find their way to the newly formed tubers, and, infecting them, complete the seasonal cycle of the parasite.