Nearly all varieties of apples suffer to a certain extent from Fruit Spots of one kind or another, but on some varieties like the Tolman Sweet and the Yellow Bellflower there is a specific kind of spotting which is now recognized as an important disease. Other varieties of apples also show this disease, but to a less degree than those mentioned. Sometimes the Baldwin is considerably spotted and thus the disease has been called brown spot of the Baldwin. It has been erroneously referred to as stippen and Dry Rot.
In 1892 a Fruit Spot was noted on quince in New Jersey. It seems likely that this is the first authoritative record of the disease in the United States. Subsequent reports indicate that it is widely distributed throughout the northeastern United States. It apparently does not occur west of Michigan nor south of Virginia. It is said to be common in Canada, and is also reported from Germany. No doubt the greatest losses are incurred by Fruit Spot in New England; and often the disease is very appropriately called the New England Fruit Spot and the New Hampshire Fruit Spot. In these sections of the United States 50 to 90 per cent of the fruit is spotted in epiphytotic years. In the state of Maine the disease is so common on the Yellow Bellflower that the spots have come to be regarded as characteristic markings of this variety, and thus affected fruits have been awarded prizes at fairs and fruit shows. But affected fruit must be graded as a second-rate product, thereby incurring considerable loss to apple - growers concerned.
The disease has been described on the Baldwin fruit somewhat as follows (Fig. 24): Spots are first seen about the middle of August. They are deeper red on the colored portion of the apple and darker green on the lighter portion. The affected tissue is at first only slightly sunken, if at all. Usually each lesion centers about a lenticel. There are from two to ten times as many spots on the blossom-end as on the stem half of the apple; this is due in a great measure to the larger number of lenticels at the blossom-end. The spots enlarge slowly and never attain great size; the color is finally dark-red, brown or black and the surface of the spot more sunken than at first. In this stage the disease has some semblance to Black Rot. At first the spots are superficial, and only in the later stages is the flesh noticeably discolored. In no case does the discoloration extend more than a small fraction of an inch into the pulp. The tissue beneath the discolored skin is rendered brown and corky. Black specks abound on the surface of the lesion: usually one appears in the center of the affected area and others are arranged radially about it. These are the fruiting pustules of the causal pathogene. On yellow varieties like the Yellow Bellflower and Tolman Sweet the spots are at first almost a carmine red. Later they turn brown in color. This change in color may occur while the apple still hangs on the tree, particularly if the weather is wet for a few weeks prior to harvesting; or the change may be delayed until the fruit is stored. Spots on affected fruit increase but little in size when placed in cold storage.
The fungus Phoma pomi produces this apple Fruit Spot disease. The pathogene probably hibernates in the form of sclerotial masses and as peculiar thick-walled cells known as chlamydospores. These structures winter over on fallen fruit. With the advent of the growing season conidia originate from both the sclerotia and the pycnidia on the fallen fruit, and these spores bring about the first infections of the year. Observations show that a larger number of the inoculations take place in July or early August. None occur after the last of August. Entrance into the fruit is gained by way of the stomata. After the spores are deposited on the apple fruit fifty days may elapse before there are any signs of the disease visible to the unaided eye. Owing to this condition of affairs fruits may be inoculated before picking but show no evidence of Fruit Spot at the time of harvest. In due time, however, the disease develops in storage or in market. On entering the apple-tissue the germtube of the fungus rapidly develops a system of mycelium; the threads grow between the cells of the fruit. The large flesh-cells react against the invasion by the fungus as evidenced by their thickened walls. Within the center of such an affected region a small pocket is produced by the collapse of one or two cells. The rate of spreading varies with the variety. After the fungus is well established within the fruit, mycelial masses are developed beneath the epidermis. Ultimately these break through the protective layer and expose themselves as cup - shaped fruiting structures within which conidia are produced. Sometimes pycnidia are developed on affected areas; in these are formed pycnospores which assist in disseminating the fungus.
Fig. 24. - Apple Fruit Spot.
This apple Fruit Spot is readily controlled by the use of fungicides. Two applications give satisfactory results: the first should be made about the last of June and a subsequent application should be made as late as the middle of July. Earlier and later applications as a rule have little value, while those made early in July are very efficient. If spraying or dusting for apple scab is practiced, the second and third applications for the scab disease will suffice for Fruit Spot. These applications are made (1) when about two - thirds of the petals have fallen and (2) about three weeks later.
Brooks, Charles. The fruit spot of apples. New Hampshire Agr.
Exp. Sta. Rept. 19-20:332-365. (Also published in Bulletin Torr. Bot. Club 35: 423 - 456.) 1908. Brooks, Charles. Some apple diseases. The fruit spot of apples.
New Hampshire Agr. Exp. Sta. Bul. 144: 116-119. 1909. Brooks, Charles. Some apple diseases and their treatment. Fruit spot. New Hampshire Agr. Exp. Sta. Bul. 157: 10 - 13. 1912. Brooks, Charles and Black, C. A. Apple fruit spot and quince blotch.
Phytopath. 2: 63 - 72. 1912. Clinton, G. P. Report of the station botanist. Apple fruit spot.
Connecticut Agr. Exp. Sta. Rept. 1909-1910: 723 - 724. 1911.