500 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



CONTRIBUTIONS FROM THE WISLEY LABORATORY. 



I. Apple -leaf Spot. 

 By Fred. J. Chittenden, F.L.S. 



The occurrence of brown dead spots on the leaves of the apple was more 

 than usually frequent during the summer of 1907, and specimens have 

 been received at the laboratory from a considerable number of widely 

 separated localities. At the same time there have been repeated references 

 to the blotching and death of apple leaves and the consequent lessening 

 of the crop, particularly as regards weight, in the horticultural Press, 

 thus emphasising the fact that the trouble has been met with in nearly 

 all parts of the country. 



Many different reasons have been assigned by growers as the cause 

 of the appearance of the leaf-spftt, e.g. the prevalence of adverse weather 

 conditions during the growing season, particularly of frost or of exces- 

 sively low temperatures closely approaching the freezing-point ; the use 

 of certain spray fluids against the attacks of aphides and similar pests ; 

 the attacks of insects themselves ; and, very rarely indeed, the ' attack 

 of a fungus, which was actually the direct cause of the trouble. 



In every case of apple-leaf spot brought to our notice (and some 

 hundreds of leaves have been examined) — with the exception of a very 

 few in which the spot had been caused by a small burrowing larva 

 feeding on the soft tissue between the two skins of the leaf, leaving the 

 skins intact — the spot was associated with the presence of a fungus. This 

 form of trouble in apple culture appears to merit greater attention than it 

 has hitherto received in this country, as it is capable of causing consider- 

 able damage both in the year of the attack and in the years subsequent to 

 it, even if it should not recur with equal virulence in those years. 



Description of the Spots. — As a rule the spots on the apple leaves 

 are more or less rounded (fig. 89, a) ; but as their growth is limited by 

 the main veins, they may have one or more straight sides, They never 

 include large veins. They vary considerably in size, being from 1 mm. 

 to 6 mm. in diameter (i.e. from one-twelfth to half an inch), but the largest 

 of these measurements is very rarely attained. They are brown in colour, 

 and each is surrounded by a narrow purplish line. The tissue of the part 

 of the leaf attacked is perhaps a little thinner than the rest of the leaf 

 and is brittle.* Several spots frequently occur near together on a leaf, 

 and, where they meet, run together, so that a considerable area of the leaf 

 is killed. In the worst instances the leaf appears as if scorched with fire. 



* Spots very similar in appearance to these are produced by the fungus PJiyllosticta 

 jyrunicola, and the microsclerotia on the spots bear a superficial resemblance to the 

 perithecia of that fungus, so that at first the spots might be attributed to it ; but spots 

 caused by /'. jirunicola lack the narrow purple margin and are surrounded by a line 

 of the same colour as the spot. Injury to the leaves of apples very similar to that 

 here described has been caused in severe attacks of Fusicladium dendriticum, the 

 fungus producing the too well-known apple-scab. 



