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THE FARMERS HANDBOOK. 



Brown Fleck. 



This is a fairly common condition. 



Brown Fleck. 



Tubers that to all external appear- 

 ances are healthy and sound, when 

 cut open show rusty markings of 

 various sizes and shapes. These 

 discoloured areas consist of dea<l 

 tissue, and must not be confounded 

 with blight. In the latter case the 

 diseased parts are, at least at 

 first, immediately beneath the skin, 

 whereas in Brown Fleck the mark- 

 ings are more internal, scattered. 

 and usually not in contact with the 

 skin. No signs of disease are to be 

 found on the stem or leaf parts of 

 the plants which produce such tubers, 

 and the cause of Brown Fleck is 

 not known. The disease does not 

 necessarily appear if seed with Brown 

 Fleck is planted. Many conditions 

 relating to the nature and composi- 

 tion of the soil, amount of moisture, 

 and weather have been investigated 

 as contributing causes, but no definite 

 conclusions have been arrived at. 



Scab. 



" scab " are to be found on potatoes, three of which are 

 : — 1. Scab due to thread- forming bacteria, Actinomyces 



Several forms of 

 dealt with here, viz. 

 chromog enus ; 2. Rhizoctonia scab, due to a fungus, Rhizoctonia solani ; and 

 3. Eelworm scab, due to a nematode worm, Heterodera radicicola. 



The skin of a potato is really of the nature of cork; it is only a thin 

 layer, but so long as it is intact it is highly protective. When the skin is 

 injured the potato endeavours to repair the damage by producing an extra 

 amount of corky cell substance around the seat of injury. In this way 

 a scab is produced, and scabbing may be regarded as the manifestation 

 of the efforts of the plant to repair injury and to protect itself from further 

 attack. Various conditions have 'been at times suggested as the cause of 

 scab, such as the presence of lime, ashes, fresh stable manure, cinders or 

 grit in the soil; also the dryness of a season, and the nature of the soil 

 (whether acid, alkaline, sandy, heavy, &c). Where experiments, carefully 

 conducted with yroper scientific precautions, have been carried out. it is 

 becoming evident that some living organism is always the cause, and that 

 the above conditions may influence its growth. In New South Wales the 

 best known scab producers are eelworms and the fungus Rhizoctonia. 



Scab Due to Actinomyces Chromog enus. — This disease is skin-deep only, 

 and its attacks are limited to the tubers. Spots consisting of accumulated 

 cork tissue are formed; these may he readily removed. This form of scab 

 does not impair the germination of the seed, but it reduces the yield, as 

 well as prejudicing the keeping qualities of the tubers. It does not in any 

 way impair the edible quality. 



