232 Journal of Agriculture, Victoria. [10 Apkil. 1919. 



been destroyed, in the country districts the disease has made consider- 

 able progress, and diseased plants have been found in nearly all parts of 

 Victoria. 



Symptoms of the Disease. 



The affected plants usually show the earliest evidence of attack on 

 the young terminal leaves, from which it spreads rapidly to the lower 

 leaA^es, 



Affected leaves at first show a slight discoloration on the surface, 

 and later numerous distinct blackish, or brownish, spots develop on their 

 upper surfaces, sometimes, though not often, appearing also on the 

 under side. If a leaf be held up to the light the spots will be seen dis- 

 tinctly as a pattern between the main veins. They may, however, be 

 confluent when the whole leaf, with the exception of the main veins, is 

 opaque. 



A more critical examination disclosed the fact that the veins in some 

 cases, as well as the mesophyll of the leaf, are also discoloured. Some- 

 times one-half of the veins are black or brown along some part of their 

 length, and the other part clear or semi-transparent. Sections show that 

 the vascular bundles are affected, but the injury to the leaf is almost 

 always confined to the upper cells only. 



The remarkable feature of the disease is the rapidity of its action. 

 Plants, apparently healthy, develop within a few hours slight spotting of 

 the leaves, and completely wilt from the tops downward in about 24 to 

 30 hours. Occasionally affected plants may last for several days, but in 

 the majority of cases wilting and death take place rapidly. 



Spotting is not confined to the leaves, but occurs on petioles and 

 stem, appearing either as minute spots or thin brownish black lines or 

 streaks, varying from 5 to 8 m.m. long by 1 to 5 m.m. broad. 



On the stem and petioles the spotting is entirely superficial, and 

 restricted to the outermost cortical and epidermal cells. The fruit is 

 affected also, and the spots vary from a few to a number so great as to 

 become confluent. The spots are of different size and shape, varying 

 chiefly from circular to oblong, sunken or superficial, and in colour 

 from brown to brownish black. In severe cases the tissue beneath 

 the spots is injured and discoloured for a considerable depth into the 

 flesh of the fruit. Diseased fruits, as a rule, fall, and those remaining 

 on the plant do not ripen, or only redden in part, and are quite unfit for 

 market. As a rule, however, affected plants fail to produce fruit, except 

 in those cases where the plants have been affected late in the season. Very 

 young fruits when affected turn brown, shrivel, and fall. 



If the stem of the affected plants be split or cut through, no dis- 

 coloration will be observed. The pith in parts is normal, and in others 

 dry, shrunken, and cracked, forming numerous partitions, Avith spaces 

 between, leaving sections of the stem more or less hollow. The disease 

 does not affect the root system, which is usually of normal development, 

 and in no way discoloured, and showing no signs of attack, either by 

 fungi or nematodes. Careful microscopical examination has, so far, 

 failed to reveal either fungi or bacteria within the plant tissue, and 

 cultural methods have also failed to produce any organism. 



Strong, vigorous, and luxuriant plants seem, on the whole, to be more 

 subject to attack, but thrifty and unthrifty are liable to infection. 



