Relationship of the apple aphis and other insects to the dissemination of fire blight in apple orchards, with a view to methods of blight control
Our present knowledge, as indicated by the above data, proves that fire blight is due to a specific organism, a bacterium known in science as Bacillus amylovorus, which manifests itself in the following ways: 1. Blossom blight, due to the bacterial infecting the nectary of the flower and multiplying therein, later passing down into the stem by way of the ovary and pedicel. Apparent by the browning of the flowers, which later become blackened. 2. Twig blight, due to inoculation infections through wounds made by insects or other agencies on the young and tender shoots, blighting from the tips downward, the leaves turning brown and appearing as though scorched by fire. 3. Fruit blight, due to the progress of the bacteria up the pedicel into the pulp of the fruit; appearing as brownish or much darkened areas, later involving the entire fruit. 4. Canker blight, due to the entering of the larger branches by bacteria, conveyed by insect, bird, or mechanical agents, or by the bacillus working back from infected twigs, fruit-spurs, or water-sprouts. These cankers vary in size from barely visible areas to a girdling of the entire limb; they appear roughened and depressed with a distinct line separating the canker from the apparently healthy tissue. They are called "hold-over cankers." 5. Collar blight is nothing more than canker blight which attacks the base of the trunk usually through an infected water-sprout, borers, or mechanical injury. Trees afflicted with collar blight soon take on the sickly appearance of half starved trees, prematurely defoliating (partially or wholly), and finally dying. 6. Leaf blight, due largely to insect injury, the majority of infections the margin, either lateral or terminal, although central lesions are found. The blighted portion of the leaf being a light or yellowish brown with a faint purplish border at the advancing edge, which when active shows a narrow watery zone.
During the warm sunny days soon after rains, when the trees are in blossom and the pollen gathering insects are busy visiting the flowers, and the hold-over cankers are exuding drops of the blight bacterial, fire blight begins its havoc. Insects come in contact with this gummy substance and later visit the blossoms, thereby inoculating the flowers with the active organism, blossom blight resulting. Bees are awarded first place in spreading the blight to the flowers but flies and all other insects which visit the bloom are also liable to spread the germ. Later on in the season, as long as the tree is in a vigorous stage of growth, the aphids, ants, leaf hoppers, borers, beetles, and in fact any of the biting or sucking insects which inhabit the apple tree, may spread the organism to the succulent tissue, causing twig blight, fruit blight, leaf blight, or body blight.
It is a fact that the insect, which in one section of the country is most troublesome in disseminating blight-bacillus, may not be so important an agent in another section. Different varieties of apple trees may also vary in different sections as to their powers of resistance or susceptibility; the York Imperial being listed as a susceptible variety in Pennsylvania and Virginia while in West Virginia it seems to be quite resistant.
We cannot control weather conditions which play so important a part in the spread of blight. The weather may put a tree in a responsive or susceptible condition to inoculation and may also control insect life to a great extent. And it is indeed difficult to try to "harden up" a tree by the application of phosphate and potash fertilizers and by the discontinuing of cultivation, especially when the soil is strong and rains are frequent. Hence the more practical methods must be followed: 1. Fight the insects which spread the disease. Spray with nicotine sulphate or some other tobacco extracts in combination with the early scab or codling moth sprays. Where "Black leaf 40" is used, the recommended strength is 1 to 800 parts of water. 2. Cut out the source of infection,- the hold over cankers--cutting back an inch or two into healthy bark, and disinfect the wound with bichloride of mercury (1-1000). Prune out all blighted branches and if possible cut out all blighted twigs, cutting back several inches into the healthy wood. A severe winter often kills the germs in the twigs. All the waste material should be hauled out of the orchard and burned. 3. It is well to discontinue cultivation and application of manure or nitrogenous fertilizers as soon as blight its appearance. 4. The trees are most susceptible to the attack when planted in low wet ground; hence drain the low spots in the orchard.
Many bulletins have stated that winter pruning is inducive to wood and thus favors fire blight. Little attention should be paid to such recommendations for we need trees with good growth which will carry a crop of fruit and allow the sun to penetrate throughout the tree and color up the apples.
Many fruit growers have bearing orchards and are not anticipating new ones, so to such,the talk of susceptible and resistant varieties means little. Then too, some of the susceptible varieties are the best commercial apples to raise, and when such is the case it means careful inspection for the hold-over cankers. It is however well to pay some attention to this fact when planting an orchard.
What is needed badly is community spirit or cooperation in this difficult work of controlling fire blight. For if one's neighbor does not keep the disease out of his orchard, it is sure to spread to the adjoining orchard. So everyone should "pull together" and control blight which has and is now causing the apple grower heavy losses.