As the spots grow older the delicate fungal coating becomes extinct at the center, and exposed at this point is the brown corky layer of the host-tissue. This frequently becomes checked and cracked. In some cases the whole spot thus becomes corky so that the pathogene disappears entirely, leaving a russeted scar on the side of the fruit. Scab spots often cause a dwarfing of the side of the apple on which they occur. When the attack is severe, the fruit shrivels and falls. The removal of the cuticle permits the entrance of rot-producing pathogenes, and consequently decay of the apple-flesh appears beneath scab spots. Late infections, which appear at picking time, are much smaller, being scarcely more than mere specks; they are also very black and no marked depression nor dwarfing of the fruit accompanies such infections. The scab spots on twigs vary with the variety affected. In some cases the affected portion becomes somewhat swollen, and the twig as a whole becomes prominently blistered. Severely attacked young twigs may appear blistered over the surface to such an extent that the bark will subsequently peel in flakes. This type of injury is common in Europe, and is called grind or scurf by the Germans. In other cases the shoots are not swollen and do not present a blistered appearance. A third type of symptom on the twigs is found in cases where the scab spots are isolated, pock - like markings.
The apple scab pathogene is a fungus known by the name Venturia inaequalis. It passes the winter in the old fallen leaves as immature perithecia. With the advent of spring these sexual fruiting bodies come to full maturity by the time the blossom-buds are ready to open. These spring activities of the parasite are characterized chiefly by the formation, within the perithecia, of ascospores which constitute the principal inoculum for the first infections. Inoculation is brought about through the agency of the wind. The ascospores are forcibly discharged into the air from the perithecia imbedded in the old leaves on the ground, and these spores, being extremely light, are carried to the opening buds. Spore discharge is conditioned by (1) the maturity of the ascospores themselves. They are maturing during a period of about one month beginning approximately at the time the blossom-buds are ready to open. (2) The occurrence of rain-periods when the ascospores are mature. It has been estimated that in forty-five minutes of wet weather at the proper time no less than eight billion ascospores might be discharged from the old fallen leaves under a large apple-tree. These figures indicate how one may account for even the most abundant primary infection of young leaves and blossoms that has ever been recorded. Since the lower surfaces of the unfolding leaves and the young fruit-pedicels are most exposed at the time of first ascospore-discharge, these constitute the first points of attack. It has been frequently observed that the leaves of the blossom-buds suffer more severely than those of the leaf-buds. This is explained on the grounds that the former buds open first; the leaf-buds, opening later, often escape the early inoculation to which the leaves of the blossom - buds are exposed.
It has been noted that the ascospores are discharged under rainy conditions; and unless this condition prevails for several hours the ascospores cannot germinate, a process preliminary to penetration of the leaves and pedicels. Following a rain any conditions favoring the retention of moisture also favor spore-germination. Several factors acting in this connection are: dense foliage, which prevents prompt evaporation after rains; good air drainage favors this evaporation and accordingly those trees on the hill-tops are less liable to scab-infection than those in low pockets; showers followed by winds are unfavorable to infection since the moisture is quickly removed from the leaves, while showers in the evening followed by a calm night are highly favorable to infection. Spore-germination occurs by the emission of a mycelial thread, called a germ-tube, which penetrates the leaf to a slight extent, usually not going deeper than the cuticle as long as the leaf hangs on the tree. Sometimes the first layer of leaf-cells, the epidermis, is invaded. Within a very few days the pathogene establishes a food relation with the host, the lesion becoming visible to the naked eye within two weeks or less. The germtube, as a result of extensive growth, becomes a dense system of branching threads - the mycelium. These threads grow radially from the point of invasion, partially dissolving the cuticle as they proceed. The undissolved cuticle at the advancing margin is uplifted and gradually breaks away in flakes, forming a protective covering in the form of a papery rim about the border of the spot. While the pathogene undoubtedly gets most of its food from the cuticle, water is drawn from adjacent cells as evidenced by the one - sidedness of affected apples and in the puffing of the leaves under the scab spots.
The mycelium of the pathogene on the leaves and fruits modifies its form slightly by transforming into a stroma, a spore-bearing cushion. From this cushion arise spore-stalks, conidiophores, which cut successively from the free outer end several conidia - asexual summer spores. These conidia mature rapidly and are blown to other leaves and fruits, where they produce scab spots just as described for the ascospores. From these spots, caused by the growth from the conidia, arise again the stromata on which a second crop of conidia are borne. These are likewise capable of further spread of the pathogene. This process repeats itself throughout the growing-season, depending on conditions of moisture. If fruits are inoculated just prior to picking, the spots appear in storage; their appearance has been described under Symptoms. Fruits which fall carry the fungus with them, but the pathogene never develops further. Leaves, on the other hand, furnish the chief hibernating quarters for the fungus. As soon as the foliage falls, the mycelium of the organism penetrates all parts of the leaves. During the autumn and early winter the perithecia are partially formed, these organs lying dormant until spring. At this time growth is resumed and the life-cycle is again initiated. There is considerable evidence that the conidia hibernate on the twigs or in the bud - scales, but the old leaves on the ground are to be regarded as the chief source of the primary inoculum.