408 Agricultural Gazette of N.S.TF. [Ji^^ 2, 1908. 



The Growth of Black-spot on Cased Fruit. 



il. M. NICHOLLS, 



HiKiii Itiver, Tasmania. 



TnK (|uestiuu arises t'loin tiiiu' to time as tu whether the (Usease known as 

 Black-spot or Apple-scab (Ftisirladin/n (letidriticiim), is capable of (Irscloping- 

 (111 apples after they liave been jiickeil and placed iii cases, and ijuite recently 

 inanv growers have claimed that alt li(iiiL;h tlieir apples were ipiife clean wIumi 

 packed, they were spotted with the disease when tlie cases were opened after 

 a steamer voyage. This has been strenuously d(>nied by some authorities, 

 who contend that directly the apple is picke(l the dis(>ase ceases to develojj, 

 so that at present there is considerable doubt in the minds of fruit-growers on 

 the subject. With a view of finding out the true facts of the ease, the wri'ei- 

 recently undertook an extended investigation, with an uidiniited supply of 

 material at hand, and the results arri\ed at show that the apple-growers are, 

 to a certain extend, right in their contention. Tt must be borne in mind that 

 the disease is jjroduced by a minute fungus, w hieh grows immediately beneath 

 the skin of the apple. At first it consists of a sinyle filament, the germ-tube 

 of the infecting spore, which penetrates the skin, and commences to deve'op 

 beneath it. Tt soon e.xpands into a mycelium of flattened cells, which 

 brandies out into tree-like growths. As the fungus matures, dark-bi'ov\n 

 cushion-like masses of cells are foi-med on the mycelium, and these put out- 

 dense clusters of conidiophores, each of which bears a single terminal spore. 

 The conidiophores rupture the skin of the apple, which disappears from the 

 diseased area, and the closely-packed masses of dark-brown spores give the 

 characteristic black-colour to the spots or scabs. Where the conidiophores 

 are densely packed together, they ai-e long, and colourless in the middle of 

 their length for .some distance ; but where they are not .so crowded, as in tl.e 

 rejuvenescence of old spots, they are very short, and dark-brown througliout, 

 and often curiously clubbed at the apex. As the disea.se progres.ses, fresh 

 masses of spores are jiroduced ivtund the edges of the spot, which enlarges in 

 a roughly circular manner. As hot and dry weather comes on, the fungus 

 ceases to jiroduce s|)ores, and as those alreadv ]>roduced have fallen away, the 

 dark colour of the disea.sed spot gradually (hsappears, and nothing is to be 

 seen but a brow^nish surface of dead cells, the pulp-cells of the apple which 

 have been destroyed by the operations of the fungus. 'I'his is the means by 

 which the fungus destroys the value of the fruit, as the remoxal of the skin 

 and the killing of the surface layer of cells })revents its developiiuMit, and 

 leads to cracking and distortion. 



The mycelium extends for a comparatively long distance from the spot, all 

 round the edges, underneath the skin, but its pi'e.sence is not ijerceptible 

 to the unaided eye until it produces conidiophores. It retains its vitality, 



