60 



BITTER PIT INVESTIGATION. 



damage to ripe fruit, and recently has been found on maize, causing it to wither, but without leaving 

 any marks at all. As early as 1892, Dr. Cobb (15) mentions the punctures of insects, such as the 

 Harlequin Bug, as a probable cause, but in 1896 he stated definitely, "I am now of the opinion 

 that the insect Dindymus versicolor is not the cause of this disease." Bitter Pit often occurs where 

 there is no evidence of the insect, and this theory has now been given up. 



But, although the Harlequin Bug was discredited, the theory of insect punctures and the 

 injection of a poison into the fruit was not dropped. And this seemed to have a more scientific 

 basis to rest upon, for Stigmonose, as it was called by Woods (120), was shown to be a disease of 

 carnations, causing " spotting " of the leaves, and produced by the punctures of insects. The 

 spots were mostly produced by Aphides and Thrips, and in making a section of the leaf, the sucking 

 apparatus of the insect was found in the tissues. 



In 1903, Dr. Cobb (19) called attention in the Agricultural Gazette of New South Wales to a 

 disease common on apples, pears, and quinces, but less so in the last, characterized by irregularities 

 of the surface and browning of the tissue beneath the skin. The illustrations given are identical 

 with those of the pear (Fig. 18) and the apples in Plate VII. The disease is there named " Stigmonose," 

 and, from the description and drawings, there is no doubt that, in the case of the apple, it is " Crinkle " 

 — " A short distance under the skin there appears a layer of spongy brown tissue, characteristic 

 of this disease as it appears in full-grown apples "—and, in the case of the pear, Bitter Pit — " from 

 end to end the peculiar malformations are to be seen, though they are most numerous at the blossom 

 end." 



All the apple varieties affected were found to be infested with the so-called red spider, and 

 that was then regarded as the probable cause. But it is interesting to note that, in a communication 

 to me from Dr. Cobb, dated 17th April, 1901, the following remarks occur about Bitter Pit : — " The 

 theory that gives me the greatest stimulus is that we have in Bitter Pit a kind of Stigmonose — that 

 the starting point is some mechanical injury, such as an insect puncture, and one is driven to suppose 

 the cause to be one that escapes ordinary observation. I have some pretty strong circumstantial 

 evidence against the red spider as a cause of Stigmonose in apples. I have often thought of mosquitoes 

 in this connexion, and I certainly should not overlook them in a search for the cause of Bitter Pit. 

 I now recall the fact that an orchard I had charge of 25 years ago suffered most severely (on 

 Baldwins) on the side next to a swamp that bred mosquitoes, but no recent opportunity has occurred 

 for me to make any observations in this connexion." This mosquito theory has been recently 

 revived by Dr. Greig-Smith (36), who considered that these pits might very well have been caused 

 by the injection of fluid into the fruit by insects such as the mosquito. Their piercing apparatus 

 would reach about as deep as the centre of most of these brown spots, and, moreover, these spots 

 only grew to a certain size. The sterile nature of these spots and their limited growth fitted in 

 with the injection theory, for if the disease were produced by microbes, one would expect the spots 

 to continue growing. The deep-seated spots are not hereby accounted for, and in the very numerous 

 sections of the surface of the apple leading to the spots I have never found any trace of the sucking 

 apparatus of any insect. Nevertheless, experiments were carried out to test this theory. It is not 

 such a simple matter as it looks to enclose an apple tree in mosquito netting, so that no insect will 

 gain access to the tree before the fruit has set. In the first place the variety must be self-fertile, 

 so that fertilization will occur in the absence of insect life, and, besides, in the still air beneath the 

 net there may not be sufficient motion to scatter the pollen. Further, the rain might rot it, and 

 hailstones pierce it so that all these contingencies had to be guarded against. Trees of a convenient 

 size of the variety Jonathan were chosen, and covered with mosquito netting and cheese-cloth 

 respectively, and wire netted over to prevent the net being damaged by the weather (Fig. 110). 

 A solitary apple grew on the enclosed tree, while those alongside, under normal conditions, bore 

 freely, and it was removed on 6th March, 1912. The fruit was normal, but not too brightly coloured, 

 on account of being deprived of a certain amount of light, There were a few distinct pits of a greenish 



