1903.] PUBLIC DOCUMENT — No. 33. 33 



Investigation of affected leaves failed at all times to show 

 any fungus or other organism which could be regarded as 

 the cause of the spotting, but revealed peculiarities which 

 point to a cause of quite a different nature ; namely, the 

 occurrence of freezing temperature and frost at the time the 

 leaves were unfolding, and subsequent cold, wet weather to 

 an unusual extent throughout the season. At the time when 

 apple trees were leaving out, a period of very low tempera- 

 ture came on, with frost and ice. Inmiediately following 

 this the first spotting of the leaves appeared, being most 

 noticeable in the most frosty places. Affected leaves showed 

 numerous dead spots, especially near the veins, where would 

 be the largest amount of water. In these spots the tissue 

 was dead and ruptured. No organism was to be found as 

 the cause of the injury, and from the sequence of events 

 there could be no reasonable doubt that the frost was the 

 destructive agency. As the season progressed, these leaves 

 gradually dropped off, as might be expected. Further than 

 this, however, the spotting of the leaves gradually increased 

 through the summer, so that in the latter part of the season 

 trees were affected which had not shown the trouble at first, 

 while those originally affected lost almost all their leaves. 

 This at first sight seemed to render it impossiljle that the 

 trouble Avas due to the spring frost, since much of the spot- 

 ting did not appear until August, particularly in well-cared- 

 for orchards. In all these cases, however, the injurious 

 effects were undoubtedly due to the same original cause. 

 Careful microscopic examination of leaves when first af- 

 fected showed not only the actual dead spots, but also many 

 other portions affected in a peculiar manner. Here and 

 there on the leaf could be found minute, blister-like spots, 

 retaining at first the natural green color. In these places 

 freezing had evidently occurred, causing more or less me- 

 chanical injury to the tissues, but not sufficient to cause 

 immediate death. The epidermis became separated from 

 the underlying cells, and more or less ruptured. In such 

 spots, apparently, originated the trouble manifested later in 

 the summer. The weather, being abnormally wet and cold, 

 produced a low state of vitality, so that the tissue gradually 

 died away in these injured places, and visible dead spots 



