Probably no one who has had anything whatever to do with apples is absolutely ignorant of the Soft rot. It is the bane of the apple dealer and consumer, and of any one who attempts to hold this fruit in either common or cold storage. Of all the rots of the barreled apple none is so ruinous, none so common, and none so absolutely destructive. It is world-wide in its geographical range, and it is variously called Soft Rot, Blue Mold, Bin Rot and Penicillium Rot. These names are all significant and perhaps are desirable in the order listed. If the very peculiar and characteristic odor, which is never absent from a diseased fruit, could be accurately described in a single word, doubtless a new and more appropriate common name could be derived. The odor is the first noticeable feature of this fruit decay, but the softness of the affected tissue will never be overlooked by the careful observer, so that the name Soft rot is very desirable.
The odor given off from the barrel or bin by apples affected with Soft rot has been mentioned; this is a very accurate diagnostic symptom so far as determining the presence of the disease in a lot of fruit is concerned. Apples which do not show Soft rot lesions may, however, carry the peculiar characteristic odor of the disease, owing to contact or proximity with affected fruit. The rotten area itself is very soft, watery and light or yellowish brown in color. In the case of lesions involving a considerable portion of the apple the skin becomes wrinkled (Fig. 25), sometimes in a concentric manner. These changes are accompanied by a characteristic moldy taste which is decidedly unpleasant. Young spots may begin anywhere on the surface of the fruit where there is a rupture in the skin. A single lesion may develop at such a rate as to involve the whole fruit in two weeks or less. The rot is primarily one of ripe apples; green fruits are, as a rule, not affected. Under conditions of high relative humidity bluish or greenish blue mold tufts develop profusely over the diseased area (Fig. 25). Cause.
The most important organism concerned in Soft rot is the fungus Penicillium expansum. It is very likely, however, that several other species of Penicillium may at times be responsible. The above-mentioned pathogene is generally familiar as an enemy of fruits. It grows as a saprophyte on a large number of dead organic materials and produces a vast number of spores which are omnipresent. These spores float abundantly in the air and ultimately come to rest on various objects, fruits included. Whenever one of these spores falls into a wound of any sort on the surface of an apple, it germinates and its mycelium proceeds to feed on the ripe fruit. Among the common types of wounds through which P. expansum gains entrance may be noted, finger-nail cuts, bruises, worm-holes, scab spots, and spray-injured places. The fact that the Soft rot fungus gains entrance to the apple through such injuries is highly important in the matter of controlling the disease. It is further worthy of attention in this connection that this pathogene cannot enter normal, unbroken skin. Accordingly it will be learned on observation that in barreled apples of a good quality those fruits at the heads of the barrel are rotted more extensively than those in the center of the barrel.
Fig. 25. - Soft rot, or Blue Mold.
Like many fungi, P. expansum develops an extensive system of mycelium in the lesion. From the germ tube, hyphse grow in all directions into the flesh. The threads dissolve their way between the cells of the fruit-pulp, as a result of which the latter are easily pushed out of position. They slip over each other at the slightest pressure. Thus when the surface of a rotted area is pressed by the finger the tissue quickly and easily gives way, offering no resistance whatever. The disease is well named Soft rot. As already mentioned, under conditions of high relative humidity greenish cushions or pustules appear on the surface of the affected area. These are tufts of fruiting stalks of the fungus which arise from the mycelium within. Numerous hyphse grow in erect fashion at the same point; their general arrangement is not unlike that of an inverted broom without the handle. The tips of these hyphae, or conidiophores, become branched in a digitate fashion, and at the end of each stalk is developed a chain of spores, or conidia. The number of conidia which might be produced on the surface of a single rotten apple would aggregate millions. As soon as mature these spores are easily broken from the conidiophore and, being extremely light in weight, float about in the air for some time.